How to convert a string of bytes into an int?
Question:
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.
UPDATE:
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()
0.36242198944091797
>>> Timer("int('yxccxa6xbb'.encode('hex'), 16)").timeit()
1.1432669162750244
My hacky method:
>>> Timer("sum(ord(c) << (i * 8) for i, c in enumerate('yxccxa6xbb'[::1]))").timeit()
2.8819329738616943
FURTHER UPDATE:
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 structnstruct.unpack(">L", "yxccxa6xbb")[0]""").timeit()
0.98822188377380371
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)nstruct.unpack(">L", "yxccxa6xbb")[0]""", 'import struct').timeit()
68.474128007888794
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.
Answers:
You can also use the struct module to do this:
>>> struct.unpack("<L", "yxccxa6xbb")[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 = 'yxccxa6xbb'
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.
import array
integerValue = array.array("I", 'yxccxa6xbb')[0]
Warning: the above is strongly platformspecific. 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 3.2 and later, use
>>> int.from_bytes(b'yxccxa6xbb', byteorder='big')
2043455163
or
>>> int.from_bytes(b'yxccxa6xbb', byteorder='little')
3148270713
according to the endianness of your bytestring.
This also works for bytestringintegers of arbitrary length, and for two’scomplement signed integers by specifying signed=True
. See the docs for from_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/convertvaluestofromintegerhex.html
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
= 'xffx10x11'
data_ints = struct.unpack('<' + 'B'*len(x), x) # [255, 16, 17]
And:
data_bytes = struct.pack('<' + 'B'*len(data_ints), *data_ints) # 'xffx10x11'
That *
is required!
See https://docs.python.org/2/library/struct.html#formatcharacters for a list of the format specifiers.
I was struggling to find a solution for arbitrary length byte sequences that would work under Python 2.x. Finally I wrote this one, it’s a bit hacky because it performs a string conversion, but it works.
Function for Python 2.x, arbitrary length
def signedbytes(data):
"""Convert a bytearray into an integer, considering the first bit as
sign. The data must be bigendian."""
negative = data[0] & 0x80 > 0
if negative:
inverted = bytearray(~d % 256 for d in data)
return signedbytes(inverted)  1
encoded = str(data).encode('hex')
return int(encoded, 16)
This function has two requirements:

The input
data
needs to be abytearray
. You may call the function like this:s = 'yxccxa6xbb' n = signedbytes(s)

The data needs to be bigendian. In case you have a littleendian value, you should reverse it first:
n = signedbytes(s[::1])
Of course, this should be used only if arbitrary length is needed. Otherwise, stick with more standard ways (e.g. struct
).
int.from_bytes is the best solution if you are at version >=3.2.
The “struct.unpack” solution requires a string so it will not apply to arrays of bytes.
Here is another solution:
def bytes2int( tb, order='big'):
if order == 'big': seq=[0,1,2,3]
elif order == 'little': seq=[3,2,1,0]
i = 0
for j in seq: i = (i<<8)+tb[j]
return i
hex( bytes2int( [0x87, 0x65, 0x43, 0x21])) returns ‘0x87654321’.
It handles big and little endianness and is easily modifiable for 8 bytes
>>> reduce(lambda s, x: s*256 + x, bytearray("yxccxa6xbb"))
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.
As mentioned above using unpack
function of struct is a good way. If you want to implement your own function there is an another solution:
def bytes_to_int(bytes):
result = 0
for b in bytes:
result = result * 256 + int(b)
return result
A decently speedy method utilizing array.array I’ve been using for some time:
predefined variables:
offset = 0
size = 4
big = True # endian
arr = array('B')
arr.fromstring("x00x00xffx00") # 5 bytes (encoding issues) [0, 0, 195, 191, 0]
to int: (read)
val = 0
for v in arr[offset:offset+size][::pow(1,not big)]: val = (val<<8)v
from int: (write)
val = 16384
arr[offset:offset+size] =
array('B',((val>>(i<<3))&255 for i in range(size)))[::pow(1,not big)]
It’s possible these could be faster though.
EDIT:
For some numbers, here’s a performance test (Anaconda 2.3.0) showing stable averages on read in comparison to reduce()
:
========================= byte array to int.py =========================
5000 iterations; threshold of min + 5000ns:
______________________________________code__________min_____________max_____________avg_______efficiency
⣿⠀⠀⠀⠀⡇⢀⡀⠀⠀⠀⠀⠀⠀⡇⠀⠀⠀⡀⠀⢰⠀⠀⠀⢰⠀⠀⠀⢸⠀⠀⢀⡇⠀⢀⠀⠀⠀⠀⢠⠀⠀⠀⠀⢰⠀⠀⠀⢸⡀⠀⠀⠀⢸⠀⡇⠀⠀⢠⠀⢰⠀⢸⠀
⣿⣦⣴⣰⣦⣿⣾⣧⣤⣷⣦⣤⣶⣾⣿⣦⣼⣶⣷⣶⣸⣴⣤⣀⣾⣾⣄⣤⣾⡆⣾⣿⣿⣶⣾⣾⣶⣿⣤⣾⣤⣤⣴⣼⣾⣼⣴⣤⣼⣷⣆⣴⣴⣿⣾⣷⣧⣶⣼⣴⣿⣶⣿⣶
val = 0 nfor v in arr: val = (val<<8)v  5373.848ns  850009.965ns  ~8649.64ns  62.128%
⡇⠀⠀⢀⠀⠀⠀⡇⠀⡇⠀⠀⣠⠀⣿⠀⠀⠀⠀⡀⠀⠀⡆⠀⡆⢰⠀⠀⡆⠀⡄⠀⠀⠀⢠⢀⣼⠀⠀⡇⣠⣸⣤⡇⠀⡆⢸⠀⠀⠀⠀⢠⠀⢠⣿⠀⠀⢠⠀⠀⢸⢠⠀⡀
⣧⣶⣶⣾⣶⣷⣴⣿⣾⡇⣤⣶⣿⣸⣿⣶⣶⣶⣶⣧⣷⣼⣷⣷⣷⣿⣦⣴⣧⣄⣷⣠⣷⣶⣾⣸⣿⣶⣶⣷⣿⣿⣿⣷⣧⣷⣼⣦⣶⣾⣿⣾⣼⣿⣿⣶⣶⣼⣦⣼⣾⣿⣶⣷
val = reduce( shift, arr )  6489.921ns  5094212.014ns  ~12040.269ns  53.902%
This is a raw performance test, so the endian powflip is left out.
The shift
function shown applies the same shiftoring operation as the for loop, and arr
is just array.array('B',[0,0,255,0])
as it has the fastest iterative performance next to dict
.
I should probably also note efficiency is measured by accuracy to the average time.
In python 3 you can easily convert a byte string into a list of integers (0..255) by
>>> list(b'yxccxa6xbb')
[121, 204, 166, 187]
For newer versions of Python a simple way is:
int(b'hello world'.hex(), 16)