python: sys is not defined

Move import sys outside of the try-except block:

import systry:    # ...except ImportError:    # ...

If any of the imports before the import sys line fails, the rest of the block is not executed, and sys is never imported. Instead, execution jumps to the exception handling block, where you then try to access a non-existing name.

sys is a built-in module anyway, it is always present as it holds the data structures to track imports; if importing sys fails, you have bigger problems on your hand (as that would indicate that all module importing is broken).

You're trying to import all of those modules at once. Even if one of them fails, the rest will not import. For example:

try:    import datetime    import foo    import sysexcept ImportError:    pass

Let's say foo doesn't exist. Then only datetime will be imported.

What you can do is import the sys module at the beginning of the file, before the try/except statement:

import systry:    import numpy as np    import pyfits as pf    import scipy.ndimage as nd    import pylab as pl    import os    import heapq    from scipy.optimize import leastsqexcept ImportError:    print "Error: missing one of the libraries (numpy, pyfits, scipy, matplotlib)"    sys.exit()

I'm guessing your code failed BEFORE import sys, so it can't find it when you handle the exception.

Also, you should indent the your code whithin the try block.

try:

import sys# .. other safe importstry:    import numpy as np    # other unsafe importsexcept ImportError:    print "Error: missing one of the libraries (numpy, pyfits, scipy, matplotlib)"    sys.exit()