python: sys is not defined

Question 1

Move import sys outside of the try-except block:

import sys
try:
    # ...
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).

Question 2

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 sys
except 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 sys
try:
    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 leastsq

except ImportError:
    print "Error: missing one of the libraries (numpy, pyfits, scipy, matplotlib)"
    sys.exit()

Question 3

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

Question 4

In addition to the answers given above, check the last line of the error message in your console. In my case, the 'site-packages' path in sys.path.append('.....') was wrong.