Python implementation of `functools.lru_cache` does not check that argument is callable

[cfbolz]

Bug report

Bug description:

The C implementation of functools.lru_cache checks whether its argument is callable and raises an exception if not. The Python version does not do that:

import sys
sys.modules['_functools'] = None

import functools
functools.lru_cache()(1) # does not raise

without blocking the import of _functools you get:

Traceback (most recent call last):
  File "/home/cfbolz/projects/cpython/functoolserror.py", line 5, in <module>
    functools.lru_cache()(1)
    ~~~~~~~~~~~~~~~~~~~~~^^^
  File "/home/cfbolz/projects/cpython/Lib/functools.py", line 598, in decorating_function
    wrapper = _lru_cache_wrapper(user_function, maxsize, typed, _CacheInfo)
TypeError: the first argument must be callable

I think this should simply be fixed in the Python implementation and will prepare a patch.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-144030: add check that argument is callable to Python version of functools.lru_cache #144031

[cfbolz]

(bug found by @zahinwahab via fuzzing and reported first here: pypy/pypy#5369 )