When testing PDB in python 3.8.1, PDB throws an exception when I call the pm() function in PDB without importing system.

[Fresh session]
>>> import pdb
>>> pdb.pm()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8/pdb.py", line 1631, in pm
    post_mortem(sys.last_traceback)
AttributeError: module 'sys' has no attribute 'last_traceback'
>>> import sys
>>> pdb.pm()
> /Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8/pdb.py(1631)pm()
-> post_mortem(sys.last_traceback)
...

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).