From the annals of bug subtlety

Posted on Fri 20 June 2014 in blog

From the annals of the truly screwed up comes an object lesson in why it’s really nice to have a debugger.

I’m porting a biggish python codebase to support multiple OSs and maya versions. As I move things around I try to use the opportunity to shore up test coverage. And it always feels like the most boring chore imaginable, until something like this crops up.

I’ve got an exporter framework that I share between projects and files, and I have to move it from Maya 2011-only to support multiple versions. It’s important code, so it’s tested - but the test is really dumb: it creates a test scene, imports the test framework, and calls one function.

But, for some reason, running the tests in 2014 never works - even though I can manually execute the exact steps in a regular copy of maya and all is well.

So I threw it under the debugger — _PyCharm FTW!__ — and started stepping through. No dice, everything seemed OK but still the test failed: it could not find my test objects. Finally, in desperation, I started stepping though the test and issuing an ls() after every step… and I found that the break wasn’t caused by running code - it was caused by importing my module. I didn’t call it - just imported it. WTF?

It turns out that importing PyMel was wiping my test scene in 2014! The tests all run under maya.standalone, and the bug only shows up there, which is why just doing it by hand in maya wasn’t showing the same symptoms.

Here’s my repro case:

import maya.cmds as cmds
# [u'pCube1', u'polyCube1']'transform')
# [u'front', u'pCube1', u'persp', u'side', u'top']

import pymel.core as pm'transform')
# [u'front', u'persp', u'side', u'top']

This is a 100% repro in maya.standalone - but not in GUI maya, where the bug does not occur.

Is this true for everybody else?

The workaround is to import pymel earlier so that the destruction doesn’t affect anything important.

But… ouch!