How to Convert Int to Bytes in Python 2 and Python 3

Conversion from int to bytes is the reverse operation of conversion from bytes to int that is introduced in the last HowTo tutorial. Most of the int-to-bytes methods introduced in this article are the reverse methods of bytes-to-int methods.

Python 2.7 and 3 Compatible int to bytes Conversion Method

You could use pack function in the Python struct module to convert the integer to bytes in the specific format.

>>> import struct
>>> struct.pack("B", 2)
>>> struct.pack(">H", 2)
>>> struct.pack("<H", 2)

The first argument in the struct.pack function is the format string that specifies the bytes format like byte length, singularity, byte order (little or big endian), etc.

Python 3 Only int to bytes Conversion Methods

Use bytes to Convert int to bytes

As indicated in the last article, bytes is a built-in data type from Python 3. You could easily use bytes to convert the integer 0~255 to bytes data type.

>>> bytes([2])
The integer must be surrounded by the parenthesis, otherwise, you will get the bytes object of size given by the parameter initialized with null bytes but not the corresponding bytes
>>> bytes(2)

Use int.to_bytes() Method to Convert int to bytes

From Python3.1, a new integer class method int.to_bytes() is introduced. It is the reverse conversion method of int.from_bytes() as discussed in the last article.

>>> (258).to_bytes(2, byteorder="little")
>>> (258).to_bytes(2, byteorder="big")
>>> (258).to_bytes(4, byteorder="little", signed=True)
>>> (-258).to_bytes(4, byteorder="little", signed=True)

The first argument is the converted bytes data length, the second argument byteorder defines the byte order to be little or big-endian, and the optional argument signed determines whether two’s complement is used to represent the integer.

Performance Comparison

In Python 3, you have 3 ways to convert int to bytes,

  • bytes() method
  • struct.pack() method
  • int.to_bytes() method

We will check the execution time of each method to compare their performance, and finally give you the recommendation if you want to increase your code execution speed.

>>> import timeint
>>> timeit.timeit('bytes([255])', number=1000000)
>>> timeit.timeit('struct.pack("B", 255)', setup='import struct', number=1000000)
>>> timeit.timeit('(255).to_bytes(1, byteorder="little")', number=1000000)
Conversion Method - -Execution Time (1 million times)
bytes() 0.31296982169325455 s
struct.pack() 0.2640925447800839
int.to_bytes() 0.5622947660224895

Therefore, please use struct.pack() function to perform the int-to-bytes conversion to get the best execution performance, although it is already introduced in Python 2 branch.