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/
