bpo-27432: added max_length param to safe_repr and wrote tests for it

[Julsy]

https://bugs.python.org/issue27432

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[serhiy-storchaka]

What is the purpose of adding a parameter to internal function if it is never used?

[gpshead]

What is the purpose of adding a parameter to internal function if it is never used?

Julsy and I were pairing on this issue at the PyCon US mentored sprint. This is the first step in the work to clean up and clarify safe_repr usage. More details in the bug.

[gpshead]

I spent a while looking at this and what it'd take to wind up with a TestCase.maxReprLength style implementation to limit the length of unittest assertion failure reprs (as described in the bpo issue). I don't see a nice path forward via safe_repr as is.

adding max_length as this PR does is easy enough, but plumbing everything through is rather gross. mostly mechanical changes I started in a client before abandoning the work:

1. change all TestCase method safe_repr calls to self._safe_repr method calls so that the method could pass self.maxReprLength in via max_length=.
2. realize that this is mostly pointless without also updating unittest.util._common_shorten_repr which calls safe_repr in a similar manner. do that...
3. and notice how we have many calls of safe_repr on safe_repr'ed things and the nesting between _common_shorten_repr and safe_repr which makes for messy uninformative potentially misleading error messages when maxReprLength was actually set to a value.

Overall I think we should backup and reconsider what we want our unittest assertions to do and how if we're going to add error message length constraining as a feature. shoehorning it in via the existing repr implementations doesn't feel right.

So I'll close this PR and drop a similar note on the bug in case anyone wants to pick this up in the future. unittest.safe_repr (aka unittest.util.safe_repr) remains an undocumented API.

[gpshead]

Thanks for working on this at PyCon this year Julia! Attention on issues is good and sometimes it takes trying to implement something before realizing it isn't the right thing to do. :)