Papouche Guinslyzinho Papouche Guinslyzinho - 27 days ago 8
Python Question

how to use the post_mortem method of pdb?

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

The post_mortem method wants a traceback object, not a Trace object. Traceback objects can be acquired from sys.exec_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.