There is a subtle bug in Python 3.4 implementation of traceback library:

>>> import traceback
>>>
>>> try:
...     1 / 0
... except Exception as e:
...     exc = e
...
>>> traceback.print_exception(exc.__class__, exc, exc.__traceback__)
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
ZeroDivisionError: division by zero
>>> 
>>> traceback.print_exception(exc.__class__, exc, None)
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
ZeroDivisionError: division by zero
>>> 
>>> traceback.print_exception(exc.__class__, exc, None, chain=False)
ZeroDivisionError: division by zero

As can be seen, giving None traceback is ignored if chain == True. This is because None is incorrectly used as a sentinel value in traceback._iter_chain (see line 135).

Note that this bug is fixed in Python 3.5 since there is a new implementation of traceback module using TracebackException objects. Will this be backported to Python 3.4? Otherwise, the bug itself should be fixed anyway.