How to convert a string of bytes into an int?

Question

In Python 3.2 and later, use

>>> int.from_bytes(b'y\xcc\xa6\xbb', byteorder='big')
2043455163

or

>>> int.from_bytes(b'y\xcc\xa6\xbb', byteorder='little')
3148270713

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.

You can also use the struct module to do this:

>>> struct.unpack("<L", "y\xcc\xa6\xbb")[0]
3148270713L

As Greg said, you can use struct if you are dealing with binary values, but if you just have a "hex number" but in byte format you might want to just convert it like:

s = 'y\xcc\xa6\xbb'
num = int(s.encode('hex'), 16)

...this is the same as:

num = struct.unpack(">L", s)[0]

...except it'll work for any number of bytes.

I use the following function to convert data between int, hex and bytes.

def bytes2int(str):
 return int(str.encode('hex'), 16)

def bytes2hex(str):
 return '0x'+str.encode('hex')

def int2bytes(i):
 h = int2hex(i)
 return hex2bytes(h)

def int2hex(i):
 return hex(i)

def hex2int(h):
 if len(h) > 1 and h[0:2] == '0x':
  h = h[2:]

 if len(h) % 2:
  h = "0" + h

 return int(h, 16)

def hex2bytes(h):
 if len(h) > 1 and h[0:2] == '0x':
  h = h[2:]

 if len(h) % 2:
  h = "0" + h

 return h.decode('hex')

Source: http://opentechnotes.blogspot.com.au/2014/04/convert-values-to-from-integer-hex.html

import array
integerValue = array.array("I", 'y\xcc\xa6\xbb')[0]

Warning: the above is strongly platform-specific. Both the "I" specifier and the endianness of the string->int conversion are dependent on your particular Python implementation. But if you want to convert many integers/strings at once, then the array module does it quickly.

In Python 2.x, you could use the format specifiers <B for unsigned bytes, and <b for signed bytes with struct.unpack/struct.pack.

E.g:

Let x = '\xff\x10\x11'

data_ints = struct.unpack('<' + 'B'*len(x), x) # [255, 16, 17]

And:

data_bytes = struct.pack('<' + 'B'*len(data_ints), *data_ints) # '\xff\x10\x11'

That * is required!

See https://docs.python.org/2/library/struct.html#format-characters for a list of the format specifiers.

>>> reduce(lambda s, x: s*256 + x, bytearray("y\xcc\xa6\xbb"))
2043455163

Test 1: inverse:

>>> hex(2043455163)
'0x79cca6bb'

Test 2: Number of bytes > 8:

>>> reduce(lambda s, x: s*256 + x, bytearray("AAAAAAAAAAAAAAA"))
338822822454978555838225329091068225L

Test 3: Increment by one:

>>> reduce(lambda s, x: s*256 + x, bytearray("AAAAAAAAAAAAAAB"))
338822822454978555838225329091068226L

Test 4: Append one byte, say 'A':

>>> reduce(lambda s, x: s*256 + x, bytearray("AAAAAAAAAAAAAABA"))
86738642548474510294585684247313465921L

Test 5: Divide by 256:

>>> reduce(lambda s, x: s*256 + x, bytearray("AAAAAAAAAAAAAABA"))/256
338822822454978555838225329091068226L

Result equals the result of Test 4, as expected.