Convert byte array to int python

Question

Python doesn't traditionally have much use for "numbers in big-endian C layout" that are too big for C. (If you're dealing with 2-byte, 4-byte, or 8-byte numbers, then struct.unpack is the answer.)

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int.from_bytes() method
How do you convert bytes to integers in Python?
How many bytes is an int?
How are bytes represented in Python?
How do you convert a string to an int in Python?

But enough people got sick of there not being one obvious way to do this that Python 3.2 added a method int.from_bytes that does exactly what you want:

int.from_bytes(b, byteorder='big', signed=False)

Unfortunately, if you're using an older version of Python, you don't have this. So, what options do you have? (Besides the obvious one: update to 3.2, or, better, 3.4…)

First, there's your code. I think binascii.hexlify is a better way to spell it than .encode('hex'), because "encode" has always seemed a little weird for a method on byte strings (as opposed to Unicode strings), and it's in fact been banished in Python 3. But otherwise, it seems pretty readable and obvious to me. And it should be pretty fast—yes, it has to create an intermediate string, but it's doing all the looping and arithmetic in C (at least in CPython), which is generally an order of magnitude or two faster than in Python. Unless your bytearray is so big that allocating the string will itself be costly, I wouldn't worry about performance here.

Alternatively, you could do it in a loop. But that's going to be more verbose and, at least in CPython, a lot slower.

You could try to eliminate the explicit loop for an implicit one, but the obvious function to do that is reduce, which is considered un-Pythonic by part of the community—and of course it's going to require calling a function for each byte.

You could unroll the loop or reduce by breaking it into chunks of 8 bytes and looping over struct.unpack_from, or by just doing a big struct.unpack('Q'*len(b)//8 + 'B' * len(b)%8) and looping over that, but that makes it a lot less readable and probably not that much faster.

You could use NumPy… but if you're going bigger than either 64 or maybe 128 bits, it's going to end up converting everything to Python objects anyway.

So, I think your answer is the best option.

Here are some timings comparing it to the most obvious manual conversion:

import binascii
import functools
import numpy as np

def hexint(b):
    return int(binascii.hexlify(b), 16)

def loop1(b):
    def f(x, y): return (x<<8)|y
    return functools.reduce(f, b, 0)

def loop2(b):
    x = 0
    for c in b:
        x <<= 8
        x |= c
    return x

def numpily(b):
    n = np.array(list(b))
    p = 1 << np.arange(len(b)-1, -1, -1, dtype=object)
    return np.sum(n * p)

In [226]: b = bytearray(range(256))

In [227]: %timeit hexint(b)
1000000 loops, best of 3: 1.8 µs per loop

In [228]: %timeit loop1(b)
10000 loops, best of 3: 57.7 µs per loop

In [229]: %timeit loop2(b)
10000 loops, best of 3: 46.4 µs per loop

In [283]: %timeit numpily(b)
10000 loops, best of 3: 88.5 µs per loop

For comparison in Python 3.4:

In [17]: %timeit hexint(b)
1000000 loops, best of 3: 1.69 µs per loop

In [17]: %timeit int.from_bytes(b, byteorder='big', signed=False)
1000000 loops, best of 3: 1.42 µs per loop

So, your method is still pretty fast…

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A bytes object can be converted to an integer value easily using Python. Python provides us various in-built methds like from_bytes() as well as classes to carry out this interconversion.

int.from_bytes() method

A byte value can be interchanged to an int value by using the int.from_bytes() method. This method requires at least Python 3.2 and has the following syntax :

Syntax: int.from_bytes(bytes, byteorder, *, signed=False)
Parameters:
bytes – A byte object
byteorder – Determines the order of representation of the integer value. byteorder can have values as either “little” where most significant bit is stored at the end and least at the beginning, or big, where MSB is stored at start and LSB at the end. Big byte order calculates the value of an integer in base 256.
signed – Default value – False . Indicates whether to represent 2’s complement of a number.
Returns – an int equivalent to the given byte

The following snippets indicate the conversion of byte to int object.

Example 1:

Python3

byte_val = b'\x00\x01'

int_val = int.from_bytes(byte_val, "big")

print(int_val)

Output:

Example 2:

Python3

byte_val = b'\x00\x10'

int_val = int.from_bytes(byte_val, "little")

print(int_val)

Output:

Example 3:

Python3

byte_val = b'\xfc\x00'

int_val = int.from_bytes(byte_val, "big", signed="True")

print(int_val)

Output:

-1024

How do you convert bytes to integers in Python?

Syntax: int.from_bytes(bytes, byteorder, *, signed=False).

Parameters:.

Returns – an int equivalent to the given byte..

How many bytes is an int?

The int and unsigned int types have a size of four bytes. However, portable code should not depend on the size of int because the language standard allows this to be implementation-specific.

How are bytes represented in Python?

In Python, a byte string is represented by a b , followed by the byte string's ASCII representation. A byte string can be decoded back into a character string, if you know the encoding that was used to encode it.

How do you convert a string to an int in Python?

To convert, or cast, a string to an integer in Python, you use the int() built-in function. The function takes in as a parameter the initial string you want to convert, and returns the integer equivalent of the value you passed. The general syntax looks something like this: int("str") .

programming python Python bytes Bytearray Python Type bytes Python

Convert byte array to int python

int.from_bytes() method

Python3

Python3

Python3

How do you convert bytes to integers in Python?

How many bytes is an int?

How are bytes represented in Python?

How do you convert a string to an int in Python?

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