[Tutor] curious struct problem

[Marcus Goldfish]

I'm trying to read a binary file using struct.  This code works
interactively in the IPython shell, but not as a function invoked from a
command line (Windows).  Can someone point out my error?  Thanks!

import struct

def demux(filename, channel, nchannels):
   "Demultiplexes a stream of shorts (signed int16) from a file."
   fmt = str(nchannels) + 'h'
   sz = struct.calcsize(fmt)

   infile = open(fname, 'rb')
   chunk = infile.read(sz)
   while chunk:
      x = struct.unpack(fmt, chunk)
      chunk = infile.read(sz)
   infile.close()

#struct.error: unpack str size does no match format
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