🐍 Understanding How Python Code Works Internally

Here’s a clean, professional documentation-style explanation of how Python works — formatted clearly, like what you’d write in Notion or technical documentation Understanding How Python Code Works Internally

When you write and run a Python program, several steps happen behind the scenes before your code executes. Let’s break down the entire process.

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⚙️ Step 1: Source Code (.py)

You start by writing your Python code in a .py file.

Example:

# first.py
def greet():
    print("Hello from first file!")

Then in another file:

# second.py
from first import greet

greet()

When you run:

python3 second.py

Python automatically creates a folder called __pycache__ and stores a compiled version of the imported module inside it (for example, first.cpython-312.pyc).

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🧩 Step 2: Compilation to Bytecode

Before execution, Python compiles your .py source code into bytecode — a low-level, platform-independent representation of your code.

• Bytecode is not machine code.

• It’s a Python-specific intermediate code that runs faster than raw source code.

• These compiled files are saved as .pyc files inside the __pycache__ folder.

💡 Example:
first.cpython-312.pyc → compiled with CPython 3.12 version.

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📦 Step 3: Execution by the Python Virtual Machine (PVM)

Once the code is compiled into bytecode, the Python Virtual Machine (PVM) takes over.

• The PVM interprets and executes the bytecode instructions line by line.

• It acts as the runtime engine of Python.

• This step is why Python is known as an interpreted language.

🧠 In short:

Python → compiles to bytecode → runs on Python Virtual Machine (PVM)

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📁 Step 4: .pyc Files and Their Behavior

• .pyc files are auto-generated only for imported modules (not for top-level scripts you run directly).

• These files help speed up execution next time — since the bytecode is already compiled.

Example:

project/
│
├── first.py
├── second.py
└── __pycache__/
     └── first.cpython-312.pyc

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🧠 Variants of the Python Interpreter

Python has multiple implementations — all follow the same concept but use different underlying technologies:

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🧾 Summary

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🔍 Key Notes

• Bytecode is platform-independent.

• .pyc files are stored in __pycache__ automatically.

• Only imported files generate .pyc files.

• PVM is the runtime engine that executes the bytecode.

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