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This code:
z = [
 ["Y" for y in None
 ] for x in range(4)
 ]
produces this stacktrace in both Python 2.7 and 3.4:
Traceback (most recent call last):
 File "/Users/edwsmith/dev/untitled4/test.py", line 7, in <module>
 ] for x in range(4)
 File "/Users/edwsmith/dev/untitled4/test.py", line 7, in <listcomp>
 ] for x in range(4)
TypeError: 'NoneType' object is not iterable
Of course my code was slightly more complex, but I lost a fair amount of time troubleshooting how the 'for x in range(4)' was evaluating to None, when really, it was the inner comprehension that was failing.
Ideally the stack trace would say:
Traceback (most recent call last):
 File "/Users/edwsmith/dev/untitled4/test.py", line 6, in <module>
 ["Y" for y in None
 File "/Users/edwsmith/dev/untitled4/test.py", line 6, in <listcomp>
 ["Y" for y in None
TypeError: 'NoneType' object is not iterable