Interacting With Python

Starting the Interpreter

In a GUI desktop environment, it is likely that the installation process placed an icon on the desktop or an item in the desktop menu system that starts Python.

For example, in Windows, there will likely be a program group in the Start menu labeled Python 3.x, and under it a menu item labeled Python 3.x (32-bit), or something similar depending on the particular installation you chose.

Clicking on that item will start the Python interpreter:

Alternatively, you can open a terminal window and run the interpreter from the command line. How you go about opening a terminal window varies depending on which operating system you’re using:

• In Windows, it is called Command Prompt.
• In macOS or Linux, it should be called Terminal.

Using your operating system’s search function to search for “command” in Windows or “terminal” in macOS or Linux should find it.

Once a terminal window is open, if paths have been set up properly by the Python install process, you should be able to just type python. Then, you should see a response from the Python interpreter.

This example is from the Windows Command Prompt window:

C:\Users\john>python
Python 3.6.0 (v3.6.0:41df79263a11, Dec 23 2016, 07:18:10) [MSC v.1900 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>>

If you are not seeing the >>> prompt, then you are not talking to the Python interpreter. This could be because Python is either not installed or not in your terminal window session’s path. It’s also possible that you just haven’t found the correct command to execute it. You can refer to our installing Python tutorial for help.

Executing Python Code

If you are seeing the prompt, you’re off and running! The next step is to execute the statement that displays Hello, World! to the console:

1. Ensure that the >>> prompt is displayed, and the cursor is positioned after it.
2. Type the command print("Hello, World!") exactly as shown.
3. Press the Enter key.

The interpreter’s response should appear on the next line. You can tell it is console output because the >>> prompt is absent:

>>> print("Hello, World!")
Hello, World!

If your session looks like the above, then you have executed your first Python code! Take a moment to celebrate.

Did something go wrong? Perhaps you made one of these mistakes:

• You forgot to enclose the string to be printed in quotation marks:

  Python

  >>> print(Hello, World!)
    File "<stdin>", line 1
      print(Hello, World!)
                        ^
  SyntaxError: invalid syntax
  

• You remembered the opening quotation mark but forgot the closing one:

  Python

  >>> print("Hello, World!)
    File "<stdin>", line 1
      print("Hello, World!)
                          ^
  SyntaxError: EOL while scanning string literal
  

• You used different opening and closing quotation marks:

  Python

  >>> print("Hello, World!')
    File "<stdin>", line 1
      print("Hello, World!')
                           ^
  SyntaxError: EOL while scanning string literal
  

• You forgot the parentheses:

  Python

  >>> print "Hello, World!"
    File "<stdin>", line 1
      print "Hello, World!"
                          ^
  SyntaxError: Missing parentheses in call to 'print'
  

• You entered extra whitespace characters before the command:

  Python

  >>>     print("Hello, World!")
    File "<stdin>", line 1
      print("Hello, World!")
      ^
  IndentationError: unexpected indent
  

(You will see in an upcoming section why this matters.)

If you got some sort of error message, go back and verify that you typed the command exactly as shown above.

Exiting the Interpreter

When you are finished interacting with the interpreter, you can exit a REPL session in several ways:

• Type exit() and press Enter:

  Python

  >>> exit()
  
  C:\Users\john>
  

• In Windows, type Ctrl+Z and press Enter:

  Python

  >>> ^Z
  C:\Users\john>
  

• In Linux or macOS, type Ctrl+D. The interpreter terminates immediately; pressing Enter is not needed.

• If all else fails, you can simply close the interpreter window. This isn’t the best way, but it will get the job done.

IDLE

Most Python installations contain a rudimentary IDE called IDLE. The name ostensibly stands for Integrated Development and Learning Environment, but one member of the Monty Python troupe is named Eric Idle, which hardly seems like a coincidence.

The procedure for running IDLE varies from one operating system to another.

Using IDLE

Once IDLE is installed and you have started it successfully, you should see a window titled Python 3.x.x Shell, where 3.x.x corresponds to your version of Python:

The >>> prompt should look familiar. You can type REPL commands interactively, just like when you started the interpreter from a console window. Mindful of the qi of the universe, display Hello, World! again:

The interpreter behaves more or less the same as when you ran it directly from the console. The IDLE interface adds the perk of displaying different syntactic elements in distinct colors to make things more readable.

It also provides context-sensitive help. For example, if you type print( without typing any of the arguments to the print function or the closing parenthesis, then flyover text should appear specifying usage information for the print() function.

One other feature IDLE provides is statement recall:

• If you have typed in several statements, you can recall them with Alt+P and Alt+N in Windows or Linux.
• Alt+P cycles backward through previously executed statements; Alt+N cycles forward.
• Once a statement has been recalled, you can use editing keys on the keyboard to edit it and then execute it again. The corresponding commands in macOS are Cmd+P and Cmd+N.

You can also create script files and run them in IDLE. From the Shell window menu, select File → New File. That should open an additional editing window. Type in the code to be executed:

From the menu in that window, select File → Save or File → Save As… and save the file to disk. Then select Run → Run Module. The output should appear back in the interpreter Shell window:

OK, that’s probably enough Hello, World!. The qi of the universe should be safe.

Once both windows are open, you can switch back and forth, editing the code in one window, running it and displaying its output in the other. In that way, IDLE provides a rudimentary Python development platform.

Although it is somewhat basic, it supports quite a bit of additional functionality, including code completion, code formatting, and a debugger. See the IDLE documentation for more details.

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Thonny

Thonny is free Python IDE developed and maintained by the Institute of Computer Science at the University of Tartu, Estonia. It is targeted at Python beginners specifically, so the interface is simple and uncluttered as well as easy to understand and get comfortable with quickly.

Like IDLE, Thonny supports REPL interaction as well as script file editing and execution:

Thonny performs syntax highlighting and code completion in addition to providing a step-by-step debugger. One feature that is particularly helpful to those learning Python is that the debugger displays values in expressions as they are evaluated while you are stepping through the code:

Thonny is especially easy to get started with because it comes with Python 3.6 built in. So you only need to perform one install, and you’re ready to go!

Versions are available for Windows, macOS, and Linux. The Thonny website has download and installation instructions.

IDLE and Thonny are certainly not the only games going. There are many other IDEs available for Python code editing and development. See our Python IDEs and Code Editors Guide for additional suggestions.