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/ >
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