Python Bytes and Bytearray

Data Types Satellite

Bytes & Bytearray

Python treats text (`str`) and binary (`bytes`) separately. Learn how to read/write binary files, encode/decode strings, and mutate buffers.

Convert between str and bytesMutate bytearray

Bytes literals

A bytes object is an immutable sequence of integers in the range 0–255. Prefix a literal with b:

data = b"Python"        # each character is one byte
data[0]                 # 80 — indexing yields an int, not a byte
data[:3]                # b'Pyt' — slicing yields bytes
b"\x00\xff"             # arbitrary bytes via \x escapes
bytes([80, 121])        # b'Py' — from a list of ints

Only ASCII characters can appear directly in a bytes literal; anything else must be escaped or produced by encoding.

Encoding & decoding

Text and bytes are not interchangeable. Convert between them explicitly:

data = "café".encode("utf-8")   # b'caf\xc3\xa9' — str -> bytes
text = data.decode("utf-8")     # 'café'          — bytes -> str

• Always name the encoding when talking to files, networks, or other systems; don't rely on the platform default.
• Mismatched encodings raise UnicodeDecodeError. Handle unknown input with errors="replace" or errors="ignore".

data.decode("ascii", errors="replace")   # substitutes � for non-ASCII bytes

Default to UTF-8 unless a data source dictates otherwise.

bytes vs bytearray

Type	Mutable?	When to use
bytes	No	Immutable binary data, network responses, file contents
bytearray	Yes	In-place edits, building/streaming buffers

buffer = bytearray(b"abc")
buffer[0] = ord("A")     # b'Abc' — edit in place
buffer.append(0x21)      # b'Abc!'
bytes(buffer)            # freeze into an immutable copy

Because bytes is immutable, repeatedly concatenating it in a loop is slow—collect chunks in a bytearray (or a list) and join once.

Common operations

from pathlib import Path

Path("image.png").read_bytes()          # read a binary file
Path("copy.bin").write_bytes(data)      # write bytes to disk

data.hex()                              # '636166c3a9' — bytes to hex string
bytes.fromhex("636166")                 # b'caf' — hex string back to bytes

Slicing, in, and len() all work on bytes just like strings, but every result stays bytes.

Next up in your learning path

File handling

Read/write binary files with confidence.

Companion

Numbers & struct module

Pack/unpack binary data.

Advanced

Networking

Use bytes with sockets and HTTP libraries.

Preview

Frequently Asked Questions

What is the difference between str and bytes in Python?

A str holds Unicode text; bytes holds raw binary data as integers 0–255. You cannot mix them—convert text to bytes with .encode("utf-8") and bytes back to text with .decode("utf-8"). Files and sockets operate on bytes.

text = "café"
data = text.encode("utf-8")   # b'caf\xc3\xa9'
data.decode("utf-8")          # 'café'

When should I use bytearray instead of bytes?

Use bytearray when you need to modify binary data in place—setting individual bytes, appending, or building a buffer incrementally. Use bytes for fixed, read-only data.

buf = bytearray(b"abc")
buf[0] = ord("A")   # b'Abc'
buf.append(0x21)    # b'Abc!'
bytes(buf)          # freeze into immutable bytes

Why do I get UnicodeDecodeError?

It means you decoded bytes with the wrong encoding—for example reading UTF-8 data as ASCII. Decode with the correct encoding, or pass errors="replace" or errors="ignore" to tolerate malformed bytes.

data = "café".encode("utf-8")
data.decode("ascii")                    # UnicodeDecodeError
data.decode("ascii", errors="replace")  # 'caf��'

Why does indexing bytes give me a number?

Indexing a bytes object returns the integer value of that byte (0–255), not a one-byte bytes object. To get a single byte back as bytes, slice instead.

data = b"Py"
data[0]     # 80  — an int
data[0:1]   # b'P' — a bytes object