Cache _is_setuptools_namespace results to avoid repetitive I/O

[correctmost]

Type of Changes

	Type
✓	🔨 Refactoring

Description

_is_setuptools_namespace can end up reading the same files over and over.

For example, when running pylint's import-error checks on yt-dlp, ~23,500 redundant open() calls were performed prior to caching.

Closes pylint-dev/pylint#9603.

Stats

Before

Command	Mean [s]	Min [s]	Max [s]	Relative
pylint --recursive=y .	17.126 ± 0.084	17.047	17.292	1.00

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
    28477    0.140    0.000    0.512    0.000 {built-in method _io.open}

After

Command	Mean [s]	Min [s]	Max [s]	Relative
pylint --recursive=y .	16.969 ± 0.065	16.882	17.079	1.00

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
     4949    0.047    0.000    0.064    0.000 {built-in method _io.open}

Cache stats at Pylint exit

CacheInfo(hits=23528, misses=1165, maxsize=1024, currsize=1024)

[Pierre-Sassoulas]

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