Additional DOC and BUG fix related to merging with mix of columns and…

[Dr-Irv]

Checklist for other PRs (remove this part if you are doing a PR for the pandas documentation sprint):

• closes BUG: New feature allowing merging on combination of columns and index levels drops levels of index #20452
• tests added / passed
  • test_join.py
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry
  • added to v0.24 related to bug fix

This PR adds some documentation about how MultiLevel index levels are dropped when doing a merge that includes levels and columns. It also fixes a small bug related to join.

[codecov]

Codecov Report

Merging #20475 into master will increase coverage by 49.74%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           master   #20475       +/-   ##
===========================================
+ Coverage   42.51%   92.25%   +49.74%     
===========================================
  Files         161      161               
  Lines       51691    51692        +1     
===========================================
+ Hits        21975    47690    +25715     
+ Misses      29716     4002    -25714

Flag	Coverage Δ
#multiple	90.66% <100%> (?)
#single	42.51% <0%> (-0.01%)	⬇️

Impacted Files	Coverage Δ
pandas/core/reshape/merge.py	94.28% <100%> (+84.55%)	⬆️
pandas/core/computation/pytables.py	92.37% <0%> (+0.3%)	⬆️
pandas/io/pytables.py	92.3% <0%> (+0.92%)	⬆️
pandas/util/_test_decorators.py	93.24% <0%> (+4.05%)	⬆️
pandas/compat/__init__.py	58.36% <0%> (+8.17%)	⬆️
pandas/core/config_init.py	99.24% <0%> (+9.84%)	⬆️
pandas/core/reshape/util.py	100% <0%> (+11.53%)	⬆️
pandas/compat/numpy/__init__.py	92.85% <0%> (+14.28%)	⬆️
pandas/core/computation/common.py	85.71% <0%> (+14.28%)	⬆️
pandas/core/api.py	100% <0%> (+14.81%)	⬆️
... and 120 more

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[jreback]

can you add this issue number here

ideally can you use the join_type fixture here (to run this for all join types)

[jreback]

you showed 3 examples in the OP, can you replicate them here

[Dr-Irv]

@jreback I made the changes you asked for and it's all green

[jreback]

@Dr-Irv this got a bit lost, can you rebase and update

[pep8speaks]

Hello @Dr-Irv! Thanks for updating the PR.

• There are no PEP8 issues in the file pandas/core/reshape/merge.py !

• There are no PEP8 issues in the file pandas/tests/reshape/merge/test_join.py !

• There are no PEP8 issues in the file pandas/tests/reshape/merge/test_merge.py !

[Dr-Irv]

@jreback merged with latest, and now all green. I now added a line in whatsnew.

[datapythonista]

@Dr-Irv sorry this wasn't merged before, can you do one more rebase please?

[datapythonista]

@jreback

[Dr-Irv]

@datapythonista now all green

[datapythonista]

Sorry @Dr-Irv, another rebase needed.

[Dr-Irv]

@datapythonista I've done the merge, but the failure is because there is something wrong on Azure. Up to you to decide how to proceed.

[Dr-Irv]

@datapythonista Current merge is now all green.

[jreback]

lgtm. @Dr-Irv just pushed a commit to fix whitespace. ping on green.

[jreback]

thanks @Dr-Irv