How to convert a string of bytes into an int?


How can I convert a string of bytes into an int in python?

Say like this: "yxccxa6xbb"

I came up with a clever/stupid way of doing it:

sum(ord(c) << (i * 8) for i, c in enumerate("yxccxa6xbb"[::-1]))

I know there has to be something builtin or in the standard library that does this more simply...

This is different from converting a string of hex digits for which you can use int(xxx, 16), but instead I want to convert a string of actual byte values.


I kind of like James" answer a little better because it doesn"t require importing another module, but Greg"s method is faster:

>>> from timeit import Timer
>>> Timer("struct.unpack("<L", "yxccxa6xbb")[0]", "import struct").timeit()
>>> Timer("int("yxccxa6xbb".encode("hex"), 16)").timeit()

My hacky method:

>>> Timer("sum(ord(c) << (i * 8) for i, c in enumerate("yxccxa6xbb"[::-1]))").timeit()


Someone asked in comments what"s the problem with importing another module. Well, importing a module isn"t necessarily cheap, take a look:

>>> Timer("""import struct
struct.unpack(">L", "yxccxa6xbb")[0]""").timeit()

Including the cost of importing the module negates almost all of the advantage that this method has. I believe that this will only include the expense of importing it once for the entire benchmark run; look what happens when I force it to reload every time:

>>> Timer("""reload(struct)
struct.unpack(">L", "yxccxa6xbb")[0]""", "import struct").timeit()

Needless to say, if you"re doing a lot of executions of this method per one import than this becomes proportionally less of an issue. It"s also probably i/o cost rather than cpu so it may depend on the capacity and load characteristics of the particular machine.

Answer rating: 336

In Python 3.2 and later, use

>>> int.from_bytes(b"yxccxa6xbb", byteorder="big")


>>> int.from_bytes(b"yxccxa6xbb", byteorder="little")

according to the endianness of your byte-string.

This also works for bytestring-integers of arbitrary length, and for two"s-complement signed integers by specifying signed=True. See the docs for from_bytes.

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