Reading integers from binary file in Python

Question

I m trying to read a BMP file in Python  I know the first two bytes  indicate the BMP firm  The next 4 bytes are the file size  When I execute   fin   open  hi bmp    rb   firm   fin read 2    file size   int fin read 4       I get      ValueError  invalid literal for int   with base 10   F  x13    What I want to do is reading those four bytes as an integer  but it seems Python is reading them as characters and returning a string  which cannot be converted to an integer  How can I do this correctly

User · Answer

As of Python 3 2   you can also accomplish this using the from bytes native int method   file size   int from bytes fin read 2   byteorder  big     Note that this function requires you to specify whether the number is encoded in big- or little-endian format  so you will have to determine the endian-ness to make sure it works correctly

User · Answer

Except struct you can also use array module  import array values   array array  l     array of long integers values read fin  1    read 1 integer file size    values 0

User · Answer

As you are reading  the binary file  you need to unpack it into a integer  so use struct module for that  import struct fin   open  hi bmp    rb   firm   fin read 2    file size    struct unpack  i  fin read 4

User · Answer

An alternative method which does not make use of  struct unpack    would be to use NumPy   import numpy as np  f   open  file bin    r   a   np fromfile f  dtype np uint32     dtype  represents the datatype and can be int   uint   float   complex  or a user defined type  See numpy fromfile   Personally prefer using NumPy to work with array matrix data as it is a lot faster than using Python lists

User · Answer

The read method returns a sequence of bytes as a string  To convert from a string byte-sequence to binary data  use the built-in struct module  http   docs python org library struct html    import struct  print struct unpack  i   fin read 4      Note that unpack always returns a tuple  so struct unpack  i   fin read 4   0  gives the integer value that you are after   You should probably use the format string   lt i    lt  is a modifier that indicates little-endian byte-order and standard size and alignment - the default is to use the platform s byte ordering  size and alignment   According to the BMP format spec  the bytes should be written in Intel little-endian byte order

User · Answer

When you read from a binary file  a data type called bytes is used  This is a bit like list or tuple  except it can only store integers from 0 to 255   Try   file size   fin read 4  file size0   file size 0  file size1   file size 1  file size2   file size 2  file size3   file size 3    Or   file size   list fin read 4     Instead of   file size   int fin read 4

[python] Reading integers from binary file in Python

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