[Tutor] newb - Problem reading binary files

[Elizabeth Finn]

This is probably a newbie question, and I apologize for the length – but I have consulted several books / sites and haven’t found a good answer. I need to read a file that is in binary format, and then convert some of the values into integer values. These values can range from 1 to 4 bytes.
   
  First question – is there an easy way to do this? I finally wrote my own little utility to handle multi-byte integers because I couldn’t find a built-in way (except for ord() which works only for single bytes)
   
  def getnum(num_str):
              """
              Given a string representing a binary number, return the number.
              If string is more than one byte, calculate the number to return.
              Assume that each byte is signed magnitude
              """                    
              x = len(num_str)
              ans = 0
              for i in range( x ):
                          nextstr = num_str[i:i+1]
                          ans = ans * 256
                          ans = ans + ord(nextstr)
              return ans
   
  This “brute force” method usually works, but - now here is the other question -sometimes the code does not pick up two full bytes when it is supposed to. I open the file and read a block that I want into a string:
              
                          f=open(fname, 'rb')
  f.seek(offset, 0)
  block = f.read(2000)
   
  Then for each number I pull the bytes from the string, then call getnum() to calculate the number.
   
              test = block[0:1]             # 1 byte 
              test = block[1:4]             # 3 bytes
              test = block[4:6]             # 2 bytes
              test = block[20:12]             # 2 bytes
              test = block[1996:2000]       #4 bytes
   
  This method usually works, except that for some of the 2-byte numbers I get only the first byte and first half of the second byte – for instance: 'x06\x33’ comes out as ‘x063’. This is very confusing especially because one 2-byte substring – “00 01” comes out as expected, but “06 52” becomes “065”. Any ideas?

 
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