how to use the post_mortem method of pdb?

Question

I am trying to understand how to use the pdb.post_mortem() method.

for this given file

# expdb.py
import pdb
import trace

def hello():
  a = 6 * 9
  b = 7 ** 2
  c = a * b
  d = 4 / 0
  print(c)

tracer = trace.Trace()

Command prompt

'''
# first Try

λ python -i expdb.py
>>> pdb.post_mortem()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Program Files\Anaconda3\lib\pdb.py", line 1590, in post_mortem
    raise ValueError("A valid traceback must be passed if no "
ValueError: A valid traceback must be passed if no exception is being handled
'''

'''
# Second Try

λ python -i expdb.py
>>> pdb.post_mortem(traceback=tracer.run('hello()') )
 --- modulename: trace, funcname: _unsettrace
trace.py(77):         sys.settrace(None)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Program Files\Anaconda3\lib\trace.py", line 500, in run
    self.runctx(cmd, dict, dict)
  File "C:\Program Files\Anaconda3\lib\trace.py", line 508, in runctx
    exec(cmd, globals, locals)
  File "<string>", line 1, in <module>
  File "expdb.py", line 8, in hello
    d = 4 / 0
ZeroDivisionError: division by zero
>>>

Answer 1

The post_mortem method wants a traceback object, not a Trace object. Traceback objects can be acquired from sys.exc_info()[2] inside of an except block, or you can simply call pdb.post_mortem() with no arguments directly (in the except block).

But either way, you must catch the exception before you can debug it.