It's not clear to me why people prefer an extra function which would be exactly equivalent to lru_cache in the expected use case (i.e. decorating a function without arguments). It seems like a good way to cause confusion, especially for beginners. Based on the Zen, there should be one obvious way to do it.
On Sun, Apr 26, 2020 at 6:33 PM Tom Forbes <t...@tomforb.es> wrote: > What if the functions requires arguments? How to cache calls with > different arguments? What if some arguments are not hashable? > > Then I think lru_cache is perfectly suitable for that use case. `once()` > would only be useful if you’re calling a function with no arguments and > therefore return a constant value. I originally thought that an exception > could be raised if `@once()` was used with a function that accepted > arguments, but it might be better to instead simply ignore arguments > instead? It could help with some situations where your method accepts a > single “self” argument, or another value, that you know will be constant > across calls. > > Why ``functools``? Why not your own library or a package at PyPI. Like > https://pypi.org/project/cachetools/ ? > > Because `lru_cache` fits the use case almost perfectly, is available in > the stdlib and is very, very fast. As such people are using it like they > would use `once()` which to me feels like a good argument to either special > case `lru_cache()` to account for this or explicitly add a complimentary > `once()` method alongside `lru_cache`. Adding a complimentary method seems > better. > > On Sun, Apr 26, 2020 at 4:26 PM Oleg Broytman <p...@phdru.name> wrote: > >> On Sun, Apr 26, 2020 at 03:03:16PM +0100, Tom Forbes <t...@tomforb.es> >> wrote: >> > Hello, >> > 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. >> My rationale for suggesting this addition is twofold: >> > >> > First: It???s fairly common to use `lru_cache()` to implement this >> behaviour. We use this inside Django (example < >> https://github.com/django/django/blob/77aa74cb70dd85497dbade6bc0f394aa41e88c94/django/forms/renderers.py#L19>), >> internally in other projects at my workplace, inside the stdlib itself < >> https://github.com/python/cpython/blob/2fa67df605e4b0803e7e3aac0b85d851b4b4e09a/Lib/ipaddress.py#L1324> >> and in numerous other projects. In the first few pages of a Github code >> search < >> https://github.com/search?l=Python&q=%22functools.lru_cache%22&type=Code> >> it is fairly easy to find examples, any decorated method with no parameters >> is using `lru_cache()` like `once()`. 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. >> > >> > Second: Implementing this in Python, in my opinion, crosses the line of >> ???annoying and non-trivial enough to not want to repeatedly do it???. >> While a naive (untested) implementation might be: >> > >> > def 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() >> >> What if the functions requires arguments? How to cache calls with >> different arguments? What if some arguments are not hashable? >> >> Why ``functools``? Why not your own library or a package at PyPI. Like >> https://pypi.org/project/cachetools/ ? >> >> > return obj >> > return inner >> > >> > While to the people who are likely going to be reading this mailing >> this the code above is understandable and potentially even somewhat simple. >> However to a lot of people who might not have had experience with writing >> decorators or understand sentinel objects and their use the above code >> might be incomprehensible. A much more common, and in my opinion worse, >> implementation that I???ve seen is something along the lines of this: >> > >> > _value = None >> > def get_value(): >> > nonlocal _value >> > if _value is None: >> > _value = some_function() >> > return _value >> > >> > Which is not ideal for obvious reasons. And these are not even >> including a potentially key feature: locking the wrapped function so that >> it is only called once if it is invoked from multiple threads at once. >> > >> > So, I???d like to propose adding a `once()` decorator to functools that: >> > 1. Has a C implementation, keeping the speed on-par with `lru_cache()` >> > 2. Ensures that the wrapped function is only called once when invoked >> by multiple threads >> > >> > For some related discussion about this idea and lru_cache, please see >> my thread on < >> https://discuss.python.org/t/reduce-the-overhead-of-functools-lru-cache-for-functions-with-no-parameters/3956 >> >discuss.python.org <http://discuss.python.org/>. >> >> Oleg. >> -- >> Oleg Broytman https://phdru.name/ >> p...@phdru.name >> Programmers don't die, they just GOSUB without RETURN. >> _______________________________________________ >> Python-ideas mailing list -- python-ideas@python.org >> To unsubscribe send an email to python-ideas-le...@python.org >> https://mail.python.org/mailman3/lists/python-ideas.python.org/ >> Message archived at >> https://mail.python.org/archives/list/python-ideas@python.org/message/74ZGZRB4JF6EPVB5E7WHL44KZUGEUELV/ >> Code of Conduct: http://python.org/psf/codeofconduct/ >> > _______________________________________________ > Python-ideas mailing list -- python-ideas@python.org > To unsubscribe send an email to python-ideas-le...@python.org > https://mail.python.org/mailman3/lists/python-ideas.python.org/ > Message archived at > https://mail.python.org/archives/list/python-ideas@python.org/message/2BDIXRRLGJD3JWNUJCCB227SGCQK2YN5/ > Code of Conduct: http://python.org/psf/codeofconduct/ >
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