How does CPython execute your code? | Interview Question

Q: How does CPython execute your code?

**Target Audience:** Python developers who have written functions and loops and want to know what the interpreter does with them. No C or compiler background is assumed. **Difficulty:** Hard **Python version:** Behaviour and `dis` output checked on CPython 3.11, with the differences on 3.12 and 3.13 shown where they matter. Both examples also run in the page on Pyodide 0.25.1 (CPython 3.11.3). > **How to read this doc:** Every idea is explained in plain words first. When a word appears that an interviewer expects you to know, it is tagged with `📌 Interview term:` and defined on the spot. All of them are collected in the Quick Glossary at the end. Every output shown was produced by running the code. ## 1. Why This Even Matters — A Story First Imagine you hire a translator to read a recipe written in French to a cook who only understands a short list of hand signals: "pick up", "put down", "mix", "pour". The translator does not stand next to the cook translating each French sentence

The problem

Question presented to candidate: "When I type python app.py, what actually happens between the text in that file and the program running? Is Python compiled or interpreted, what are bytecode and .pyc files, and what changed in CPython 3.11 to make it faster?"

What a strong answer should cover:

The pipeline: the tokenizer turns text into tokens, the parser builds an AST, the compiler turns the AST into a code object holding bytecode, and the interpreter's eval loop runs that bytecode
Python is "compiled to bytecode, then interpreted": the whole file is compiled first, so a SyntaxError anywhere stops even line 1, while a NameError only appears when that line runs
The eval loop is a stack machine: instructions such as LOAD_FAST, BINARY_OP, STORE_FAST push and pop a value stack inside a per-call frame; dis shows them
Imported modules cache bytecode in __pycache__/name.cpython-311.pyc (16-byte header, then a marshalled code object); the script you run directly is not cached, and a .pyc only skips compiling, it does not make code run faster
Bytecode is a CPython implementation detail that changes between versions (different instructions in 3.11, 3.12 and 3.13 for the same function)
3.11's specializing adaptive interpreter (PEP 659) rewrites hot instructions into type-specific ones such as BINARY_OP_ADD_INT; an experimental JIT followed in 3.13 (PEP 744)

Code / implementation expected: Optional — reading dis output for a small function and explaining each instruction is the usual exercise.

cpython
internals
bytecode

Try it yourself

Sketch your approach before you read the solution. That is what the interview tests.

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A short nudge toward the approach. It will not give the answer away.

Solution

Target Audience: Python developers who have written functions and loops and want to know what the interpreter does with them. No C or compiler background is assumed. Difficulty: Hard Python version: Behaviour and dis output checked on CPython 3.11, with the differences on 3.12 and 3.13 shown where they matter. Both examples also run in the page on Pyodide 0.25.1 (CPython 3.11.3).

How to read this doc: Every idea is explained in plain words first. When a word appears that an interviewer expects you to know, it is tagged with 📌 Interview term: and defined on the spot. All of them are collected in the Quick Glossary at the end. Every output shown was produced by running the code.

1. Why This Even Matters — A Story First

Imagine you hire a translator to read a recipe written in French to a cook who only understands a short list of hand signals: "pick up", "put do

The solution is hidden so you can try first

17 min read · 11 sections

Run the code

Edit the code and run it here. Each console line with an expected value becomes a test.

PythonFrom source text to bytecode: tokens, AST, code object and dis

Practice

PythonA toy eval loop running real bytecode, plus the adaptive interpreter

Practice

Discussion

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