Python Execution Model, CPython Bytecode & __name__ == '__main__'

1Concept

Python source files (.py) are parsed into an Abstract Syntax Tree (AST), compiled into CPython bytecode (.pyc in __pycache__), and executed by the CPython Virtual Machine. The idiom if __name__ == '__main__': checks if the script is run directly or imported as a module.

2Architecture Diagram

[script.py] ──► [AST Parser] ──► [Bytecode (.pyc)] ──► [CPython Virtual Machine (Eval Loop)]

3Code Example

import sys
import platform

def main():
    print("=== Python 3.12 Runtime Engine ===")
    print(f"Python Version: {platform.python_version()}")
    print(f"Implementation: {platform.python_implementation()}")
    print(f"Recursion Limit: {sys.getrecursionlimit()}")

if __name__ == "__main__":
    main()

4Expected Output

=== Python 3.12 Runtime Engine ===
Python Version: 3.12.2
Implementation: CPython
Recursion Limit: 1000

5Key Takeaways

• ✓if __name__ == '__main__' prevents code from executing automatically on import.
• ✓CPython compiles bytecode to __pycache__ to speed up subsequent load times.
• ✓Python uses dynamic typing evaluated at runtime.