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Python Bytes and bytearray Examples

Python Bytes and bytearray Examples

Bytes, bytearray. These objects store binary buffers. With them, we efficiently represent bytes (numbers 0 through 255). Programs often process many bytes.


For binary data, using a list of bytes works, but this is slow. For thousands or millions of bytes, bytearray (which can be changed) and bytes (which cannot be) are critical.


Bytearray example. Let us begin. This example creates a list called "elements." Each number in the list is between 0 and 255 (inclusive). We create a bytearray from the list.

Modify: We modify the first two elements in the bytearray. This cannot be done with a bytes object.

For: We use the for-loop to iterate over the bytearray's elements. This is the same as how we use a list.

Based on:

Python 3

Python program that creates bytearray from list

elements = [0, 200, 50, 25, 10, 255]

# Create bytearray from list of integers.
values = bytearray(elements)

# Modify elements in the bytearray.
values[0] = 5
values[1] = 0

# Display bytes.
for value in values:
    print(value)

Output

5
0
50
25
10
255

Bytes example. We now consider "bytes," bytearray's stubborn sibling. The elements of a bytes object cannot be changed. It is an immutable array of bytes.

Buffer protocol: Bytearray, bytes and memoryview act upon the buffer protocol. They all share similar syntax with small differences.

Python program that creates bytes object

elements = [5, 10, 0, 0, 100]

# Create immutable bytes object.
data = bytes(elements)

# Loop over bytes.
for d in data:
    print(d)

Output

5
10
0
0
100

Error. Now we get into some trouble—that is always fun. Here we try to modify the first element of a bytes object. Python complains—the "object does not support item assignment."

Python program that causes error

data = bytes([10, 20, 30, 40])

# We can read values from a bytes object.
print(data[0])

# We cannot assign elements.
data[0] = 1

Output

10
Traceback (most recent call last):
  File "/Users/sam/Documents/test.py", line 9, in <module>
    data[0] = 1
TypeError: 'bytes' object does not support item assignment

Literals. Bytes and bytearray objects can be created with a special string literal syntax. We prefix the literals with a "b." This prefix is required.

Tip: Buffer protocol methods require byte-prefix string literals, even for arguments to methods like replace().

Python program that uses byte literals

# Create bytes object from byte literal.
data = bytes(b"abc")
for value in data:
    print(value)

print()

# Create bytearray from byte literal.
arr = bytearray(b"abc")
for value in arr:
    print(value)

Output

97
98
99

97
98
99

Slice, bytearray. We can slice bytearrays. And because bytearray is mutable, we can use slices to change its contents. Here we assign a slice to an integer list.

Python program that uses slice, changes bytearray

values = [5, 10, 15, 20]
arr = bytearray(values)

# Assign first two elements to new list.
arr[0:2] = [100, 0, 0]

# The array is now modified.
for v in arr: print(v)

Output

100
0
0
15
20

Slice, bytes. A bytes object too supports slice syntax, but it is read-only. Here we get a slice of bytes (the first two elements) and loop over it.

Often: We can loop over a slice directly in the for-loop condition. The variable is not needed.

Python program that uses slice, bytes

data = bytes(b"abc")

# Get a slice from the bytes object.
first_part = data[0:2]

# Display values from slice.
for element in first_part: print(element)

Output

97
98

Count. Many methods are available on the buffer interface. Count is one. It loops through the bytes and counts instances matching our specified pattern.

Note: Count must loop through all elements. If another loop is needed afterwards, often we can combine loops for speed.

Argument: The argument to count() must be a byte object, like a "b" string literal or a number between 0 and 255.

Python that uses count, buffer interface

# Create a bytes object and a bytearray.
data = bytes(b"aabbcccc")
arr = bytearray(b"aabbcccc")

# The count method (from the buffer interface) works on both.
print(data.count(b"c"))
print(arr.count(b"c"))

Output

4
4

Find. This method returns the leftmost index of a matching sequence. Optionally we can specify a start index and an end index (as the second and third arguments).

Python that uses find

data = bytes(b"python")

# This sequence is found.
index1 = data.find(b"on")
print(index1)

# This sequence is not present.
index2 = data.find(b"java")
print(index2)

Output

4
-1

In operator. This tests for existence. We use "in" to see if an element exists within the bytes objects. This is a clearer way to see if a byte exists in our object.

Python that uses in operator

data = bytes([100, 20, 10, 200, 200])

# Test bytes object with "in" operator.
if 200 in data:
    print(True)

if 0 not in data:
    print(False)

Output

True
False

Combine two bytearrays. As with lists and other sequences, we can combine two bytearrays (or bytes) with a plus. In my tests, I found it does not matter if we combine two different types.

Python that uses plus on bytearrays

left = bytearray(b"hello ")
right = bytearray(b"world")

# Combine two bytearray objects with plus.
both = left + right
print(both)

Output

bytearray(b'hello world')

Convert list. A list of bytes (numbers between 0 and 256) can be converted into a bytearray with the constructor. To convert back into a list, please use the list built-in constructor.

Tip: Lists display in a more friendly way with the print method. So we might use this code to display bytearrays and bytes.

Python that uses list built-in

initial = [100, 255, 255, 0]
print(initial)

# Convert the list to a byte array.
b = bytearray(initial)
print(b)

# Convert back to a list.
result = list(b)
print(result)

Output

[100, 255, 255, 0]
bytearray(b'd\xff\xff\x00')
[100, 255, 255, 0]

Convert string. A bytearray can be created from a string. The encoding (like "ascii") is specified as the second argument in the bytearray constructor.

Decode: To convert from a bytearray back into a string, the decode method is needed.

Python that converts string, bytearray

# Create a bytearray from a string with ASCII encoding.
arr = bytearray("abc", "ascii")
print(arr)

# Convert bytearray back into a string.
result = arr.decode("ascii")
print(result)

Output

bytearray(b'abc')
abc

Append, del, insert. A bytearray supports many of the same operations as a list. We can append values. We can delete a value or a range of values with del. And we can insert a value.

Python that uses append, del, insert

# Create bytearray and append integers as bytes.
values = bytearray()
values.append(0)
values.append(1)
values.append(2)
print(values)

# Delete the first element.
del values[0:1]
print(values)

# Insert at index 1 the value 3.
values.insert(1, 3)
print(values)

Output

bytearray(b'\x00\x01\x02')
bytearray(b'\x01\x02')
bytearray(b'\x01\x03\x02')

ValueError. Numbers inserted into a bytearray or bytes object must be between 0 and 255 inclusive. If we try to insert an out-of-range number, we will receive a ValueError.

Python that causes ValueError

# This does not work.
values = bytes([3000, 4000, 5000])
print("Not reached")

Output

Traceback (most recent call last):
  File "/Users/sam/Documents/test.py", line 4, in <module>
    values = bytes([3000, 4000, 5000])
ValueError: byte must be in range(0, 256)

Replace. The buffer protocol supports string-like methods. We can use replace() as on a string. The arguments must be bytes objects—here we use "b" literals.

Python that uses replace on bytes

value = b"aaabbb"

# Use bytes replace method.
result = value.replace(b"bbb", b"ccc")
print(result)

Output

b'aaaccc'

Compare. A "b" literal is a bytes object. We can compare a bytearray or a bytes object with this kind of constant. To compare bytes objects, we use two equals signs.

Note: Two equals signs compares the individual byte contents, not the identity of the objects.

Python that compares bytes

# Create a bytes object with no "bytes" keyword.
value1 = b"desktop"
print(value1)

# Use bytes keyword.
value2 = bytes(b"desktop")
print(value2)

# Compare two bytes objects.
if value1 == value2:
    print(True)

Output

b'desktop'
b'desktop'
True

Start, end. We can handle bytes objects much like strings. Common methods like startswith and endswith are included. These check the beginning and end parts.

Argument: The argument to startswith and endswith must be a bytes object. We can use the handy "b" prefix.

Python that uses startswith, endswith

value = b"users"

# Compare bytes with startswith and endswith.
if value.startswith(b"use"):
    print(True)

if value.endswith(b"s"):
    print(True)

Output

True
True

Split, join. The split and join methods are implemented on bytes objects. Here we handle a simple CSV string in bytes. We separate values based on a comma char.

Python that uses split, join

# A bytes object with comma-separate values.
data = b"cat,dog,fish,bird,true"

# Split on comma-byte.
elements = data.split(b",")

# Print length and list contents.
print(len(elements))
print(elements)

# Combine bytes objects into a single bytes object.
result = b",".join(elements)
print(result)

Output

5
[b'cat', b'dog', b'fish', b'bird', b'true']
b'cat,dog,fish,bird,true'

Memoryview. This is an abstraction that provides buffer interface methods. We can create a memoryview from a bytes object, a bytearray or another type like an array.

Array

Tip: With memoryview we can separate our code that uses the buffer interface from the actual data type. It is an abstraction.

Python that uses memoryview

view = memoryview(b"abc")

# Print the first element.
print(view[0])

# Print the element count.
print(len(view))

# Convert to a list.
print(view.tolist())

Output

b'a'
3
[97, 98, 99]

Performance. Suppose we want to append 256 values to a list. Bytearray here is faster. So we both improve memory size and reduce time required with bytearray over list.

So: Bytearray is more complex to handle. It does not support non-ASCII characters or large numeric values.

But: In many programs where these are not required, bytearray can be used to improve speed. This benchmark supports this idea.

Python that times list, bytearray appends

import time

print(time.time())

# Version 1: append to list.
for i in range(0, 1000000):
    x = list()
    for v in range(0, 255):
        x.append(v)

print(time.time())

# Version 2: append to bytearray.
for i in range(0, 1000000):
    x = bytearray()
    for v in range(0, 255):
        x.append(v)

print(time.time())

Results

1411859925.29213
1411859927.673053    list append:      2.38 s
1411859929.463818    bytearray append: 1.79 s  [faster]

Bytes and bytearrays are an efficient, byte-based form of string. They have many of the same methods as strings, but can also be used as lists. They improve performance.


In Python, lists can become inefficient quickly. And strings, immutable, lead to excessive copying. Where we represent data in bytes, numbers from 0 to 255, these buffer types are ideal.