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   y xcc xa6 xbb   I came up with a clever stupid way of doing it   sum ord c   lt  lt   i   8  for i  c in enumerate  y xcc xa6 xbb    -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    gt  gt  gt  from timeit import Timer  gt  gt  gt  Timer  struct unpack   lt L    y xcc xa6 xbb   0     import struct   timeit   0 36242198944091797  gt  gt  gt  Timer  int  y xcc xa6 xbb  encode  hex    16    timeit   1 1432669162750244   My hacky method    gt  gt  gt  Timer  sum ord c   lt  lt   i   8  for i  c in enumerate  y xcc xa6 xbb    -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    gt  gt  gt  Timer    import struct nstruct unpack   gt L    y xcc xa6 xbb   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    gt  gt  gt  Timer    reload struct  nstruct unpack   gt L    y xcc xa6 xbb   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

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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 lt  lt 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

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In Python 2 x  you could use the format specifiers  lt B for unsigned bytes  and  lt b for signed bytes with struct unpack struct pack   E g   Let x     xff x10 x11   data ints   struct unpack   lt      B  len x   x     255  16  17    And   data bytes   struct pack   lt      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

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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 big-endian         negative   data 0   amp  0x80  gt  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 a bytearray  You may call the function like this   s    y xcc xa6 xbb  n   signedbytes s   The data needs to be big-endian  In case you have a little-endian 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

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You can also use the struct module to do this    gt  gt  gt  struct unpack   lt L    y xcc xa6 xbb   0  3148270713L

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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   gt  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   gt  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

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In python 3 you can easily convert a byte string into a list of integers  0  255  by   gt  gt  gt  list b y xcc xa6 xbb    121  204  166  187

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In Python 3 2 and later  use   gt  gt  gt  int from bytes b y xcc xa6 xbb   byteorder  big   2043455163   or   gt  gt  gt  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

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In Python 3 2 and later  use   gt  gt  gt  int from bytes b y xcc xa6 xbb   byteorder  big   2043455163   or   gt  gt  gt  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

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You can also use the struct module to do this    gt  gt  gt  struct unpack   lt L    y xcc xa6 xbb   0  3148270713L

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gt  gt  gt  reduce lambda s  x  s 256   x  bytearray  y xcc xa6 xbb    2043455163   Test 1  inverse    gt  gt  gt  hex 2043455163   0x79cca6bb    Test 2  Number of bytes   8    gt  gt  gt  reduce lambda s  x  s 256   x  bytearray  AAAAAAAAAAAAAAA    338822822454978555838225329091068225L   Test 3  Increment by one    gt  gt  gt  reduce lambda s  x  s 256   x  bytearray  AAAAAAAAAAAAAAB    338822822454978555838225329091068226L   Test 4  Append one byte  say  A     gt  gt  gt  reduce lambda s  x  s 256   x  bytearray  AAAAAAAAAAAAAABA    86738642548474510294585684247313465921L   Test 5  Divide by 256    gt  gt  gt  reduce lambda s  x  s 256   x  bytearray  AAAAAAAAAAAAAABA    256 338822822454978555838225329091068226L   Result equals the result of Test 4  as expected

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In python 3 you can easily convert a byte string into a list of integers  0  255  by   gt  gt  gt  list b y xcc xa6 xbb    121  204  166  187

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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

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gt  gt  gt  reduce lambda s  x  s 256   x  bytearray  y xcc xa6 xbb    2043455163   Test 1  inverse    gt  gt  gt  hex 2043455163   0x79cca6bb    Test 2  Number of bytes   8    gt  gt  gt  reduce lambda s  x  s 256   x  bytearray  AAAAAAAAAAAAAAA    338822822454978555838225329091068225L   Test 3  Increment by one    gt  gt  gt  reduce lambda s  x  s 256   x  bytearray  AAAAAAAAAAAAAAB    338822822454978555838225329091068226L   Test 4  Append one byte  say  A     gt  gt  gt  reduce lambda s  x  s 256   x  bytearray  AAAAAAAAAAAAAABA    86738642548474510294585684247313465921L   Test 5  Divide by 256    gt  gt  gt  reduce lambda s  x  s 256   x  bytearray  AAAAAAAAAAAAAABA    256 338822822454978555838225329091068226L   Result equals the result of Test 4  as expected

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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

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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

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You can also use the struct module to do this    gt  gt  gt  struct unpack   lt L    y xcc xa6 xbb   0  3148270713L

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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

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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

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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   gt  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   gt  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

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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   x00 x00 xff x00     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 lt  lt 8  v   from int   write   val   16384 arr offset offset size          array  B    val gt  gt  i lt  lt 3   amp 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 lt  lt 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 pow-flip is left out  The shift function shown applies the same shift-oring 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

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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

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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   x00 x00 xff x00     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 lt  lt 8  v   from int   write   val   16384 arr offset offset size          array  B    val gt  gt  i lt  lt 3   amp 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 lt  lt 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 pow-flip is left out  The shift function shown applies the same shift-oring 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

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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

User · Answer

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 lt  lt 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

User · Answer

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

User · Answer

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

User · Answer

In Python 2 x  you could use the format specifiers  lt B for unsigned bytes  and  lt b for signed bytes with struct unpack struct pack   E g   Let x     xff x10 x11   data ints   struct unpack   lt      B  len x   x     255  16  17    And   data bytes   struct pack   lt      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

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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 big-endian         negative   data 0   amp  0x80  gt  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 a bytearray  You may call the function like this   s    y xcc xa6 xbb  n   signedbytes s   The data needs to be big-endian  In case you have a little-endian 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

User · Answer

You can also use the struct module to do this    gt  gt  gt  struct unpack   lt L    y xcc xa6 xbb   0  3148270713L

User · Answer

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

[python] How to convert a string of bytes into an int?

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