Previous Thread on Similar Topic:
https://mail.python.org/archives/list/python-ideas@python.org/thread/4XXGPXWIKU2Q6DHR3GQS5SHQ74ILIYFQ/#3PFQBPPELFICAYL674HKNEQBPQL35HP3
Relevant Code:
https://github.com/python/cpython/blob/f9433ff/Lib/functools.py#L528
I have been beset by two different scenarios where having some option to modify
the input of the `lru_cache` key would be useful. These are simplified versions
of said scenarios, but emphasize why I might want this capability.
1. Scoring model on repeated input.
def score(model, feature_vector_that_includes_example_id):
...
This would be easily memorizable with a key of (model.__hash__(),
feature_vector[ID_INDEX]).
2. Multiple user-friendly wrappers around the same complex internal function
def _foo(obj, transform_fn):
# Some non-trivial logic
new_obj = transform_fn(obj)
# Some non-trivial logic
return new_obj
def foo_1(obj):
def _transform_1(obj):
...
return foo(obj, _transform_1)
def foo_2(obj):
def _transform_2(obj):
...
return foo(obj, _transform_2)
For this subset of functions, I know I can safely use (obj, fn.__name__) as a
key.
This, alongside the "Previous Thread" (wanting to use a subset of arguments for
the key) and the comment in the `_make_key` function
(https://github.com/python/cpython/blob/f9433ff/Lib/functools.py#L464),
indicate that there are other scenarios in which users might want to customize
the `_make_key` function.
The main drawbacks I see are
1. Very prone to error (see conversation in "Previous Thread")
2. For sufficiently complex `_make_key` functions, the benefit may be obscured
by the cost. For instance, casting a `list` to a `tuple` would incur an O(n)
cost, which many users may not recognize.
I would love to get community feedback before pursuing this idea further.
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