Deprecate center on df.expanding

[MBrouns]

df.expanding(center=True) currently returns non-sensical results and it is unclear what results would be expected. It was previously removed in #7934 for that same reason,but was reintroduced for unclear reasons in a later refactoring

• closes API/BUG: center=True in expanding operations returns non-sensical results #20647
• tests added / passed
• passes black pandas
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

[charlesdong1991]

thanks for active PRs! not very familiar with this deprecation scope, have a question to clear it.

[jreback]

you will either have to explicity set center=None for expanding tests, or use a @pytest.mark.filterwarnings("...") for this specific warning (this is ok to do)

[pep8speaks]

Hello @MBrouns! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

There are currently no PEP 8 issues detected in this Pull Request. Cheers! 🍻

Comment last updated at 2020-07-07 14:22:09 UTC

[MBrouns]

@jreback If I understood correctly you mean the change I made in f47dede

[jreback]

@jreback If I understood correctly you mean the change I made in f47dede

yep

[MBrouns]

@jreback anything that still needs to happen here?

[charlesdong1991]

@MBrouns pls rebase and resolve conflicts, and make CI green. otherwise, it's unlikely that @jreback will review this PR

[MBrouns]

@charlesdong1991 the test failures seem to be unrelated to this PR and also seem to happen on master

[charlesdong1991]

@MBrouns hey, after a quick skim, i think the linting failure is indeed on master, which will be fixed by #35007

but i think for this PR, still need to make changes for the test in pandas/tests/window/test_expanding.py or other test files in which the test uses expanding and raises the warning.

you need to either catch or ignore the warnings for the test, see https://github.com/pandas-dev/pandas/pull/34887/checks?check_run_id=810672133

[jreback]

thanks @MBrouns very nice