How to Run Your Python Scripts and Code

Using the python Command

To run Python scripts with the python command, you need to open a command-line window and type in the word python followed by the path to your target script:

• Windows
• Linux + macOS

PS> python .\hello.py
Hello, World!

PS> py .\hello.py
Hello, World!

$ python ./hello.py
Hello, World!

After you press Enter, you’ll see the phrase Hello, World! on your screen. If the previous command doesn’t work right, then you may need to check if Python is in your system’s PATH. You can also check where you saved hello.py.

That’s it! You’ve run your first script! Note that on Windows, you also have the option of using the py command, which triggers the py.exe launcher for console applications. This is the most basic and practical way to run Python scripts.

A cool feature of a terminal or shell application is that you can redirect the output of your commands using a straightforward syntax. This feature may be useful in those situations where you have a Python program that can generate a long output, and you’d like to save it to a file for later analysis.

In these situations, you can do something like the following:

$ python hello.py > output.txt

In this command, the > symbol tells the shell to redirect the output of your command to the output.txt file, rather than to the standard system output, your screen. This process is commonly known as redirection, and it works on both Windows and Unix-like systems, such as Linux and macOS.

If the output file doesn’t exist, then the shell automatically creates it. On the other hand, if the file already exists, then the shell overwrites its old content with the new output.

Finally, if you want to add the output of consecutive executions to the end of output.txt, then you can use two angle brackets (>>) instead of one:

$ python hello.py >> output.txt

Now, the shell app will append the current output to the end of output.txt. You’ll end up with a file containing the phrase "Hello, World!" twice.

Using the Script’s Filename Directly

On Windows, you can also run Python scripts by simply entering the name of the file containing the executable code at the command line:

PS> .\hello.py

Once you’ve written the path to your script and pressed Enter, you’ll note that a new terminal window appears on your screen for a few seconds, showing the script output. This is possible because Windows associates .py and .pyw files to python.exe and pythonw.exe, respectively.

This way of running Python scripts on Windows may be annoying because the code runs in a new terminal window that automatically closes after the execution ends. In most cases, you won’t be able to check the program’s output.

On Linux and macOS, you can also run your scripts directly. However, things are a bit different here, and you need some setup steps. Go ahead and run the following command:

• Linux
• macOS

$ ./hello.py
bash: ./hello.py: Permission denied

$ ./hello.py
zsh: permission denied: ./hello.py

Unix systems prioritize security, which means that you can’t go around executing any file as a program. So, you get a permission denied error when you try to run hello.py directly. To fix this issue, you need to explicitly tell the system that the file is executable. To do this, you can use the chmod command:

$ chmod +x hello.py

After running this command, your hello.py file will be executable. However, that’s not enough for the script to run properly:

$ ./hello.py
./hello.py: line 1: syntax error near unexpected token `"Hello, World!"'
./hello.py: line 1: `print("Hello, World!")'

Why are you getting another error now? The problem is that your operating system (OS) doesn’t know which program to use for running your script and is trying to run it with the shell itself. You can fix that by making a small addition to your hello.py file:

#!/usr/bin/env python3

print("Hello, World!")

You’ve added a new line at the beginning of hello.py. It now starts with a Unix-style shebang, which is a special kind of comment that you can include in your scripts to tell the operating system which program to use for running the content of this file. In this case, you tell the OS to use Python.

Now you can run the script directly from your command line:

$ ./hello.py
Hello, World!

Wow! That was a long road! However, the effort was worth it. Now when you create a Python script to automate tasks in a Unix operating system, you know how to make it executable and run it from your command line.

Running Modules With the -m Option

The python command has a series of command-line options that can be useful in specific situations. For example, if you want to run a Python module, then you can use the command python -m <module-name>. The -m option searches Python’s module search path, sys.path, for the module name and runs its content:

$ python -m hello
Hello, World!

In this example, you run the hello.py file as a module. This is possible because Python automatically adds the current directory to its sys.path list. Note that the module-name argument needs to be the name of a module object, not a file name. In other words, you don’t include the .py suffix.

If the target module isn’t in sys.path, then you get an error:

$ python -m missing
.../python: No module named missing

In this example, the missing name isn’t in the sys.path list, so Python isn’t able to execute it, and therefore it returns an error.

Getting to Know the Python Interpreter

Python is a high-level programming language with a clean and readable syntax. Python and its wide ecosystem of packages and libraries can boost your productivity in a variety of fields. The name Python also refers to a piece of software called the interpreter, which is the program that allows you to run Python code.

The interpreter is a layer of software that works between your program and your computer hardware to get your code running. Depending on the Python implementation that you use, the interpreter can be a program written in:

• C, like CPython, which is the core implementation of the language
• Python itself, like PyPy, which is a fast implementation with a just-in-time (JIT) compiler
• Java, like Jython, which can take advantage of the Java ecosystem
• .NET, like IronPython, which uses the .NET ecosystem

Whatever interpreter you use, the code that you write will run in this program. Therefore, the first condition to be able to run scripts and code is to have the interpreter correctly installed on your operating system.

The Python interpreter can run code in two different modes:

1. Script, or program
2. Interactive, or REPL

In script mode, you use the interpreter to run a source file as an executable program, just like you learned in the previous section. In this case, Python loads the file content and runs the code line by line, following the program’s execution flow.

Alternatively, interactive mode is when you launch the interpreter and use it as a platform to run code that you type in directly. This mode is pretty useful for learning Python as well as for developing, testing, and debugging your applications.

Running Python Code Interactively

Interactive sessions are a widely used tool for running Python code. To start a Python interactive session, or REPL, open a command-line window, type in the python command, and then press Enter.

These steps will take you into the Python interpreter, which looks something like the following:

• Windows
• Linux
• macOS

PS> python
Python 3.11.5 (tags/v3.11.5:cce6ba9, Aug 24 2023, 14:38:34) [MSC v.1936 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>>

$ python
Python 3.11.5 (main, Sep 13 2023, 18:12:18) [GCC 11.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>>

$ python
Python 3.11.5 (main, Aug 31 2023, 19:03:43) [Clang 14.0.3 (clang-1403.0.22.14.1)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>>

The standard primary prompt for the interactive mode consists of three right angle brackets, >>>. So, as soon as you see these characters, you’ll know that you’re in.

The Python interpreter is an interactive way to talk to your computer using the language. It’s like live chat. It’s also known as the REPL because it goes through four steps that run under the hood:

1. Reading your input, which consists of Python code as expressions and statements
2. Evaluating your Python code, which generates a result or causes side effects
3. Printing any output so that you can check your code’s results and get immediate feedback
4. Looping back to step one to continue the interaction

This feature of Python is a powerful tool that you’ll wind up needing in your Python coding adventure, especially when you’re learning the language or when you’re in the early stages of a development process.

Once you’ve started a REPL session, you can write and run Python code as you wish. The only drawback is that when you close the session, your code will be gone. This is another difference between the script and interactive modes. Scripts are persistent.

When you work interactively, Python evaluates and executes every expression and statement immediately:

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

>>> 2 + 5
7

>>> print("Welcome to Real Python!")
Welcome to Real Python!

An interactive session will allow you to test every piece of code that you execute. That’s why this tool is an awesome development helper and an excellent space to experiment with the language and test ideas on the fly.

To leave interactive mode and jump back to the system shell, you can use one of the following options:

• Executing the built-in quit() or exit() functions
• Pressing the Ctrl+Z and Enter key combination on Windows, or the Ctrl+D combination on Unix systems, such as Linux and macOS

Go ahead and give the Python REPL a try. You’ll see that it’s a great development tool that you must keep in your tool kit.

Taking Advantage of import Statements

When you import a module from another module, script, or interactive session, what really happens is that Python loads its contents for later access and use. The interesting point is that the import statement runs any executable code in the imported module.

When the module contains only class, function, variable, and constant definitions, you probably won’t be aware that the code was run. However, when the module includes calls to functions, methods, or other statements that generate visible results, then you’ll witness its execution.

This provides you with another option to run scripts:

>>> import hello
Hello, World!

You’ll note that import runs the code only once per session. After you first import a module, successive imports do nothing, even if you modify the content of the module. This is because import operations are expensive, and Python takes some extra steps to optimize overall performance:

>>> import hello  # Do nothing
>>> import hello  # Do nothing again

These two imports do nothing because Python knows that the hello module was already imported. Therefore, Python skips the import. This behavior may seem annoying, especially when you’re working on a module and trying to test your changes in an interactive session. However, it’s an intentional optimization.

Using the importlib Standard-Library Module

In the Python standard library, you can find the importlib module. This module provides the import_module() function, which allows you to programmatically import modules.

With import_module(), you can emulate an import operation and, therefore, execute any module or script. Take a look at this example:

>>> import importlib
>>> importlib.import_module("hello")
Hello, World!
<module 'hello' from '/home/username/hello.py'>

The import_module() function imports a module, bringing its name to your current namespace. It also runs any executable code that the target module contains. That’s why you get Hello, World! on your screen.

You already know that once you’ve imported a module for the first time, you won’t be able to import it again using another import statement. If you want to reload the module and run it once again, then you can use the reload() function, which forces the interpreter to import the module again:

>>> import hello
Hello World!

>>> import importlib
>>> importlib.reload(hello)
Hello World!
<module 'hello' from '/home/username/hello.py'>

An important point to note here is that the argument of reload() has to be the name of a module object, not a string. So, to use reload() successfully, you need to provide a module that’s already imported.

Leveraging the Power of the Built-in exec() Function

So far, you’ve learned about some handy ways to run Python scripts. In this section, you’ll learn how to do that by using the built-in exec() function, which supports the dynamic execution of Python code.

The exec() function provides an alternative way to run your scripts from inside your code:

>>> with open("hello.py") as hello:
...     exec(hello.read())
...
Hello, World!

In this example, you use the with statement to open the hello.py file for reading. Then, you read the file’s content with the .read() method. This method returns a string that you pass to exec() for execution.

You must be careful when using the exec() function because it implies some important security risks, especially if you’re using it for running external code. To learn more about this function, check out Python’s exec(): Execute Dynamically Generated Code.