What Really Happens When You Run a Python Script?

Ever wondered what happens behind the scenes when you run a Python file? Let’s unlock the magic beneath the hood and see how your Python code goes from a simple script to a running program.

1. Python Script & Interpreter

To run a Python program, you need two key things:

• Your Python script file (e.g., hello.py)

• The Python interpreter software (the program called python that runs your script)

When you type python hello.py, you are instructing the Python interpreter to execute your script.

2. Source Code to Bytecode

Before your script runs, Python first compiles your source code into something called bytecode. Bytecode is an intermediate, low-level, platform-independent set of instructions that the Python interpreter can understand. Think of it as a simplified translation of your code.

3. Python Virtual Machine (PVM)

The bytecode is then executed by the Python Virtual Machine (PVM), which is part of the Python interpreter. Unlike machine code that talks directly to your computer’s hardware, bytecode is meant for the PVM, which interprets and runs it.

4. .pyc Files & pycache Folder

To save time, Python stores the compiled bytecode in .pyc files inside a special folder called __pycache__. This way, the next time you run the same script, Python can quickly load the bytecode instead of recompiling the source code.

The file names in __pycache__ include the original script name, the Python implementation (like cpython), and the version number (e.g., 3.12), so Python keeps everything organized.

5. Other Python Implementations

While CPython is the most common and standard Python interpreter, there are others like:

• Jython (Python running on Java)

• IronPython (.NET framework)

• Stackless Python

• PyPy (a faster alternative with Just-In-Time compilation)

Why Is This Important?

Understanding what happens when you run a Python script helps you appreciate how Python balances ease of use with portability and efficiency. It also aids in debugging and optimizing your code.

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Did you know?

If you change your Python script, the bytecode automatically regenerates in a new .pyc file, speeding up future runs!

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Next Steps

Keep exploring the Python ecosystem! Next, you might want to learn about how Python imports modules or how memory management works under the hood.