bytearray()

bytearray([source[, encoding[, errors]]])

The bytearray type is a mutable sequence of bytes. Unlike immutable bytes objects, you can modify bytearray objects in place — useful for building or processing binary data incrementally.

Syntax

bytearray()
bytearray(source)
bytearray(source, encoding)
bytearray(source, encoding, errors)

Parameters

Parameter	Type	Default	Description
source	iterable, int, or str	—	Initial bytes or string to convert
encoding	str	—	Encoding (required if source is a string)
errors	str	—	Error handling scheme (see below)

Encoding Error Handling

Value	Behavior
'strict'	Raise UnicodeDecodeError on failure (default)
'ignore'	Ignore unencodable characters
'replace'	Replace with ?
'backslashreplace'	Use Python escape sequences

Examples

Creating an empty bytearray

data = bytearray()
print(data)
# bytearray(b'')

Creating with initial size

# Useful for buffering
buffer = bytearray(1024)  # 1KB zero-initialized
print(len(buffer))
# 1024

From a string with encoding

text = "Hello"
data = bytearray(text, 'utf-8')
print(data)
# bytearray(b'Hello')

From bytes or bytearray

original = b"binary data"
copy = bytearray(original)
print(copy)
# bytearray(b'binary data')

# Modify the copy
copy[0] = 66  # Change 'b' to 'B'
print(copy)
# bytearray(b'Binary data')

From a list of integers

# Each integer must be 0-255
data = bytearray([72, 101, 108, 108, 111])
print(data)
# bytearray(b'Hello')

Modifying in place

data = bytearray(b"hello")

# Change a single byte
data[0] = 72  # 'H'
print(data)
# bytearray(b'Hello')

# Append bytes
data.extend(b' world')
print(data)
# bytearray(b'Hello world')

# Append integer (single byte)
data.append(33)  # '!'
print(data)
# bytearray(b'Hello world!')

Common Patterns

Building HTTP response

response = bytearray(b"HTTP/1.1 200 OK\r\n")

# Add headers
response.extend(b"Content-Type: text/html\r\n")
response.extend(b"Content-Length: 13\r\n")
response.extend(b"\r\n")

# Add body
response.extend(b"Hello, World!")

print(bytes(response))
# b'HTTP/1.1 200 OK\r\nContent-Type: text/html\r\n...'

Reading fixed-size binary records

import struct

# Simulate binary data
record_size = 12
raw_data = bytearray(b"John\x00\x00\x00\x00\x00\x00\x00\x00\x0025")

# Parse: 8-char name + 4-byte age
name = raw_data[:8].rstrip(b'\x00').decode('utf-8')
age = struct.unpack('>I', raw_data[8:12])[0]

print(f"{name}, {age} years old")
# John, 25 years old

Network packet construction

# Build a simple packet: header (4 bytes) + payload
packet = bytearray()

# Header: version (1) + type (1) + length (2)
packet.append(1)        # version
packet.append(5)        # type
packet.extend(struct.pack('>H', 10))  # payload length

# Payload
packet.extend(b"Hello12345")

print(bytes(packet))
# b'\x01\x05\x00\nHello12345'

Mutability Notes

Unlike bytes, bytearray supports item assignment and in-place operations:

data = bytearray(b"test")

# This works with bytearray
data[0] = 84  # Change 't' to 'T'
print(data)
# bytearray(b'Test')

# This would fail with bytes:
# TypeError: 'bytes' object does not support item assignment

See Also

• bytes()
• memoryview()
• open()