I recently started learning Python more deeply.

Not just writing Python, but understanding how Python actually works internally.

When we run a Python file it feels simple.

python app.py

And the program runs.

But what actually happens between typing that command and the program executing on your machine?

Let’s break it down from first principles.

The Goal of a Programming Language

At the most basic level a programming language does one thing.

It converts human readable instructions into something a computer can execute.

Computers only understand machine code.

Humans write high level code.

So every programming language needs a process to translate our code into something the computer can run.

Python does this in a few steps.

Python Code (.py)

↓

Bytecode

↓

Python Virtual Machine

↓

Program Execution

Step 1. Writing Python Code

Everything starts with the code we write.

Example:

print("Hello World")

This code lives inside a file like:

app.py

At this stage it is just text. The computer cannot execute it yet.

Python first converts this code into something called bytecode.

Step 2. Compilation to Bytecode

When you run a Python script, Python compiles the code into bytecode.

Bytecode is a lower level representation of your program.

It is not machine code. It is an intermediate format that Python understands.

Sometimes Python stores this compiled version in a folder called:

__pycache__

Inside that folder you might see files like:

app.cpython-311.pyc

These files contain the compiled bytecode so Python can reuse them later and start faster.

Step 3. Python Virtual Machine

After the code becomes bytecode it is executed by something called the Python Virtual Machine (PVM).

You can think of the PVM as the engine that runs Python programs.

It reads bytecode instructions and executes them one by one.

Conceptually it works like this:

read instruction

execute instruction

repeat

Each instruction tells Python what to do.

Examples:

print text
create variables
run calculations
call functions

Why Python Works This Way

This design makes Python very portable.

The same Python code can run on:

Linux
macOS
Windows

Because the Python interpreter handles the interaction with the operating system.

Your code stays the same.

CPython

When most people say Python they mean CPython.

CPython is the main implementation of Python written in the C programming language.

It includes:

the Python compiler
the Python virtual machine
memory management
the standard library

There are other implementations like PyPy and Jython but CPython is the most common.

Final Thoughts

When we run a Python file it looks simple.

But internally Python follows a structured process.

Python source code is compiled to bytecode
The bytecode is executed by the Python Virtual Machine
The program runs on your machine

Understanding this makes Python feel less like magic and more like a well designed system.

I recently started documenting my Python learning journey here on the blog.

In the next post I will explore mutable and immutable objects in Python, and why understanding this is important when working with lists, dictionaries, and variables.

How Python Executes Code - Explained Using First Principles

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