coverage.py doesn't seem to work with pdb.set_trace()

[nedbat]

Originally reported by Anonymous

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First, I've been a long-time user of coverage.py - thanks so much for creating it!

I recently have been working on a personal project of mine and uncovered a behavior with coverage.py that has me stumped.

Without going into too many gory details, my project is an extension to Python's pdb debugger that allows you to debug non-interactive Python programs over a telnet session. I've written some tests for it and am attempting to use coverage.py, but I can't get coverage.py to properly measure coverage even though I know certain lines of code are being hit in my tests.

It seems to come down to coverage.py seemingly not playing well with pdb.set_trace(). Here's a very simple script that illustrates the problem I'm having:

#!python

import pdb

class FriendlyPdb(pdb.Pdb):

    def cmdloop(self):
        print("Look, a custom pdb!")
        return pdb.Pdb.cmdloop(self)

if __name__ == '__main__':
    FriendlyPdb().set_trace()

If I run this Python script, it behaves as you'd expect:

$ coverage run example.py
--Return--
> /Users/ryanpetrello/example.py(11)<module>()->None
-> FriendlyPdb().set_trace()
Look, a custom pdb!
(Pdb) c

Yet the coverage report says that lines 6-7 (the contents of the cmdloop function) are missing:

$ coverage report -m
Name         Stmts   Miss  Cover   Missing
------------------------------------------
example.py       7      2    71%   6-7

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• Bitbucket: https://bitbucket.org/ned/coveragepy/issue/647
• This issue had attachments: Screen Shot 2018-03-01 at 12.07.32 AM.png. See the original issue for details.

[nedbat]

Original comment by ryanpetrello (Bitbucket: ryanpetrello, GitHub: ryanpetrello)

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Apologies for creating this anonymously - I didn't realize Bitbucket allowed that. I'm the person who filed this issue.

[nedbat]

Both coverage.py and debuggers use the trace function feature. If everyone was careful to chain the trace functions, it might work, but I don't think pdb invokes a previous trace function in its function. I'm not sure coverage.py can do anything to make this work.

[nedbat]

Original comment by ryanpetrello (Bitbucket: ryanpetrello, GitHub: ryanpetrello)

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Ned,

Thanks for taking the time to reply and confirming I wasn't missing something obvious. Please feel free to close this ticket :).