MATLAB vs Python: Why and How to Make the Switch

Changing the Default Window Layout in Spyder

The default window in Spyder looks like the image below. This is for version 3.3.4 of Spyder running on Windows 10. It should look quite similar on macOS or Linux:

Before you take a tour of the user interface, you can make the interface look a little more like MATLAB. In the View → Window layouts menu choose MATLAB layout. That will change the window automatically so it has the same areas that you’re used to from MATLAB, annotated on the figure below:

In the top left of the window is the File Explorer or directory listing. In this pane, you can find files that you want to edit or create new files and folders to work with.

In the top center is a file editor. In this editor, you can work on Python scripts that you want to save to re-run later on. By default, the editor opens a file called temp.py located in Spyder’s configuration directory. This file is meant as a temporary place to try things out before you save them in a file somewhere else on your computer.

In the bottom center is the console. Like in MATLAB, the console is where you can run commands to see what they do or when you want to debug some code. Variables created in the console are not saved if you close Spyder and open it up again. The console is technically running IPython by default.

Any commands that you type in the console will be logged into the history file in the bottom right pane of the window. Furthermore, any variables that you create in the console will be shown in the variable explorer in the top right pane.

Notice that you can adjust the size of any pane by putting your mouse over the divider between panes, clicking, and dragging the edge to the size that you want. You can close any of the panes by clicking the x in the top of the pane.

You can also break any pane out of the main window by clicking the button that looks like two windows in the top of the pane, right next to the x that closes the pane. When a pane is broken out of the main window, you can drag it around and rearrange it however you want. If you want to put the pane back in the main window, drag it with the mouse so a transparent blue or gray background appears and the neighboring panes resize, then let go and the pane will snap into place.

Once you have the panes arranged exactly how you want, you can ask Spyder to save the layout. Go to the View menu and find the Window layouts flyout again. Then click Save current layout and give it a name. This lets you reset to your preferred layout at any time if something gets changed by accident. You can also reset to one of the default configurations from this menu.

Running Statements in the Console in Spyder

In this section, you’re going to be writing some simple Python commands, but don’t worry if you don’t quite understand what they mean yet. You’ll learn more about Python syntax a little later on in this article. What you want to do right now is get a sense for how Spyder’s interface is similar to and different from the MATLAB interface.

You’ll be working a lot with the Spyder console in this article, so you should learn about how it works. In the console, you’ll see a line that starts with In [1]:, for input line 1. Spyder (really, the IPython console) numbers all of the input lines that you type. Since this is the first input you’re typing, the line number is 1. In the rest of this article, you’ll see references to “input line X,” where X is the number in the square brackets.

One of the first things I like to do with folks who are new to Python is show them the Zen of Python. This short poem gives you a sense of what Python is all about and how to approach working with Python.

To see the Zen of Python, type import this on input line 1 and then run the code by pressing Enter. You’ll see an output like below:

In [1]: import this
The Zen of Python, by Tim Peters

Beautiful is better than ugly.
Explicit is better than implicit.
Simple is better than complex.
Complex is better than complicated.
Flat is better than nested.
Sparse is better than dense.
Readability counts.
Special cases aren't special enough to break the rules.
Although practicality beats purity.
Errors should never pass silently.
Unless explicitly silenced.
In the face of ambiguity, refuse the temptation to guess.
There should be one-- and preferably only one --obvious way to do it.
Although that way may not be obvious at first unless you're Dutch.
Now is better than never.
Although never is often better than *right* now.
If the implementation is hard to explain, it's a bad idea.
If the implementation is easy to explain, it may be a good idea.
Namespaces are one honking great idea -- let's do more of those!

This code has import this on input line 1. The output from running import this is to print the Zen of Python onto the console. We’ll return to several of the stanzas in this poem later on in the article.

In many of the code blocks in this article, you’ll see three greater-than signs (>>>) in the top right of the code block. If you click that, it will remove the input prompt and any output lines, so you can copy and paste the code right into your console.

Many Pythonistas maintain a healthy sense of humor. This is displayed in many places throughout the language, including the Zen of Python. For another one, in the Spyder console, type the following code, followed by Enter to run it:

In [2]: import antigravity

That statement will open your web browser to the webcomic called XKCD, specifically comic #353, where the author has discovered that Python has given him the ability to fly!

You’ve now successfully run your first two Python statements! Congratulations 😃🎉

If you look at the History Log, you should see the first two commands you typed in the console (import this and import antigravity). Let’s define some variables and do some basic arithmetic now. In the console, type the following statements, pressing Enter after each one:

In [3]: var_1 = 10
In [4]: var_2 = 20
In [5]: var_3 = var_1 + var_2
In [6]: var_3
Out[6]: 30

In this code, you defined 3 variables: var_1, var_2, and var_3. You assigned var_1 the value 10, var_2 the value 20, and var_3 the sum of var_1 and var_2. Then you showed the value of the var_3 variable by writing it as the only thing on the input line. The output from that statement is shown on the next Out line, and the number on the Out line matches the associated In line.

There are two main things for you to notice in these commands:

1. If a statement does not include an assignment (with an =), it is printed onto an Out line. In MATLAB, you would need to include a semicolon to suppress the output even from assignment statements, but that is not necessary in Python.

2. On input lines 3, 4, and 5, the Variable explorer in the top right pane updated.

After you run these three commands, your Variable explorer should look like the image below:

In this image, you can see a table with four columns:

1. Name shows the name that you gave to var_1, var_2, and var_3.
2. Type shows the Python type of the variable, in this case, all int for integer numbers.
3. Size shows the size of the data stored variable, which is more useful for lists and other data structures.
4. Value shows the current value of the variable.

Running Code in Files in Spyder

The last stop in our brief tour of the Spyder interface is the File editor pane. In this pane, you can create and edit Python scripts and run them using the console. By default, Spyder creates a temporary file called temp.py which is intended for you to temporarily store commands as you’re working before you move or save them in another file.

Let’s write some code into the temp.py file and see how to run it. The file starts with the following code, which you can just leave in place:

 1# -*- coding: utf-8 -*-
 2"""
 3Spyder Editor
 4
 5This is a temporary script file.
 6"""

In this code, you can see two Python syntax structures:

• Line 1 has a comment. In Python, the comment character is the hash or pound sign (#). MATLAB uses the percent symbol (%) as the comment character. Anything following the hash on the line is a comment and is usually ignored by the Python interpreter.

• Starting on line 2 is a string that provides some context for the contents of the file. This is often referred to as a documentation string or docstring for short. You’ll learn more about docstrings in a later section.

Now you can start adding code to this file. Starting on line 8 in temp.py, enter the following code that is similar to what you already typed in the console:

 8var_4 = 10
 9var_5 = 20
10var_6 = var_4 + var_5

Then, there are three ways to run the code:

1. You can use the F5 keyboard shortcut to run the file just like in MATLAB.
2. You can click the green right-facing triangle in the menu bar just above the Editor and File explorer panes.
3. You can use the Run → Run menu option.

The first time you run a file, Spyder will open a dialog window asking you to confirm the options you want to use. For this test, the default options are fine and you can click Run at the bottom of the dialog box:

This will automatically execute the following code in the console:

In [7]: runfile('C:/Users/Eleanor/.spyder-py3/temp.py',
   ...:         wdir='C:/Users/Eleanor/.spyder-py3')

This code will run the file that you were working on. Notice that running the file added three variables into the Variable explorer: var_4, var_5, and var_6. These are the three variables that you defined in the file. You will also see runfile() added to the History log.

In Spyder, you can also create code cells that can be run individually. To create a code cell, add a line that starts with # %% into the file open in the editor:

11# %% This is a code cell
12var_7 = 42
13var_8 = var_7 * 2
14
15# %% This is a second code cell
16print("This code will be executed in this cell")

In this code, you have created your first code cell on line 11 with the # %% code. What follows is a line comment and is ignored by Python. On line 12, you are assigning var_7 to have the value 42 and then line 13 assigns var_8 to be var_7 times two. Line 15 starts another code cell that can be executed separately from the first one.

To execute the code cells, click the Run Current Cell or Run Current Cell and Go to the Next One buttons next to the generic Run button in the toolbar. You can also use the keyboard shortcuts Ctrl+Enter to run the current cell and leave it selected, or Shift+Enter to run the current cell and select the next cell.

Spyder also offers easy-to-use debugging features, just like in MATLAB. You can double-click any of the line numbers in the Editor to set a breakpoint in your code. You can run the code in debug mode using the blue right-facing triangle with two vertical lines from the toolbar, or the Ctrl+F5 keyboard shortcut. This will pause execution at any breakpoints you specify and open the ipdb debugger in the console which is an IPython-enhanced way to run the Python debugger pdb. You can read more in Python Debugging With pdb.

Summarizing Your Experience in Spyder

Now you have the basic tools to use Spyder as a replacement for the MATLAB integrated development environment. You know how to run code in the console or type code into a file and run the file. You also know where to look to see your directories and files, the variables that you’ve defined, and the history of the commands you typed.

Once you’re ready to start organizing your code into modules and packages, you can check out the following resources:

• Python Modules and Packages – An Introduction
• How to Publish an Open-Source Python Package to PyPI
• How to Publish Your Own Python Package to PyPI

Spyder is a really big piece of software, and you’ve only just scratched the surface. You can learn a lot more about Spyder by reading the official documentation, the troubleshooting and FAQ guide, and the Spyder wiki.

Comments Start With # in Python

In MATLAB, a comment is anything that follows a percent sign (%) on a line. In Python, comments are anything that follow the hash or pound sign (#). You already saw a Python comment in the earlier section about Spyder. In general, the Python interpreter ignores the content of comments, just like the MATLAB interpreter, so you can write whatever content you want in the comment. One exception to this rule in Python is the example you saw earlier in the section about Spyder:

# -*- coding: utf-8 -*-

When the Python interpreter reads this line, it will set the encoding that it uses to read the rest of the file. This comment must appear in one of the first two lines of the file to be valid.

Another difference between MATLAB and Python is in how inline documentation is written. In MATLAB, documentation is written at the start of a function in a comment, like the code sample below:

function [total] = addition(num_1,num_2)
% ADDITION  Adds two numbers together
%   TOTAL = ADDITION(NUM_1,NUM_2) adds NUM_1 and NUM_2 together
%
%   See also SUM and PLUS

However, Python does not use comments in this way. Instead, Python has an idea called documentation strings or docstrings for short. In Python, you would document the MATLAB function shown above like this:

def addition(num_1, num_2):
    """Adds two numbers together.

    Example
    -------
    >>> total = addition(10, 20)
    >>> total
    30

    """

Notice in this code that the docstring is between two sets of three quote characters ("""). This allows the docstring to run onto multiple lines with the whitespace and newlines preserved. The triple quote characters are a special case of string literals. Don’t worry too much about the syntax of defining a function yet. You’ll see more about that in a later section.

Whitespace at the Beginning of a Line Is Significant in Python

When you write code in MATLAB, blocks like if statements, for and while loops, and function definitions are finished with the end keyword. It is generally considered a good practice in MATLAB to indent the code within the blocks so that the code is visually grouped together, but it is not syntactically necessary.

For example, the following two blocks of code are functionally equivalent in MATLAB:

 1num = 10;
 2
 3if num == 10
 4disp("num is equal to 10")
 5else
 6disp("num is not equal to 10")
 7end
 8
 9disp("I am now outside the if block")

In this code, you are first creating num to store the value 10 and then checking whether the value of num is equal to 10. If it is, you are displaying the phrase num is equal to 10 on the console from line 2. Otherwise, the else clause will kick in and display num is not equal to 10. Of course, if you run this code, you will see the num is equal to 10 output and then I am now outside the if block.

Now you should modify your code so it looks like the sample below:

 1num = 10;
 2
 3if num == 10
 4    disp("num is equal to 10")
 5else
 6    disp("num is not equal to 10")
 7end
 8
 9disp("I am now outside the if block")

In this code, you have only changed lines 3 and 5 by adding some spaces or indentation in the front of the line. The code will perform identically to the previous example code, but with the indentation, it is much easier to tell what code goes in the if part of the statement and what code is in the else part of the statement.

In Python, indentation at the start of a line is used to delimit the beginning and end of class and function definitions, if statements, and for and while loops. There is no end keyword in Python. This means that indentation is very important in Python!

In addition, in Python the definition line of an if/else/elif statement, a for or while loop, a function, or a class is ended by a colon. In MATLAB, the colon is not used to end the line.

Consider this code example:

 1num = 10
 2
 3if num == 10:
 4    print("num is equal to 10")
 5else:
 6    print("num is not equal to 10")
 7
 8print("I am now outside the if block")

On the first line, you are defining num and setting its value to 10. On line 2, writing if num == 10: tests the value of num compared to 10. Notice the colon at the end of the line.

Next, line 3 must be indented in Python’s syntax. On that line, you are using print() to display some output to the console, in a similar way to disp() in MATLAB. You’ll read more about print() versus disp() in a later section.

On line 4, you are starting the else block. Notice that the e in the else keyword is vertically aligned with the i in the if keyword, and the line is ended by a colon. Because the else is dedented relative to print() on line 3, and because it is aligned with the if keyword, Python knows that the code within the if part of the block has finished and the else part is starting. Line 5 is indented by one level, so it forms the block of code to be executed when the else statement is satisfied.

Lastly, on line 6 you are printing a statement from outside the if/else block. This statement will be printed regardless of the value of num. Notice that the p in print() is vertically aligned with the i in if and the e in else. This is how Python knows that the code in the if/else block has ended. If you run the code above, Python will display num is equal to 10 followed by I am now outside the if block.

Now you should modify the code above to remove the indentation and see what happens. If you try to type the code without indentation into the Spyder/IPython console, you will get an IndentationError:

In [1]: num = 10
In [2]: if num == 10:
   ...: print("num is equal to 10")
  File "<ipython-input-2-f453ffd2bc4f>", line 2
    print("num is equal to 10")
        ^
IndentationError: expected an indented block

In this code, you first set the value of num to 10 and then tried to write the if statement without indentation. In fact, the IPython console is smart and automatically indents the line after the if statement for you, so you’ll have to delete the indentation to produce this error.

When you’re indenting your code, the official Python style guide called PEP 8 recommends using 4 space characters to represent one indentation level. Most text editors that are set up to work with Python files will automatically insert 4 spaces if you press the Tab key on your keyboard. You can choose to use the tab character for your code if you want, but you shouldn’t mix tabs and spaces or you’ll probably end up with a TabError if the indentation becomes mismatched.

Conditional Statements Use elif in Python

In MATLAB, you can construct conditional statements with if, elseif, and else. These kinds of statements allow you to control the flow of your program in response to different conditions.

You should try this idea out with the code below, and then compare the example of MATLAB vs Python for conditional statements:

 1num = 10;
 2if num == 10
 3    disp("num is equal to 10")
 4elseif num == 20
 5    disp("num is equal to 20")
 6else
 7    disp("num is neither 10 nor 20")
 8end

In this code block, you are defining num to be equal to 10. Then you are checking if the value of num is 10, and if it is, using disp() to print output to the console. If num is 20, you are printing a different statement, and if num is neither 10 nor 20, you are printing the third statement.

In Python, the elseif keyword is replaced with elif:

 1num = 10
 2if num == 10:
 3    print("num is equal to 10")
 4elif num == 20:
 5    print("num is equal to 20")
 6else:
 7    print("num is neither 10 nor 20")

This code block is functionally equivalent to the previous MATLAB code block. There are 2 main differences. On line 4, elseif is replaced with elif, and there is no end statement to end the block. Instead, the if block ends when the next dedented line of code is found after the else. You can read more in the Python documentation for if statements.

Calling Functions and Indexing Sequences Use Different Brackets in Python

In MATLAB, when you want to call a function or when you want to index an array, you use round brackets (()), sometimes also called parentheses. Square brackets ([]) are used to create arrays.

You can test out the differences in MATLAB vs Python with the example code below:

>> arr = [10, 20, 30];
>> arr(1)

ans =

    10

>> sum(arr)

ans =

    60

In this code, you first create an array using the square brackets on the right side of the equal sign. Then, you retrieve the value of the first element by arr(1), using the round brackets as the indexing operator. On the third input line, you are calling sum() and using the round brackets to indicate the parameters that should be passed into sum(), in this case just arr. MATLAB computes the sum of the elements in arr and returns that result.

Python uses separate syntax for calling functions and indexing sequences. In Python, using round brackets means that a function should be executed and using square brackets will index a sequence:

In [1]: arr = [10, 20, 30]
In [2]: arr[0]
Out[2]: 10

In [3]: sum(arr)
Out[3]: 60

In this code, you are defining a Python list on input line 1. Python lists have some important distinctions from arrays in MATLAB and arrays from the NumPy package. You can read more about Python lists in Lists and Tuples in Python, and you’ll learn more about NumPy arrays in a later section.

On the input line 2, you are displaying the value of the first element of the list with the indexing operation using square brackets. On input line 3, you are calling sum() using round brackets and passing in the list stored in arr. This results in the sum of the list elements being displayed on the last line. Notice that Python uses square brackets for indexing the list and round brackets for calling functions.

The First Index in a Sequence Is 0 in Python

In MATLAB, you can get the first value from an array by using 1 as the index. This style follows the natural numbering convention and starts how you would count the number of items in the sequence. You can try out the differences of MATLAB vs Python with this example:

>> arr = [10, 20, 30];
>> arr(1)

ans =

    10

>> arr(0)
Array indices must be positive integers or logical values.

In this code, you are creating an array with three numbers: 10, 20, and 30. Then you are displaying the value of the first element with the index 1, which is 10. Trying to access the zeroth element results in an error in MATLAB, as shown on the last two lines.

In Python, the index of the first element in a sequence is 0, not 1:

In [1]: arr = [10, 20, 30]
In [2]: arr[0]
Out[2]: 10

In [3]: arr[1]
Out[3]: 20

In [4]: a_string = "a string"
In [5]: a_string[0]
Out[5]: 'a'

In [6]: a_string[1]
Out[6]: ' '

In this code, you are defining arr as a Python list with three elements on input line 1. On input line 2, you are displaying the value of the first element of the list, which has the index 0. Then you are displaying the second element of the list, which has the index 1.

On input lines 4, 5, and 6, you are defining a_string with the contents "a string" and then getting the first and second elements of the string. Notice that the second element (character) of the string is a space. This demonstrates a general Python feature, that many variable types operate as sequences and can be indexed, including lists, tuples, strings, and arrays.

The Last Element of a Sequence Has Index -1 in Python

In MATLAB, you can get the last value from an array by using end as the index. This is really useful when you don’t know how long an array is, so you don’t know what number to access the last value.

Try out the differences in MATLAB vs Python with this example:

>> arr = [10, 20, 30];
>> arr(end)

ans =

    30

In this code, you are creating an array with three numbers, 10, 20, and 30. Then you are displaying the value of the last element with the index end, which is 30.

In Python, the last value in a sequence can be retrieved by using the index -1:

In [1]: arr = [10, 20, 30]
In [2]: arr[-1]
Out[2]: 30

In this code, you are defining a Python list with three elements on input line 1. On input line 2, you are displaying the value of the last element of the list, which has the index -1 and the value 30.

In fact, by using negative numbers as the index values you can work your way backwards through the sequence:

In [3]: arr[-2]
Out[3]: 20

In [4]: arr[-3]
Out[4]: 10

In this code, you are retrieving the second-to-last and third-to-last elements from the list, which have values of 20 and 10, respectively.

Exponentiation Is Done With ** in Python

In MATLAB, when you want to raise a number to a power you use the caret operator (^). The caret operator is a binary operator that takes two numbers. Other binary operators include addition (+), subtraction (-), multiplication (*), and division (/), among others. The number on the left of the caret is the base and the number on the right is the exponent.

Try out the differences of MATLAB vs Python with this example:

>> 10^2

ans =

    100

In this code, you are raising 10 to the power of 2 using the caret resulting an answer of 100.

In Python, you use two asterisks (**) when you want to raise a number to a power:

In [1]: 10 ** 2
Out[1]: 100

In this code, you are raising 10 to the power of 2 using two asterisks resulting an answer of 100. Notice that there is no effect of including spaces on either side of the asterisks. In Python, the typical style is to have spaces on both sides of a binary operator.

The Length of a Sequence Is Found With len() in Python

In MATLAB, you can get the length of an array with length(). This function takes an array as the argument and returns back the size of the largest dimension in the array. You can see the basics of this function with this example:

>> length([10, 20, 30])

ans =

    3

>> length("a string")

ans =

    1

In this code, on the first input line you are finding the length of an array with 3 elements. As expected, length() returns an answer of 3. On the second input line, you are finding the length of the string array that contains one element. Notice that MATLAB implicitly creates a string array, even though you did not use the square brackets to indicate it is an array.

In Python, you can get the length of a sequence with len():

In [1]: len([10, 20, 30])
Out[1]: 3

In [2]: len("a string")
Out[2]: 8

In this code, on the input line 1 you are finding the length of a list with 3 elements. As expected, len() returns a length of 3. On input line 2, you are finding the length of a string as the input. In Python, strings are sequences and len() counts the number of characters in the string. In this case, a string has 8 characters.

Console Output Is Shown With print() in Python

In MATLAB, you can use disp(), fprintf(), and sprintf() to print the value of variables and other output to the console. In Python, print() serves a similar function as disp(). Unlike disp(), print() can send its output to a file similar to fprintf().

Python’s print() will display any number of arguments passed to it, separating them by a space in the output. This is different from disp() in MATLAB, which only takes one argument, although that argument can be an array with multiple values. The following example shows how Python’s print() can take any number of arguments, and each argument is separated by a space in the output:

In [1]: val_1 = 10
In [2]: val_2 = 20
In [3]: str_1 = "any number of arguments"
In [4]: print(val_1, val_2, str_1)
10 20 any number of arguments

In this code, the input lines 1, 2, and 3 define val_1, val_2, and str_1, where val_1 and val_1 are integers, and str_1 is a string of text. On input line 4, you are printing the three variables using print(). The output below this line the value of the three variables are shown in the console output, separated by spaces.

You can control the separator used in the output between arguments to print() by using the sep keyword argument:

In [5]: print(val_1, val_2, str_1, sep="; ")
10; 20; any number of arguments

In this code, you are printing the same three variables but setting the separator to be a semicolon followed by a space. This separator is printed between the first and second and the second and third arguments, but not after the third argument. To control the character printed after the last value, you can use the end keyword argument to print():

In [6]: print(val_1, val_2, str_1, sep="; ", end=";")
10; 20; any number of arguments;

In this code, you have added the end keyword argument to print(), setting it to print a semicolon after the last value. This is shown in the output on line below the input.

Like disp() from MATLAB, print() cannot directly control the output format of variables and relies on you to do the formatting. If you want more control over the format of the output, you should use f-strings or str.format(). In these strings, you can use very similar formatting style codes as fprintf() in MATLAB to format numbers:

In [7]: print(f"The value of val_1 = {val_1:8.3f}")
The value of val_1 =   10.000

In [8]: # The following line will only work in Python 3.8

In [9]: print(f"The value of {val_1=} and {val_2=}")
The value of val_1=10, and val_2=20

In this code, input line 7 includes an f-string, indicated by the f to start the string. This means that Python will substitute the value of any variables it encounters between {}, or curly braces, within the string. You can see that in the output, Python has replaced {val_1:8.3f} with a floating point number with 8 columns in the output and 3 digits of precision.

Input line 9 demonstrates a new feature in Python 3.8. If a variable name is immediately followed by an equals sign inside curly braces, the name of the variable and the value will be printed automatically.

You can take a deep dive into Python’s print() by checking out The Ultimate Guide to Python Print.

Function Definitions Start With def and return Values in Python

In MATLAB, you can define a function by placing the function keyword at the start of a line. This is followed by the name of any output variables, an equals (=) sign, then the name of the function and any input arguments in parentheses. Within the the function you have to assign to any variables you specified in the definition line as outputs. A simple example MATLAB function is shown below:

 1function [total] = addition(num_1,num_2)
 2total = num_1 + num_2;
 3end

In this code, you see the function definition on line 1. There is only one output variable, called total, for this function. The name of the function is addition and it takes two arguments, which will be assigned the names num_1 and num_2 in the function body. Line 2 is the implementation of the function. The value of total is set equal to the sum of num_1 and num_2. The last line of the function is the end keyword that tells the MATLAB interpreter the definition of the function has finished.

To use this function in MATLAB, you should save it in a file called addition.m, matching the name of the function. Alternatively, it can be placed in file with other commands provided that the function definition is the last thing in the file and the file is not named addition.m. Then, you can run the function by typing the following code in the MATLAB console:

>> var_1 = 20;
>> var_2 = 10;
>> sum_of_vars = addition(var_1,var_2)

sum_of_vars =

    30

In this code, you have defined two variables called var_1 and var_2 that hold the values 20 and 10, respectively. Then you created a third variable called sum_of_vars that stores the output from addition(). Check out the Variable explorer, and you’ll see that sum_of_vars has the value 30, as expected. Notice that the name sum_of_vars did not have to be the same name as the output variable used in the function definition, which was total.

MATLAB does not require a function to provide an output value. In this case, you would remove the output variable and the equals sign from the function definition. Modify your addition.m file so that the code looks like this:

 1function addition(num_1,num_2)
 2total = num_1 + num_2;
 3end

The only change in this code from the earlier code is that you deleted the [total] = from line 1, the other lines are exactly the same. Now if you try to assign the result of calling this function to a variable, MATLAB will generate an error in the console:

>> var_1 = 20;
>> var_2 = 10;
>> sum_of_vars = addition(var_1,var_2);
Error using addition
Too many output arguments.

In this code, you defined the same two variables var_1 and var_2 as before and called addition() in the same way as before. However, since addition() no longer specifies an output variable, MATLAB generates an error message that there are too many output arguments. Clicking on the word addition will open the definition of the function for you to edit or view the source code to fix the problem.

In Python, the def keyword starts a function definition. The def keyword must be followed by the name of the function and any arguments to the function inside parentheses, similar to MATLAB. The line with def must be ended with a colon (:).

Starting on the next line, the code that should be executed as part of the function must be indented one level. In Python, the function definition ends when a line of code starts at the same indentation level as the def keyword on the first line.

If your function returns some output back to the caller, Python does not require that you specify a name for an output variable. Instead, you use the return statement to send an output value from the function.

An equivalent function in Python to your first addition() example with an output variable is shown below:

 1def addition(num_1, num_2):
 2    total = num_1 + num_2
 3    return total

In this code, you see the def keyword followed by the function name and the two arguments num_1 and num_2 on line 1. On line 2 you can see the creation of a new variable total to store the sum of num_1 and num_2, and on line 3 the value of total is returned to the point where this function was called. Notice that lines 2 and 3 are indented by 4 spaces because they make up the body of the function.

The variable that stores the sum of num_1 and num_2 can have any name, it doesn’t have to be called total. In fact, you don’t need to create a variable there at all. You can simplify your previous function definition by eliminating total and simply returning the value of num_1 + num_2:

 1def addition(num_1, num_2):
 2    return num_1 + num_1

Line 1 in this code is the same as it was before, you have only changed line 2 and deleted line 3. Line 2 now computes the value of num_1 + num_2 and returns that value back to the caller of the function. Line 2 is indented by 4 spaces because it makes up the body of the function.

To use this function in Python, you do not need to save it in a file with a special name. You can place the function definition in any Python file, at any point in the file. There is no restriction that the function definition has to be last. In fact, you can even define functions right from the console, which is not possible in MATLAB.

Open Spyder and in the Console pane type:

In [1]: def addition(num_1, num_2):

On this line of code you are creating the function definition. In the Spyder/IPython console, once you start a function definition and press Enter, the start of the line becomes three dots and the cursor is automatically indented. Now you can type the remainder of the function definition. You’ll have to press Enter twice to complete the definition:

In [1]: def addition(num_1, num_2):
   ...:     return num_1 + num_2
   ...:

In this code, you have the definition of the function on the first line and the body of the function on the second line. The console automatically adds the ...: at the start of the lines to indicate these are continuation lines that apply to the function definition.

Once you’ve completed the definition, you can execute the function from the console as well. You should type this code:

In [2]: var_1 = 20
In [3]: var_2 = 10
In [4]: sum_of_vars = addition(var_1, var_2)
In [5]: sum_of_vars
Out[5]: 30

In this code, you first create two variables var_1 and var_2 that store the values you want to add together. Then, on input line 4, you assign sum_of_vars to the result that is returned from addition(). On input line 5, you are outputting the value of sum_of_vars to the console screen. This displays 30, the sum of 10 and 20.

In Python, if you do not explicitly put a return statement, your function will implicitly return the special value None. You should change your Python definition of addition() to see how this works. In the Spyder/IPython console, type the following:

In [6]: def addition(num_1, num_2):
   ...:     total = num_1 + num_2
   ...:

In this code, you have the same def line on input line 6. You have changed the first continuation line to assign the result of the addition to total instead of returning. Now you should see what happens when we execute this modified function:

In [7]: sum_of_vars = addition(var_1, var_2)
In [8]: sum_of_vars
In [9]:

In this code, on input line 7 you are assigning sum_of_vars to be the returned value from addition(). Then, on input line 8, you are showing the value of sum_of_vars on the console screen, just like before. This time though, there is no output! By default, Python prints nothing when it outputs a variable whose value is None. You can double check the value of the sum_of_vars variable by looking at the Variable explorer. In the Type column, it should list NoneType, telling you that sum_of_vars is the special None value.

Functions Accept Positional and Keyword Arguments in Python

In MATLAB, functions have input arguments specified on the first line, in the function definition. When you call a function in MATLAB, you can pass from zero up to the number of arguments that are specified. In the body of the function, you can check the number of input arguments the caller actually passed to execute different code. This is useful when you want different arguments to have different meaning, like in the example below:

 1function [result] = addOrSubtract(num_1,num_2,subtract)
 2% ADDORSUBTRACT  Add or subtract two value
 3%   RESULT = addOrSubtract(NUM_1,NUM_2) adds NUM_1 and NUM_2 together
 4%
 5%   RESULT = addOrSubtract(NUM_1,NUM_2,true) subtracts NUM_2 from NUM_1
 6
 7    switch nargin
 8        case 2
 9            result = num_1 + num_2;
10        case 3
11            result = num_1 - num_2;
12        otherwise
13            result = 0;
14    end
15end

In this code, you are defining a function with three possible input arguments. On line 7, you are starting a switch/case block that determines how many input arguments were passed to the function by using the special variable nargin. This variable stores the actual number of arguments the caller passed into the function.

In your code above, you are defining three cases:

1. If the number of input arguments is 2, you are adding num_1 and num_2 together.
2. If the number of input arguments is 3, you are subtracting num_2 from num_1.
3. If fewer than 2 arguments are passed, the output will be 0.

If more than 3 arguments are passed, MATLAB will raise an error.

Now you should experiment with this function. Save the code above into a file called addOrSubtract.m and then on the MATLAB console, try the version with two input arguments:

>> addOrSubtract(10,20)

ans =
    30

In this code, you are calling addOrSubtract() with two arguments, so the arguments are added together, resulting in an answer of 30. Next, try calling addOrSubtract() with three arguments:

>> addOrSubtract(10,20,true)

ans =
   -10

In this code, you used three input arguments, and found that the second argument was subtracted from the first, resulting in an answer of -10. Third, try calling addOrSubtract() with one argument:

>> addOrSubtract(10)

ans =
     0

In this code, you used one input argument and found the answer was 0, because MATLAB only found one argument to the function and used the otherwise case. Finally, try calling addOrSubtract() with four arguments:

>> addOrSubtract(10,20,true,30)
Error using addOrSubtract
Too many input arguments.

In this code, you find that MATLAB raises an error because there were more input arguments passed than were defined in the function line.

There are four key takeaways from this example with MATLAB:

1. There is only one kind of argument in a function definition.
2. The meaning of an argument in the code is determined by its position in the function definition.
3. The maximum number of arguments that can be passed to a function is determined by the number of arguments specified in the function definition.
4. Any number of arguments up to the maximum can be passed by the caller.

In Python, there are two kinds of arguments you can specify when defining a function. These are required and optional arguments. The key difference between these is that required arguments must be passed when a function is called, while optional are given a default value in the function definition.

You can see the differences between these two styles in the next example:

 1def add_or_subtract(num_1, num_2, subtract=False):
 2    """Add or subtract two numbers, depending on the value of subtract."""
 3    if subtract:
 4        return num_1 - num_2
 5    else:
 6        return num_1 + num_2

In this code, you are defining a function called add_or_subtract() that has three arguments: num_1, num_2, and subtract. In the function definition, you can see the two types of arguments. The first two arguments, num_1 and num_2, are required arguments.

The third argument, subtract, has a default value assigned to it by specifying a value after an equals sign in the function definition. This means that when the function is called, passing a value for subtract is optional. If no value is passed, the default as defined in the function definition line will be used. In this case, the default value is False.

In the body of the function, you are testing the value of subtract with the if statement to determine whether addition or subtraction should be performed. If subtract is True, num_2 will be subtracted from num_1. Otherwise, if subtract is False, then num_1 will be added to num_2. In either case, the result of the arithmetic operation will be returned to the caller.

In addition to the two types of arguments you can use when defining a function, there are two kinds of arguments you can specify when calling a function. These are called positional and keyword arguments. You can see the difference between these in the following example. First, try passing only two arguments to the function:

In [1]: add_or_subtract(10, 20)
Out[1]: 30

In this code, you passed only two arguments to add_or_subtract(), 10 and 20. In this case, you passed these values as positional arguments, and the meaning of the arguments is defined by their position in the function call.

Since only the two required arguments were passed, subtract will take on the default value, which is False. Therefore, 10 and 20 will be added together, which you can see on the output line. Next, try passing a value for subtract:

In [2]: add_or_subtract(10, 20, False)
Out[2]: 30
In [3]: add_or_subtract(10, 20, True)
Out[3]: -10

In this code, you passed three arguments to add_or_subtract(), with two different values for the subtract argument. First, you passed False on input line 2. The result was the addition of 10 and 20. Then, you passed True on input line 3, resulting in the difference between 10 and 20, or -10.

In these examples, you saw that it is possible in Python to define default values for arguments to a function. This means when you call the function, any arguments with default values are optional and do not have to be passed. If no value is passed for any default arguments, the default value will be used. However, you must pass a value for every argument without a default value. Otherwise, Python will raise an error:

In [4]: add_or_subtract(10)
Traceback (most recent call last):

  File "<ipython-input-4-f9d1f2ae4494>", line 1, in <module>
    add_or_subtract(10)

TypeError: add_or_subtract() missing 1 required positional argument: 'num_2'

In this code, you have only passed one of the two required arguments to add_or_subtract(), so Python raises a TypeError. The error message tells you that you did not pass a value for num_2, because it does not have a default value.

In these last three example, you have used positional arguments, so which parameter is assigned to the variables in the function depends on the order they are passed. There is another method to pass arguments to functions in Python, called keyword arguments. To use keyword arguments, you specify the name of the argument in the function call:

In [5]: add_or_subtract(num_1=10, num_2=20, subtract=True)
Out[5]: -10

In this code, you have used keyword arguments for all three arguments to add_or_subtract(). Keyword arguments are specified by stating the argument name, then an equals sign, then the value that argument should have. One of the big advantages of keyword arguments is that they make your code more explicit. (As the Zen of Python says, explicit is better than implicit.) However, they make the code somewhat longer, so it’s up to your judgement when to use keyword arguments or not.

Another benefit of keyword arguments is that they can be specified in any order:

In [6]: add_or_subtract(subtract=True, num_2=20, num_1=10)
Out[6]: -10

In this code, you have specified the three arguments for add_or_subtract() as keyword arguments, but the order is different from in the function definition. Nonetheless, Python connects the right variables together because they are specified as keywords instead of positional arguments.

You can also mix positional and keyword arguments together in the same function call. If positional and keyword arguments are mixed together, the positional arguments must be specified first, before any keyword arguments:

In [7]: add_or_subtract(10, 20, subtract=True)
Out[7]: -10

In this code, you have specified the values for num_1 and num_2 using positional arguments, and the value for subtract using a keyword argument. This is probably the most common case of using keyword arguments, because it provides a good balance between being explicit and being concise.

Finally, there is one last benefit of using keyword arguments and default values. Spyder, and other IDEs, provide introspection of function definitions. This will tell you the names of all of the defined function arguments, which ones have default arguments, and the value of the default arguments. This can save you time and make your code easier and faster to read.

There Are No switch/case Blocks in Python

In MATLAB, you can use switch/case blocks to execute code by checking the value of a variable for equality with some constants. This type of syntax is quite useful when you know you want to handle a few discrete cases. Try out a switch/case block with this example:

num = 10;
switch num
    case 10
        disp("num is 10")
    case 20
        disp("num is 20")
    otherwise
        disp("num is neither 10 nor 20")
end

In this code, you start by defining num and setting it equal to 10 and on the following lines you test the value of num. This code will result in the output num is 10 being displayed on the console, since num is equal to 10.

This syntax is an interesting comparison of MATLAB vs Python because Python does not have a similar syntax. Instead, you should use an if/elif/else block:

num = 10
if num == 10:
    print("num is 10")
elif num == 20:
    print("num is 20")
else:
    print("num is neither 10 nor 20")

In this code, you start by defining num and setting it equal to 10. On the next several lines you are writing an if/elif/else block to check the different values that you are interested in.

Namespaces Are One Honking Great Idea in Python

In MATLAB, all functions are found in a single scope. MATLAB has a defined search order for finding functions within the current scope. If you define your own function for something that MATLAB already includes, you may get unexpected behavior.

As you saw in the Zen of Python, namespaces are one honking great idea. Namespaces are a way to provide different scopes for names of functions, classes, and variables. This means you have to tell Python which library has the function you want to use. This is a good thing, especially in cases where multiple libraries provide the same function.

For instance, the built-in math library provides a square root function, as does the more advanced NumPy library. Without namespaces, it would be more difficult to tell Python which square root function you wanted to use.

To tell Python where a function is located, you first have to import the library, which creates the namespace for that library’s code. Then, when you want to use a function from the library, you tell Python which namespace to look in:

In [1]: import math
In [2]: math.sqrt(4)
Out[2]: 2.0

In this code, on input line 1 you imported the math library that is built-in to Python. Then, input line 2 computes the square root of 4 using the square root function from within the math library. The math.sqrt() line should be read as “from within math, find sqrt().”

The import keyword searches for the named library and binds the namespace to the same name as the library by default. You can read more about how Python searches for libraries in Python Modules and Packages – An Introduction.

You can also tell Python what name it should use for a library. For instance, it is very common to see numpy shortened to np with the following code:

In [3]: import numpy as np
In [4]: np.sqrt(4)
Out[4]: 2.0

In this code, input line 3 imports NumPy and tells Python to put the library into the np namespace. Then, whenever you want to use a function from NumPy, you use the np abbreviation to find that function. On input line 4, you are computing the square root of 4 again, but this time, using np.sqrt(). np.sqrt() should be read as “from within NumPy, find sqrt().”

There are two main caveats to using namespaces where you should be careful:

1. You should not name a variable with the same name as one of the functions built into Python. You can find a complete list of these functions in the Python documentation. The most common variable names that are also built-in functions and should not be used are dir, id, input, list, max, min, sum, str, type, and vars.

2. You should not name a Python file (one with the extension .py) with the same name as a library that you have installed. In other words, you should not create a Python file called math.py. This is because Python searches the current working directory first when it tries to import a library. If you have a file called math.py, that file will be found before the built-in math library and you will probably see an AttributeError.

The Most Recent Unassigned Result Is Available as _ in Python

The MATLAB console uses ans to store the result of the most recent calculation if that result was not assigned to a variable. This is really useful when you forgot to assign the result of a calculation to a variable or when you just want to chain a few simple calculations together. To see the differences between MATLAB vs Python, try out this example:

>> sum([10, 20, 30])

ans =

    60

>> ans + 10

ans =

    70

In this code, you use sum() to calculate the sum of the array. Since there is no equal sign with a variable name on the left, MATLAB assigns the output from sum() to ans. You can then use that variable in further calculations, as you do here by adding 10 to the last result. Note that this will only work in the MATLAB console, not in a script file.

In the Python console (including the IPython/Spyder console), the output from the most recent calculation is stored in the underscore character (_). Try the following code:

In [1]: sum([10, 20, 30])
Out[1]: 60

In [2]: _ + 10
Out[2]: 70

In this code, you use sum() to calculate the sum of the list. Since there is no equal sign with a variable name on the left, Python assigns the output from sum() to the underscore (_), in addition to printing it on the output line. You can then use that variable in further calculations, as you do here by adding 10 to the last result. Note that this will only work in the Python console, not in a script file.

In the IPython console, there is one additional feature enabled. You can append a number after the underscore to retrieve the result of any previous line. Try the following code:

In [3]: _1 + 20
Out[3]: 80

In [4]: _2 + 20
Out[4]: 90

In this code on input line 3 you are using _1 to mean the value of output line 1, the sum() line. To that result (60) you are adding 20, producing a result of 80. On input line 4, you are adding 20 to the value of output line 2, accessed with _2, so the result is 90.

Notice that the Spyder Variable explorer does not show this variable by default, whereas ans is shown in the MATLAB Variable explorer. In a few sections, you’ll see why the underscore isn’t shown by default and how you can see it.

Anonymous Functions Are Created With the lambda Keyword in Python

MATLAB uses the the at-symbol (@) to indicate that what follows is the definition of an anonymous function. Anonymous functions are functions that are not defined in a program file and do not use the function keyword. A program file is a MATLAB file with a filename ending in .m. Anonymous functions are limited to a single statement so they are intended for simple computations.

You can try out the differences of anonymous functions in MATLAB vs Python with this example:

>> sayHello = @(x) fprintf("Hello, %s\n",x);
>> sayHello("Eleanor")
Hello, Eleanor

In this code, the first input line defines the anonymous function with one input parameter, x. The body of the function follows, which uses fprintf() to format the input into a string. This function is assigned to sayHello. On the second input line, sayHello() is executed and passed "Eleanor" as the value. The result the string Hello, Eleanor printed on the console.

Anonymous functions are most often used when you need to pass one function into another function. In these cases, it is often not necessary to assign the function definition to a variable:

>> integral(@(x) x.^2,0,9)

ans =

    243

In this code, the first input line executes integral(), a function that computes the definite integral of a given function. The first argument to integral() must be a function, so this is a perfect place to use an anonymous function. Here, your anonymous function squares whatever the input value is. The other two arguments to integral() are the limits of the integration, such that the result of integrating x.^2 from 0 to 9 is 243.

Python uses the lambda keyword to define anonymous functions. Other than this syntax difference, anonymous functions work the same way in Python as in MATLAB:

In [1]: say_hello = lambda x: print(f"Hello, {x:s}")
In [2]: say_hello("Eleanor")
Hello, Eleanor

In this code, input line 1 defines the lambda function with one parameter, x. You use print() within the function definition to show an f-string with the value of the input parameter. The function is then stored in say_hello(). Input line 2 evaluates say_hello() with the input string "Eleanor" and produces the Hello, Eleanor output.

In Python, the official style guide called PEP 8 specifically disrecommends assigning lambda expressions to variable names, as you saw in the last example. If you want to give a function a name to refer to it several times, the preference is to use the def syntax and define a full function, even for one-line functions.

However, lambda functions are still useful when they are passed as arguments into another function:

In [3]: from scipy import integrate
In [4]: integrate.quad(lambda x: x ** 2, 0, 9)
Out[4]: (243.0, 2.6978419498391304e-12)

In this code, input line 3 imports the scipy.integrate library and stores it in the integrate namespace. On input line 4, you are using quad() from scipy.integrate to compute the integral using quadrature, very similar to integral() in MATLAB. The first argument to quad() is the function to be integrated, and you use a lambda function to specify that x ** 2 should be integrated. The second and third arguments to quad() specify that the integral should be conducted from 0 to 9.

You can see that the result on output line 4 has two values, 243.0 and 2.6978419498391304e-12. The first value is the result of the integration, and is equal to the result from MATLAB. The second value is an estimate of the absolute error in the result. An error this small is approximately the precision of the numbers used to store the result, so the answer is about as accurate as it can be.

You can read more about lambda in How to Use Python lambda Functions or watch the videos in the How to Use Python Lambda Functions course.

Class Definitions Start With class in Python

MATLAB has two ways to define a class. With the first way, you can put all of the class definition in a single file with the name of the class as the filename. Then within the file, you can use the classdef keyword to define the properties and methods that belong to the class.

With the second way, you can create a folder that starts with @ and has the same name as the class. In that folder, you can create a single file that has the same name as the class. The function definition in that file will be used as the class initializer, and it should call class() to instantiate the class. Methods of the class can be defined in other files in the same folder, where the name of each file must be the same as the name of the method.

Python only has one way to define a class, using the class keyword. Since Python uses indentation to find the end of the class definition, the entire definition must be contained in a single file:

class MyClass:
    # The rest of the class definition goes here

In this code, the first line defines the name of the class. It starts with the class keyword, followed by the name of the class and a colon. Underneath this line, all of the code that is part of the class definition (methods and attributes) must be indented. Once a line of code starts in the same column as the c in class, the class definition will be ended.

The second line in this code is a comment to note that the rest of the class definition would follow the class line.

As in all object-oriented code, Python classes can inherit from superclasses. The superclass of a given class can be given as a parameter in the class definition, as shown below:

class MyClass(MySuperClass):
    # The rest of the class definition goes here

In this code, the only change is that the name of the superclass is listed inside round brackets before the colon.

There Are No Private Properties or Methods in Python

MATLAB allows class properties and method to be set as one of four Access options:

• public: Access to the property or method is unrestricted.
• protected: Access to the property or method is only allowed in this class or subclasses.
• private: Access to the property or method is only allowed in this class.
• meta.class or {meta.class}: Access to the property or method is only allowed in the listed class or classes.

This allows you to specifically control the ways that a property or class method can be accessed.

In Python, there is no way to set a class or instance attribute or method as protected or private. All classes and class instances can have their attributes and methods changed at runtime. Python’s convention is that attributes and methods that start with an underscore (_) are intended to be private, or at least non-public. However, this convention is not enforced by any checks in the language and all attributes and methods can be modified by the user at runtime.

As you saw in the section about using the underscore to retrieve values in the console, the underscore is not visible by default in the Spyder Variable explorer. This is because Spyder and other tools respect the convention that underscore indicates something should be non-public. However, underscores can be shown in the Spyder Variable explorer if you click the gear icon in the top right of that pane and uncheck the Exclude private variables item. This will also show other non-public variables as well.

Python uses several special methods that start with a double-underscore (__), called dunder methods, to implement specific behavior for classes. The most commonly used dunder method is __init__(), which is the class initializer or constructor. You can read a lot more about dunder methods in Enriching Your Python Classes With Dunder (Magic, Special) Methods.

If you want more information about Python’s classes, you can read Object-Oriented Programming in Python vs Java. Even though that article is about Java, Java is similar to the MATLAB OOP paradigm in terms of the nature of attributes and methods.

A Class Refers to Itself as self in Python

MATLAB uses the name obj when a class wants to refer to the current instance of itself. The obj should be the first argument passed to an ordinary method. MATLAB also defines static methods that have no reference to the class instance.

Python uses the name self when a class wants to refer to the current instance of itself, but this is actually only a convention. You can call the first argument to an instance method any name you want, but self is the most common convention. Python also defines static methods that don’t take an argument of the class instance and class methods that take an argument of the class object instead of the instance. You can read more about instance, static, and class methods in Python’s Instance, Class, and Static Methods Demystified.

There Is One String Type in Python

In MATLAB, strings of characters are stored in string arrays when you use double quotes (") or in character arrays if you use single quotes ('). If you use both single and double quotes in an array assignment, the array will be promoted to a string array.

In character arrays, each character in the string occupies one column in the array. For multidimensional character arrays, each row of the array must have the same number of characters, which is to say, the same number of columns. This is shown in the example below:

 1>> charArray = ['Real'; 'Python'];
 2Dimensions of arrays being concatenated are not consistent.
 3
 4>> charArray = ['MATLAB'; 'Python'];
 5>> size(charArray)
 6
 7ans =
 8
 9     2     6

In this example, line 1 shows an attempt to define a 2-row character array using single quotes. However, the number of characters in Real is not the same as in Python, so MATLAB shows an error message that the dimensions are not consistent.

On line 4, you successfully create a character array, and on the third input line you are checking the size of the array. The output shows that there are 2 rows, as expected, and 6 columns, since the length of both MATLAB and Python are 6 characters.

This is not the case for string arrays. In string arrays, each string occupies one column in the array and each row in a multidimensional array must have the same number of strings, although each string can have different length. This is shown in the example below:

 1>> stringArray = ["Real", "Python"; "Real"];
 2Error using vertcat
 3Dimensions of arrays being concatenated are not consistent.
 4
 5>> stringArray = ["Real"; "Python"];
 6>> size(stringArray)
 7
 8ans =
 9
10     2     1

In this code, line 1 shows an attempt to define a 2-row string array using double quotes. However, the number of strings on the first row (2) does not match the number of strings in the second row (1), so MATLAB raises an error.

On line 5, you successfully create a string array. Notice that even though the number of characters is different between Real and Python, MATLAB is able to create the string array. On line 6 you are checking the size of the string array, which shows that there are 2 rows and 1 column, as expected.

In Python, there is only one string literal type, called str. You can create a string literal using single quotes (') or double quotes ("), there is no difference between the two definitions. However, there are some good arguments to prefer double quotes when defining string literals in Python, which are well expressed by the Black code formatting library.

There is one additional way to define strings in Python, using triple-single-quotes (''') or triple-double-quotes ("""). This method of creating strings allows the strings to be defined across multiple lines with the newline characters retained. You can see an example of this in the section about comments and docstrings.

You can read a lot more about defining strings in Basic Data Types in Python and Strings and Character Data in Python.

You can generate similar data structures to the string arrays and character arrays in MATLAB using NumPy in Python. NumPy has several data types, or dtypes, that are related to strings. In Python 3, the default string dtype for arrays is a fixed-width Unicode string:

In [1]: import numpy as np
In [2]: arr = np.array(("Real", "Python"))
In [3]: arr
Out[3]: array(['Real', 'Python'], dtype='<U6')

In this code, you are importing the NumPy library on input line 1 and assigning it to the np abbreviation. On input line 2, you are creating a NumPy array with 2 string elements, Real and Python, and assigning the array to arr.

On input line 3, you are showing the value of arr. The output from the third line shows that arr is storing an array that has 2 elements, 'Real' and 'Python', as expected. Notice that although you defined the array with double-quote strings, Python is displaying them with single-quote strings. Remember that there is no difference between single and double quotes in Python.

Output line 3 also shows the dtype of the data in the array. For this array, the dtype is <U6. The three characters here represent the aspects of how the strings are arranged in memory. The < means that the byte order of the array is little endian. The U means the string is of the Unicode type. Finally, the 6 means the maximum length of an element is 6 characters. This was chosen as the length of the longest string in the input.

Notice that the string Real only has 4 characters. In NumPy string dtype arrays, elements can have fewer than the maximum number of characters without problems, but assigning to elements with strings that are longer than the maximum length will truncate the input:

In [4]: arr[0] = "My favorite language is Python"
In [5]: arr
Out[5]: array(['My fav', 'Python'], dtype='<U6')

In this code, you are attempting to reassign the first element of the array with the string My favorite language is Python. Clearly, this string is longer than 6 characters, so it is truncated to only 6 when it is assigned, My fav. (The space counts as 1 character.)

If you want to create an array that can hold strings of any length, you should pass the object dtype when you create the array:

In [6]: arr_2 = np.array(("Real", "Python"), dtype=object)
In [7]: arr_2
Out[7]: array(['Real', 'Python'], dtype=object)

In this code, you are creating a new array, arr_2, with two elements again, but this time you specified the dtype to be object, which you confirmed by showing the output on the console. Now you should see how the object dtype affects assigning long strings to an element:

In [8]: arr_2[0] = "My favorite language is Python"
In [9]: arr_2
Out[9]: array(['My favorite language is Python', 'Python'], dtype=object)

In this code, you are again assigning the first element of the array to have the value My favorite language is Python. You can see from the output line that that string is stored as the first element of the array without truncation, because the dtype is object. The disadvantage of using the object dtype is that it is usually much slower than the more specific U dtype, because it has to create a a whole Python object for each element rather than just a NumPy-optimized Unicode object.

One other difference you will notice from MATLAB is how the shape or size of the array is determined:

In [10]: arr.shape
Out[10]: (2,)
In [11]: arr_2.shape
Out[11]: (2,)

In this code, we are printing the shape of arr and arr_2. Notice that they both have the same shape, two elements in a one-dimensional array. This is similar to the string array from MATLAB, where each string counts as one element in the array. However, the fact that NumPy arrays with the U dtype have a fixed maximum size behaves more like the character array from MATLAB. You’ll see more about differences in how MATLAB and NumPy compute the shape of arrays in a later section.

Libraries Are Not Automatically Reloaded in Python

When executing a function or script, MATLAB will always use the most up-to-date copy of the file on the disk. Therefore, as you’re developing a script, you can run it in the console many times and new changes that you make will automatically be picked up.

Python operates somewhat differently. Remember that when you want to access code from a file, you have to import it into a namespace. When Python imports a file or module, it only reads the code the first time it is imported. This saves quite a bit of time if you’re importing the same file several times. However, if you’re testing your code in the interactive console prompt as you work on it, Python will not pick up any changes if you import it again.

When you’re developing a module, you have a few options to have Python reload your code when it is imported. If you’re using the Spyder IDE, this is not a problem at all, since Spyder has an automatic User Module Reloading feature enabled by default.

Otherwise, if you’re using the IPython console outside of Spyder, or the Jupyter Notebook, you can use a magic command defined in those interpreters called autoreload:

In [1]: %load_ext autoreload
In [2]: %autoreload 2

In this code, you are using the load_ext magic command to load the autoreload extension. In IPython and Jupyter Notebooks, commands prefixed with the percent sign % are magic commands. The autoreload extension defines the autoreload magic function, which you use on input line 2. You are passing the parameter 2 to the autoreload magic function, which means that all modules should be reloaded every time a line of code is executed.

Lists

Python lists are mutable sequences of values. Lists can contain heterogeneous data, which means that each element of the list can be of a different type. Because lists are mutable, you can change the value of any element in the list, or add or remove values from the list, without creating a new list object.

Since lists are sequences, you can create loops that iterate over them. In Python, you do not need to access each element of a list with an index in a for loop, as you would do in MATLAB:

>> arr_1 = 1:2:6;
>> for i = 1:length(arr_1)
    disp(arr_1(i))
end
1
3
5

In this code, you are creating an array arr_1 with the integers from 1 to 6, taking every other number. Then you are creating a for loop where the loop variable goes from 1 to the length of arr_1. Finally, you are displaying the value of the element of arr_1 at the loop variable on each step by using the loop variable i to index arr_1.

In Python, you should not use an index for the list when you loop over it. Instead, you should loop directly over the items in a list:

In [1]: lst_1 = [1, "b", 3.0]
In [2]: for item in lst_1:
   ...:     print(item)
   ...:
1
b
3.0

In this code, on input line 1 you are first creating a Python list with three elements:

1. The integer 1
2. The string "b"
3. The float 3.0

This list is assigned to lst_1. Then you are using a for loop to access each item in the list in turn. On each iteration, the next value in the list is put into the variable item that you specified on the for line. Then, you are printing the value of item on each iteration.

Notice in the previous example that you could loop over the value of each element in the list without using an index. Nonetheless, sometimes you want to access the index of each item in the list as you’re looping over it. For those cases, Python provides enumerate() that returns the index and the value of the item:

In [3]: for index, item in enumerate(lst_1):
   ...:     print(f"The index is {index} and the item is {item}")
   ...:
The index is 0 and the item is 1
The index is 1 and the item is b
The index is 2 and the item is 3.0

In this code, you are looping over lst_1 again, but this time, you are using enumerate() to get both the index and the item. Then you are printing the value of the index and item on each loop iteration. As you can see from the result, the index values start at 0 as expected, but you do not need to use the index to access the item from the list.

In summary, you should not write Python code like this:

for i in range(len(lst_1)):
    print(lst_1[i])

In this code, you are creating a range of integers from 0 to the length of lst_1 and then you are accessing each element in the list by its index. This can lead to off-by-one and fencepost errors. Instead, you should write code that loops over the list directly:

for item in lst_1:
    print(item)

You can read a lot more about lists in Lists and Tuples in Python and about for loops and iteration in Python “for” Loops (Definite Iteration). There is also a more advanced concept called list comprehensions that you can learn about in Using List Comprehensions Effectively.

Dictionaries

In MATLAB, you can create a map data type with containers.Map(). This kind of data structure is useful when you have two pieces of data that are always related to each other and you want to connect them together. For instance, you can map cities to their population with a containers.Map():

>> cities = containers.Map({'Cleveland';'Pittsburgh';'Hartford'},
                           [383793,301048,122587]);
>> cities('Cleveland')

ans =

     383793

In this code, you are creating a containers.Map() on the first line. The first argument is a cell array of character arrays with the city names. These are called the keys of the map. The second argument is an array of populations. These are called the values of the map. Then, you are accessing the value of the population in Cleveland by indexing the map with a character array.

You can assign new values into the map by assigning to an undefined key value:

>> cities('Providence') = 180393;

If you try to access a key that does not exist, you will receive an error message:

>> cities('New York City')
Error using containers.Map/subsref
The specified key is not present in this container.

Python has an equivalent data structure called a dictionary. To create a Python dictionary, you can use curly braces and specify the keys and values with each other:

In [1]: cities = {
   ...: "Cleveland": 383_793,
   ...: "Pittsburgh": 301_048,
   ...: "Hartford": 122_587}
In [2]: cities["Cleveland"]
Out[2]: 383793

In this code, on input line 1 you are creating the dictionary of cities using curly braces. Notice that the key and value are specified together, separated by a colon. The values are specified with _ in the numbers, a feature available since Python 3.6. This does not change the value of the number, it only makes it easier to read very large numbers. Then, you are accessing the value at the "Cleveland" key using square brackets, similar to the indexing syntax for lists and arrays.

You can add new keys to the dictionary by assigning to them:

In [3]: cities["Providence"] = 180_393

In this code, you assigned a new key "Providence" to the dictionary with a value of 180,393. If you try to access a key that is not in the dictionary, you will get a KeyError:

In [4]: cities["New York City"]
Traceback (most recent call last):

  File "<ipython-input-4-6ebe5b35f3ea>", line 1, in <module>
    cities["New York City"]

KeyError: 'New York City'

In this code, you are trying to access the dictionary using the "New York City" key. However, this key does not exist in the dictionary so Python raises a KeyError letting you know that "New York City" is not an option.

You can read a lot more about Python dictionaries in Dictionaries in Python and KeyError exceptions in Python KeyError Exceptions and How to Handle Them. You can also iterate through dictionaries and use dictionary comprehensions, similar to list comprehensions. You can read about these topics in How to Iterate Through a Dictionary in Python.

NameError

Python NameError exceptions are usually the result of a variable being undefined. When you see a NameError, check your code for typos and misspelled variable names. You can use the debugging features and the variable explorer in Spyder to find out which variables are defined.

SyntaxError

Python SyntaxError exceptions mean that you have input some improper syntax. This is usually caused by mismatched brackets, when you have only the opening bracket or closing bracket but not the matching one. These exceptions usually point to the line after the place where the problem is located.

Another common SyntaxError is using only one equals sign in an if statement. In this case, you either meant not-equals (!=) or equals (==), so you can correct the line. For more on SyntaxError exceptions, check out Invalid Syntax in Python: Common Reasons for SyntaxError.

KeyError

Python KeyError exceptions occur when you try to access a key in a dictionary that does not exist. You can use .get() to retrieve a key from a dictionary if it exists, or return a default value if the key does not exist. You can read more about KeyError exceptions in Python KeyError Exceptions and How to Handle Them.

IndexError

Python IndexError exceptions occur when you are trying to access the index of an array or list that does not exist. This usually means the array or list you are trying to access has fewer elements than the index you are trying to access. You can use the debugging features and the variable explorer in Spyder to see the size of lists and arrays and make sure you’re only accessing indices that exist.

ImportError/ModuleNotFoundError

Python ImportError and ModuleNotFoundError exceptions occur when you try to import a module that Python cannot find. This might be because it is installed in a different conda environment or virtualenv, or it might be because you forgot to install the package.

The solution to this error is usually to conda install or pip install the package and make sure the correct environment is activated. If you’re not using the base environment in conda, you also need to make sure to install Spyder or Jupyter into your environment.

TypeError/ValueError

Python TypeError exceptions happen when an argument is of the wrong type. This happens most commonly when you pass an argument of the wrong type into a function. For instance, a function that works with numbers would raise a TypeError if a string were passed in.

A related exception is the ValueError. This exception happens when an argument is of the correct type, but has an incorrect value. For instance, a function that works only with positive numbers would raise a ValueError if a negative number were passed in.

AttributeError

Python AttributeError exceptions happen when you try to access an attribute of an object when the object does not have that attribute. You will often see this error associated with the message NoneType object has no attribute. This message most likely means that a function has returned None instead of the object you were expecting and you are trying to access an attribute that would be present on the real object, but is not defined for None.