COMPAT: Remove use of private re attribute

[TomAugspurger]

Closes #20551

[TomAugspurger]

This is a bit slower. 3.6

In [3]: t1 = typing.re.Pattern

In [4]: t2 = type(re.compile(''))

In [5]: %timeit isinstance('', t1)
424 ns ± 8.76 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)

In [6]: %timeit isinstance('', t2)
99.5 ns ± 1.58 ns per loop (mean ± std. dev. of 7 runs, 10000000 loops each)

3.7

In [1]: import re

In [2]: import typing

In [3]: t1 = typing.re.Pattern

In [4]: t2 = type(re.compile(''))

In [5]: %timeit isinstance('', t1)
741 ns ± 9.73 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)

In [6]: %timeit isinstance('', t2)
105 ns ± 1.62 ns per loop (mean ± std. dev. of 7 runs, 10000000 loops each)

So just use type(re.compile('')) everywhere?

I think the timings are small enough, that I'd rather just use typing.re.Pattern since it's a bit clearer.

[jreback]

can you add a comment / pep ref here

[codecov]

Codecov Report

Merging #20553 into master will increase coverage by 0.02%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master   #20553      +/-   ##
==========================================
+ Coverage   91.81%   91.83%   +0.02%     
==========================================
  Files         152      152              
  Lines       49259    49264       +5     
==========================================
+ Hits        45229    45244      +15     
+ Misses       4030     4020      -10

Flag	Coverage Δ
#multiple	90.22% <83.33%> (+0.02%)	⬆️
#single	41.9% <83.33%> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/core/dtypes/inference.py	98.41% <100%> (ø)	⬆️
pandas/compat/__init__.py	58.43% <80%> (+0.69%)	⬆️
pandas/util/testing.py	84.52% <0%> (-0.21%)	⬇️
pandas/core/series.py	93.85% <0%> (ø)	⬆️
pandas/plotting/_converter.py	66.81% <0%> (+1.73%)	⬆️

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[jreback]

shouldn't this be for PY37 then?

[TomAugspurger]

typing.re was added in Python 3.5. I assume we want to use the officially documented way to check for regex pattern types.

[jreback]

sorry i only read the top comment. ok then.

[jreback]

lgtm.

[jreback]

thanks!