Merge asof with nans

[svenharris]

Fix to raise a more meaningful error message when attempting to merge_asof with missing values

• closes BUG: Merge_asof error message unclear when attempting to join on missing values #23189
• tests added / passed
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

[pep8speaks]

Hello @svenharris! Thanks for updating the PR.

• In the file pandas/core/reshape/merge.py, following are the PEP8 issues :

Line 816:9: E731 do not assign a lambda expression, use a def
Line 817:9: E731 do not assign a lambda expression, use a def
Line 1470:9: E731 do not assign a lambda expression, use a def
Line 1617:5: E731 do not assign a lambda expression, use a def

• In the file pandas/tests/reshape/merge/test_merge_asof.py, following are the PEP8 issues :

Line 439:9: E731 do not assign a lambda expression, use a def

Comment last updated on October 16, 2018 at 20:11 Hours UTC

[svenharris]

Not sure why the pep8 speaks is flagging lines that weren't updated in this PR, I think I may have messed up the git history somewhere

[codecov]

Codecov Report

Merging #23190 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #23190      +/-   ##
==========================================
+ Coverage   92.19%   92.19%   +<.01%     
==========================================
  Files         169      169              
  Lines       50959    50964       +5     
==========================================
+ Hits        46980    46985       +5     
  Misses       3979     3979

Flag	Coverage Δ
#multiple	90.61% <100%> (ø)	⬆️
#single	42.29% <11.11%> (-0.01%)	⬇️

Impacted Files	Coverage Δ
pandas/core/reshape/merge.py	93.94% <100%> (+0.04%)	⬆️

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[jreback]

@svenharris very nice. can you merge master. ping on green. the CI was giving some trouble before.

[jreback]

actually this is fine, the CI failure is not yet fixed (but unrelated to this PR)

[jreback]

keep em coming!