Python bytes() Function

Python’s built-in bytes(source) function creates an immutable bytes object initialized as defined in the function argument source. A bytes object is like a string but it uses only byte characters consisting of a sequence of 8-bit integers in the range 0<=x<256. The returned byte object is immutable—you cannot change it after creation. If you plan to change the contents, use the bytearray() method to create a mutable bytearray object.

Here’s a minimal example that creates a byte from three integers stored in a list:

>>> bytes([1, 2, 3])
b'\x01\x02\x03'

The prefix \x escape sequence means the next two characters are interpreted as hex character codes. For instance, the hex code \x01 is the same as chr(0x01)=16*0+1=1 that’s simply a start of heading SOH character. (source, ASCII table)

Syntax: bytes([source[, encoding[, errors]]])

Here are some basic usages of the function:

Input : bytes(4)
Output : b'\x00\x00\x00\x00'

Input : bytes([1, 2, 3])
Output : b'\x01\x02\x03'

Input : bytes('hi', 'UTF-8')
Output : b'hi'

Want to learn more? We’re going to dive into more examples next!

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Create Bytes Object From Single Integer Argument — Examples

The following code shows you how to use the bytes() function on simple integer arguments.

# Single Integer Input Argument
print(bytes())
print(bytes(2))
print(bytes(4))

'''
b''
b'\x00\x00'
b'\x00\x00\x00\x00'
'''

If you provide only one input argument, it uses this input argument to determine how many bytes should be created. It just uses bytes with value 0, in byte notation x00 to fill the byte.

Create Bytes Object From Iterable of Integers — Examples

You can also provide an iterable argument to obtain a new byte object:

# Iterable Input Argument
print(bytes([1, 1, 1]))
print(bytes([14]))
print(bytes({9, 8, 7}))

'''
b'\x01\x01\x01'
b'\x0e'
b'\x08\t\x07'
'''

The iterable must consist of a number of integers between 0 and 256. If you fail to do so, Python will throw a ValueError:

How to Fix “ValueError: byte must be in range(0, 256)”

If you use the bytes() function on an iterable that contains at least one integer greater than the maximum number representable by 8 bits, namely 256, or smaller than 0, Python will throw a ValueError: byte must be in range(0, 256). You can fix it by ensuring that each number in your iterable can actually be represented by 8 bits and falls into the interval 0 to 256.

Here’s an example of the ValueError where you use a number larger or equal than 256:

>>> bytes([999])
Traceback (most recent call last):
  File "<pyshell#15>", line 1, in <module>
    bytes([999])
ValueError: bytes must be in range(0, 256)

Another example when using a number smaller than 0:

>>> bytes([-10])
Traceback (most recent call last):
  File "<pyshell#16>", line 1, in <module>
    bytes([-10])
ValueError: bytes must be in range(0, 256)

Fix it by modifying the numbers to lie within the interval 0 to 256:

>>> bytes([255])
b'\xff'

Summary

Python’s built-in function bytes() allows you to initialize the byte in four different ways (from simple to more complex):

👉 integer –> array has this size and is initialized with 0 bytes:

>>> bytes(4)
b'\x00\x00\x00\x00'

👉 iterable –> integers in the range 0 <= x < 256 are initial byte contents:

>>> bytes([1, 2, 3])
b'\x01\x02\x03'

👉 string and you provide the encoding (and optionally, errors) arguments –> bytes() converts string to bytes using str.encode():

>>> bytes('hi', 'UTF-8')
b'hi'

👉 object implementing the buffer interface –> initializes the byte object via a read-only object buffer.

🌍 Recommended Tutorial: Python Bytes vs ByteArray

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