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

This is a specific case of the more general problem discussed in issue 12458.