gh-141831:Document behavior of functools.lru_cache with concurrent access

[SkvProgrammer]

updating functools documentation

changes made:Clarified threading behavior in lru_cache. 

from:

The cache is threadsafe so that the wrapped function can be used in multiple threads. This means that the underlying data structure will remain coherent during concurrent updates. It is possible for the wrapped function to be called more than once if another thread makes an additional call before the initial call has been completed and cached. 

to

The cache is threadsafe so that the wrapped function can be used in multiple threads. This means that the underlying data structure will remain coherent during concurrent updates. It is possible for the wrapped function to be called more than once if another thread makes an additional call before the initial call has been completed and cached. In particular, when the same key is computed concurrently in multiple threads, each thread may execute the wrapped function independently before the first result is stored in the cache. As a consequence, the return values from these concurrent calls may be distinct objects even though they correspond to the same cache key. This behavior means that ``lru_cache`` should not be relied upon for singleton object creation or other scenarios requiring that only one instance be constructed for a given key without external synchronization. 

• Issue: Document behavior of functools.lru_cache with concurrent access #141831

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📚 Documentation preview 📚: https://cpython-previews--141869.org.readthedocs.build/

[python-cla-bot]

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[rhettinger]

This is a lot of extra text and is distracting from the core uses of lru_cache. Can you make a more succinct edit perhaps just saying that call-once behavior is not guaranteed because no locks are held during the function call.

[SkvProgrammer]

I have updated it , please check

[rhettinger]

Add a space after the period.

[rhettinger]

Remove conflict markers.

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