The context manager is badly named it does not actually catch any warnings it just resets the warning filter when leaving the context. I think you have to do:

with warnings.catch_warnings(record=True):
    warnings.simplefilter("ignore", UserWarning)
    _apply_style(rcParamsDefault)

If I understand this correctly the with warnings.catch_warnings(record=True) means that the warning is only showed once but it is still showed exactly once

If I understand this correctly the with warnings.catch_warnings(record=True) means that the warning is only showed once but it is still showed exactly once

For me, at least, the warnings aren't shown at all, due to the record=True. But on further consideration, a think a new kwarg is better -- it will be more efficient, as there's no need to generate the warning object in the first place, only to catch and suppress it elsewhere. Updating this PR...

It's shown once in https://travis-ci.org/matplotlib/matplotlib/jobs/99718127 but that might be do to other reasons.

Let's see if the latest commit here fixes that warning in the doc build. If not, there's probably something else to track down.

There is still a single /home/travis/build/matplotlib/matplotlib/lib/matplotlib/style/core.py:53: UserWarning: Style includes a parameter, 'backend', that is not related to style. Ignoring so that must presumably be triggered by some other code path.