Python3 pass lists to function with functools.lru_cache

python python-3.x functools
redfast00 picture redfast00 · Mar 10, 2018 · Viewed 9.2k times · Source

I want to cache a function that takes a list as a parameter, but when I try to do so with the functools.lru_cache decorator, it fails with TypeError: unhashable type: 'list'.


import functools

@functools.lru_cache()
def example_func(lst):
    return sum(lst) + max(lst) + min(lst)


print(example_func([1, 2]))

Answer

redfast00 picture redfast00 · Mar 10, 2018

This fails because a list is unhashable. This would make it hard for Python to know what values are cached. A way to fix this is by converting lists to tuples before passing them to a cached function: since tuples are immutable and hashable, they can be cached.

TL;DR

Use a tuple instead of a list:

>>> @lru_cache(maxsize=2)
... def my_function(args):
...     pass
...
>>> my_function([1,2,3])
Traceback (most recent call last):
  File "<input>", line 1, in <module>
    my_function([1,2,3])
TypeError: unhashable type: 'list'

>>> # TO FIX: use a tuple 

>>> my_function(tuple([1,2,3]))
>>>

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