Try varying the order of what you're doing there:

>>> import pdb, sys
>>> pdb.pm()
Traceback ...
>>> pdb.pm()
<post-mortem of the exception raised by the first call to pdb.pm()>

See https://docs.python.org/3/library/sys.html#sys.last_traceback, which notes that this attribute is not always present (it won't be if no exception has been raised).