[issue35300] Bug with memoization and mutable objects

[bolorsociedad]

Change by bolorsociedad :


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components: Library (Lib)
nosy: bolorsociedad
priority: normal
severity: normal
status: open
title: Bug with memoization and mutable objects
type: behavior
versions: Python 3.4, Python 3.5, Python 3.6, Python 3.7, Python 3.8

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[issue35300] Bug with memoization and mutable objects

[bolorsociedad]

New submission from bolorsociedad :

The decorator functools.lru_cache seems to not work properly when the function 
to be memoized returns a mutable object.

For instance:

>>> import functools
>>> @functools.lru_cache()
... def f(x):
...return [x, x + 1]
... 
>>> a = f(4)
>>> print(a)
[4, 5]
>>> a[0] = 0
>>> f(4)
[0, 5]


When the returned mutable object is modified, the cache is modified as well. In 
my opinion, functools.lru_cache should store a deep copy of the returned object.

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[issue35300] Bug with memoization and mutable objects

[bolorsociedad]

bolorsociedad  added the comment:

I understand it may be inefficient sometimes. Perhaps it would be nice to add 
an argument to lru_cache to specify that we want to deep copy? Something like

def lru_cache(..., deepcopy=False):
   ...

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