Some YB tests fail with conda Python 3.7 & Matplotlib 3.x

I’m trying to run the tests. I get 133 failures, many of them

E   AssertionError: 'Qt5Agg' != 'agg'

Does running the tests require setting a specific backend?

[rebeccabilbro]

Hey there @amueller – nice to see you here in the YB issues!

In response to your question, yes, in order to ensure our image comparison tests work across many different operating systems (or at least don’t fail with backend errors!), we’ve found that we need to use Agg.

Agg has to be set before importing pyplot, so that’s why we’ve got it on our test import (here). We also expect developers will run the tests using pytest from the root directory of YB (perhaps we should make this more clear in the contributor docs!). Running tests in this way will ensure the backend gets set properly first.

But, just out of curiosity (and to help our future contributors!)-- do you have a custom config or backend for Matplotlib? Also, what do you usually run tests with (nose? pytest?)?

[tacaswell]

A sure-fire way to fix this is to set the MPLBACKEND=agg env which overrides any other configuration files.