Python Internal Working

What is Python?

Python is a high-level, interpreted programming language.

• High-level → You write code in a human-readable format (like English) rather than low-level machine instructions.

• Interpreted → Python doesn’t directly convert your code into machine code (like C or C++), but it executes it line by line through an interpreter.

---

The Life Cycle of Python Code Execution

Let’s say you write a simple Python program in a file called hello.py:

print("Hello, World!")

Here’s how Python processes it internally:

---

Step 1: Writing the Code (Source Code)

• You write your Python code using any text editor or IDE.

• The file is saved with a .py extension, e.g., hello.py.

• This is your source code → plain text instructions written in Python syntax.

---

Step 2: Compilation into Bytecode

When you run the program by typing:

python hello.py

✅ The Python interpreter first compiles your code into bytecode.

What is Bytecode?

• Bytecode is a low-level, platform-independent set of instructions.

• It’s not machine code (binary), but a more compact form of your source code.

• Bytecode is saved as .pyc files (Python compiled files) inside the __pycache__ folder.

✔️ Example:
If you run hello.py, you’ll see __pycache__/hello.cpython-3xx.pyc (3xx as per Python version) created.

Step 3: Python Virtual Machine (PVM) Executes Bytecode

• The Python Virtual Machine (PVM) is responsible for executing the bytecode.

• It takes the .pyc file and interprets it line by line into machine instructions that the computer understands.

• This is why Python is called an interpreted language.

✅ Key Points:

• Python compiles to bytecode, but the final execution happens through an interpreter (PVM).

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

---

Behind the Scenes: Components Involved

Python uses several components to make this process work:

🔹 Python Interpreter

The Python interpreter handles the entire process:

• Compilation → From source code to bytecode.

• Execution → Interpreting the bytecode using the PVM.

Python Execution

Let’s visualize the Python execution process:

Your Code (hello.py)  
     ↓  
[ Compilation ] → Bytecode (.pyc)  
     ↓  
[ PVM Interpretation ] → Machine Instructions  
     ↓  
[ Output on Screen ] → Hello, World!

---

When using imports in python

1. When you use import, Python:

   • Compiles the module into bytecode.

   • Saves it as a .pyc file inside __pycache__.

2. The .pyc file:

   • Speeds up execution by avoiding recompilation.

   • Is version-specific (e.g., cpython-313 for Python 3.13).

3. You can:

   • Disable caching using -B or PYTHONDONTWRITEBYTECODE=1.

   • Decompile .pyc files to see the bytecode.

https://youtu.be/3HTKc-ZgZbg