What is the difference between bytes and str?

Q: What is the difference between bytes and str?

**Target Audience:** Python developers who handle files, network data or APIs, explained from zero **Difficulty:** Medium **Python version:** Behaviour checked on CPython 3.11 (and in the page's Pyodide 3.11 runtime) > **How to read this doc:** Each idea is explained in plain language first, then tagged with `📌 Interview term:` so you also learn the exact words an interviewer expects. Every output block in the **Verified** section was copied from a real run of the examples on this page. ## 1. Why This Even Matters — A Story First A piano score and a recording are both "the same song", but they are different things. The score is **meaning**: notes a musician understands. The recording is **storage**: a long list of numbers a machine can save and play. To go from one to the other you need an agreed method (performing and recording), and you can go wrong by using the wrong method in reverse. In Python, `str` is the score: text made of characters. `bytes` is the recording: a list of n

The problem

Question presented to candidate: "In Python 3, what is the difference between bytes and str? When do you get each one, how do you convert between them, and what goes wrong when people mix them up?"

What a strong answer should cover:

str is an immutable sequence of Unicode code points (text); bytes is an immutable sequence of integers 0 to 255 (binary data)
str.encode(encoding) gives bytes; bytes.decode(encoding) gives str; UTF-8 is the usual encoding, and the same text has different byte lengths in different encodings
Python 3 never converts implicitly: "a" + b"a" is a TypeError and b"abc" == "abc" is False; indexing bytes returns an int
Errors: UnicodeEncodeError, UnicodeDecodeError, and the errors= argument (strict, replace, ignore); decoding with the wrong codec can silently produce mojibake
The "Unicode sandwich": decode at the input boundary (files, sockets, HTTP), work with str inside, encode at the output boundary
Related: bytearray (mutable), memoryview (zero-copy slices), and CPython storing str at 1, 2 or 4 bytes per character (PEP 393)

Code / implementation expected: Optional — an encode/decode round trip and one error case.

bytes
str
encoding

Try it yourself

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Solution

Target Audience: Python developers who handle files, network data or APIs, explained from zero Difficulty: Medium Python version: Behaviour checked on CPython 3.11 (and in the page's Pyodide 3.11 runtime)

How to read this doc: Each idea is explained in plain language first, then tagged with 📌 Interview term: so you also learn the exact words an interviewer expects. Every output block in the Verified section was copied from a real run of the examples on this page.

1. Why This Even Matters — A Story First

A piano score and a recording are both "the same song", but they are different things. The score is meaning: notes a musician understands. The recording is storage: a long list of numbers a machine can save and play. To go from one to the other you need an agreed method (performing and recording), and you can go wrong by using the wrong method in rev

The solution is hidden so you can try first

12 min read · 11 sections

Run the code

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

Pythonencode and decode, indexing, codecs, errors and why Python 3 refuses to mix the types

Practice

PythonUnder the hood: PEP 393 storage widths, code points vs visible characters, normalisation, memoryview