[Tutor] curious struct problem

Marcus Goldfish magoldfish at gmail.com
Mon Feb 5 15:03:32 CET 2007

On 2/2/07, Danny Yoo <dyoo at hkn.eecs.berkeley.edu> wrote:
> > 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!
> Hi Marcus,
> There is something very suspicious in the code.  You don't happen to have
> any global variables in your program, do you?
> Let me underline what you might want to look at.
> > def demux(filename, channel, nchannels):
>              ^^^^^^^^

Sorry, I had a typo.  Attached below is the complete code actually copied
from my editor.  Notice that it doesn't do anything now, which is fine b.c.
I just want to troubleshoot the problem.  The error is reproduced as a
comment below the source.  As far as I know, there are no globals; I am
invoking this from a command line prompt: python demux.py foo.bin 1 1

# ---
import struct

def demux(fname, ch=1, nchan=1):
    fmt = str(nchan) + 'h'          # nchan of short (int16)
    blockSize = struct.calcsize(fmt)

    # file setup
    infile  = open(fname, 'rb')
    #outfile = open(fname + 'ch' + str(ch), 'wb')

    # iterate over data
    chunk = infile.read(blockSize)
    while chunk:
        x = struct.unpack(fmt, chunk)
        chunk = infile.read(blockSize)

    # file cleanup
    # outfile.close()

# -------------------------------------------------------------------------
#   main()
#  -------------------------------------------------------------------------
def main(argv=None):
    if argv is None:
    demux(argv[1], int(argv[2]), int(argv[3]))     # filename, ch, nchans

if __name__ == "__main__":
    import sys

# C:\python\python demux.py demux.py 1 1
# Traceback (most recent call last):
#   File "demux.py", line xx, in ?
#     sys.exit(main(sys.argv))
#   File "demux.py", line xx, in main
#     demux(argv[1], int(argv[2]), int(argv[3]))     # filename, ch, nchans
#   File "demux.py", line xx, in demux
#     x = struct.unpack(fmt, chunk)
# struct.error: unpack str size does not match format
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