Python bytes and bytearray for DSA and Binary Data

Why This Matters

Most DSA practice uses lists and strings, but real systems also move raw bytes. Hashing, parsers, file formats, network protocols, compression, and low-level encodings all need byte-level thinking.

Use this page when a problem stops being "characters in a string" and becomes "bytes in a buffer."

Core Idea

bytes is immutable. bytearray is mutable. Both are sequences of integers from 0 to 255.

Tool	Mutable?	Main use
`bytes`	no	fixed binary data, encoded text, hash inputs
`bytearray`	yes	editable byte buffer
`memoryview`	view	slice buffers without copying in many cases
`encode`	creates bytes	text to bytes
`decode`	creates str	bytes to text
`int.to_bytes`	creates bytes	pack integer into fixed byte width
`int.from_bytes`	creates int	read integer from bytes

Endianness

Endianness is byte order. Big-endian stores the most significant byte first. Little-endian stores the least significant byte first.

(258).to_bytes(2, "big") == bytes([0x01, 0x02])
(258).to_bytes(2, "little") == bytes([0x02, 0x01])

State the byte order whenever bytes represent a number.

Non-Example or Failure Mode

A Python str is text, not raw bytes. Indexing "A" gives a one-character string, while indexing b"A" gives integer 65. Confusing those two models causes parsing and encoding bugs.

Worked Example

You receive two bytes representing a length field in big-endian order:

packet = bytes([0x01, 0x2C])
length = int.from_bytes(packet, "big")

The result is 300 because 0x012C equals decimal 300.

Common Mistakes

Mistake	Correction
Treating text and bytes as the same thing.	Use `encode` and `decode` at the boundary.
Forgetting that `bytes[i]` is an integer.	Compare to `65` or use `bytes([65])` for a one-byte object.
Mutating `bytes`.	Use `bytearray` for editable buffers.
Omitting byte order.	Pass `"big"` or `"little"` to integer packing methods.
Assuming slicing always stays zero-copy.	Use `memoryview` when avoiding copies matters.

Diagnostic Questions

Question type	Question	Answer signal
Definition	What values can a byte hold?	Integers from `0` to `255`.
Example / non-example	Is `b"abc"` a Python `str`?	No. It is `bytes`.
Computation	What is `b"A"[0]`?	`65`.
Transfer	Why does endianness matter?	The same bytes can represent different numbers under different byte orders.

Runnable Drill

Python bytes and bytearray drill

Checks encoding, mutation, memoryview, hex, and integer byte order.

Output will appear here.

Exercises

Beginner:

Convert "heap" to UTF-8 bytes and back.
Explain why b"A"[0] is 65.
Mutate a bytearray in place.

Intermediate:

Pack and unpack 65535 using two bytes.
Compare big-endian and little-endian output for the same integer.

Challenge:

Write a parser for a toy packet format: two bytes of length followed by that many payload bytes.

Diagram Recommendation

Type: byte-order diagram.

Caption: "The integer 258 as two bytes: big-endian 01 02, little-endian 02 01."

Purpose: Make endianness concrete before learners parse binary data.

Next Topics

References

Python documentation. Binary sequence types: https://docs.python.org/3/library/stdtypes.html#binary-sequence-types-bytes-bytearray-memoryview
Python documentation. int.to_bytes and int.from_bytes: https://docs.python.org/3/library/stdtypes.html#int.to_bytes
Python documentation. Text encodings: https://docs.python.org/3/library/codecs.html#standard-encodings