> t...@tomforb.es wrote:
>
> I would like to suggest adding a simple “once” method to functools. As the
> name suggests, this would be a decorator that would call the decorated
> function, cache the result and return it with subsequent calls.
It seems like you would get just about everything you want with one line:
call_once = lru_cache(maxsize=None)
which would be used like this:
@call_once
def welcome():
len('hello')
> Using lru_cache like this works but it’s not as efficient as it could be - in
> every case you’re adding lru_cache overhead despite not requiring it.
You're likely imagining more overhead than there actually is. Used as shown
above, the lru_cache() is astonishingly small and efficient. Access time is
slightly cheaper than writing d[()] where d={(): some_constant}. The
infinite_lru_cache_wrapper() just makes a single dict lookup and returns the
value.¹ The lru_cache_make_key() function just increments the empty args tuple
and returns it.² And because it is a C object, calling it will be faster than
for a Python function that just returns a constant, "lambda: some_constant()".
This is very, very fast.
Raymond
¹ https://github.com/python/cpython/blob/master/Modules/_functoolsmodule.c#L870
² https://github.com/python/cpython/blob/master/Modules/_functoolsmodule.c#L809
>
> Hello,
> After a great discussion in python-ideas[1][2] it was suggested that I
> cross-post this proposal to python-dev to gather more comments from those who
> don't follow python-ideas.
>
> The proposal is to add a "call_once" decorator to the functools module that,
> as the name suggests, calls a wrapped function once, caching the result and
> returning it with subsequent invocations. The rationale behind this proposal
> is that:
> 1. Developers are using "lru_cache" to achieve this right now, which is less
> efficient than it could be
> 2. Special casing "lru_cache" to account for zero arity methods isn't trivial
> and we shouldn't endorse lru_cache as a way of achieving "call_once"
> semantics
> 3. Implementing a thread-safe (or even non-thread safe) "call_once" method is
> non-trivial
> 4. It complements the lru_cache and cached_property methods currently present
> in functools.
>
> The specifics of the method would be:
> 1. The wrapped method is guaranteed to only be called once when called for
> the first time by concurrent threads
> 2. Only functions with no arguments can be wrapped, otherwise an exception is
> thrown
> 3. There is a C implementation to keep speed parity with lru_cache
>
> I've included a naive implementation below (that doesn't meet any of the
> specifics listed above) to illustrate the general idea of the proposal:
>
> ```
> def call_once(func):
> sentinel = object() # in case the wrapped method returns None
> obj = sentinel
> @functools.wraps(func)
> def inner():
> nonlocal obj, sentinel
> if obj is sentinel:
> obj = func()
> return obj
> return inner
> ```
>
> I'd welcome any feedback on this proposal, and if the response is favourable
> I'd love to attempt to implement it.
>
> 1.
> https://mail.python.org/archives/list/python-id...@python.org/thread/5OR3LJO7LOL6SC4OOGKFIVNNH4KADBPG/#5OR3LJO7LOL6SC4OOGKFIVNNH4KADBPG
> 2.
> https://discuss.python.org/t/reduce-the-overhead-of-functools-lru-cache-for-functions-with-no-parameters/3956
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