DOC: Undocumented change in .isin behavior from 1.1.5 to 1.2.0

[StefanBRas]

Location of the documentation

https://pandas.pydata.org/docs/whatsnew/v1.2.0.html

Documentation problem

There seems to be an undocumented change with how isin works.
pandas 1.1.5:

import pandas as pd
pd.Series([0]).isin(['0']) 
# 0    True
# dtype: bool
pd.Series([1]).isin(['1']) 
# 0    True
# dtype: bool
pd.Series([1.1]).isin(['1.1']) 
# 0    True
# dtype: bool

But in pandas 1.2.0 (and 1.2.0rc):

import pandas as pd
pd.Series([0]).isin(['0']) 
# 0    False
# dtype: bool
pd.Series([1]).isin(['1']) 
# 0    False
# dtype: bool
pd.Series([1.1]).isin(['1.1']) 
# 0    False
# dtype: bool

But the release notes mentions nothing about it.
I have not posted this issue as an bug, because I think the newer implementation is the better implementation.
However I propose adding the change to the release notes for other people like me where the change broke tests in a pipeline.

Suggested fix for documentation

Add a line mentioning that isin no longer returns true when comparing floats to a string of the float.

[jreback]

hmm we did a lot of fixing on isin. likley: #37528

we would need validation tests for this as it works on master. happy to take a PR. your examples in the doc-string would be great as well.

[StefanBRas]

So adding tests for it here: https://github.com/pandas-dev/pandas/blob/master/pandas/tests/test_algos.py#L812?
I'm not sure which doc-string you're refering to here.

[jreback]

So adding tests for it here: https://github.com/pandas-dev/pandas/blob/master/pandas/tests/test_algos.py#L812?
I'm not sure which doc-string you're refering to here.

yep

https://github.com/pandas-dev/pandas/blob/master/pandas/core/series.py#L4584

[StefanBRas]

It looks like there already are tests for this:
https://github.com/pandas-dev/pandas/blob/master/pandas/tests/test_algos.py#L1047
And it looks like there has been some discussion about it in #38346
Which means that #34125 and #19356 can probably be closed.

[simonjayhawkins]

fixed in [4699c2b] BUG: isin numeric vs string (#38279), no release note added.

[omarafifii]

Hi,
I am willing to work on this issue.
I understand I should add the missing part in the release notes, but I am not sure if I should do something with the testing.

[erfannariman]

Hi,
I am willing to work on this issue.
I understand I should add the missing part in the release notes, but I am not sure if I should do something with the testing.

The tests seem to be there as mentioned by @StefanBRas , looks like only release notes are needed.