Oh, I didn't know this Python 3.8 new feature (@functools.cached_property). It does exactly what I needed, cool!
Victor Le mar. 28 avr. 2020 à 21:18, Brett Cannon <br...@python.org> a écrit : > > Victor Stinner wrote: > > Hi, > > A pattern that I used multiple times is to compute an object attribute > > only once and cache the result into the object. Dummy example: > > How is that different from > https://docs.python.org/3/library/functools.html?highlight=cached_property#functools.cached_property? > > -Brett > > > class X: > > def __init__(self, name): > > self.name = name > > self._cached_upper = None > > def _get(self): > > if self._cached_upper is None: > > print("compute once") > > self._cached_upper = self.name.upper() > > return self._cached_upper > > upper = property(_get) > > > > obj = X("victor") > > print(obj.upper) > > print(obj.upper) # use cached value > > It would be interesting to be able to replace obj.upper property with > > an attribute (to reduce the performance overhead of calling _get() > > method), but "obj.upper = value" raises an error since the property > > prevents to set the attribute. > > I understood that the proposed @called_once would store the cached > > value into the function namespace. > > Victor > > Le lun. 27 avr. 2020 à 23:44, t...@tomforb.es a écrit : > > > > > > 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: > > > > > > Developers are using "lru_cache" to achieve this right now, which is less > > > efficient > > > than it could be > > > 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 > > > Implementing a thread-safe (or even non-thread safe) "call_once" method is > > > non-trivial > > > It complements the lru_cache and cached_property methods currently > > > present in > > > functools. > > > > > > The specifics of the method would be: > > > > > > The wrapped method is guaranteed to only be called once when called for > > > the first time > > > by concurrent threads > > > Only functions with no arguments can be wrapped, otherwise an exception is > > > thrown > > > 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. > > > > > > https://mail.python.org/archives/list/python-id...@python.org/thread/5OR3LJO... > > > https://discuss.python.org/t/reduce-the-overhead-of-functools-lru-cache-for-... > > > > > > > > > Python-Dev mailing list -- python-dev@python.org > > > To unsubscribe send an email to python-dev-le...@python.org > > > https://mail.python.org/mailman3/lists/python-dev.python.org/ > > > Message archived at > > > https://mail.python.org/archives/list/python-dev@python.org/message/5CFUCM4W... > > > Code of Conduct: http://python.org/psf/codeofconduct/ > > > -- > > Night gathers, and now my watch begins. It shall not end until my death. > _______________________________________________ > Python-Dev mailing list -- python-dev@python.org > To unsubscribe send an email to python-dev-le...@python.org > https://mail.python.org/mailman3/lists/python-dev.python.org/ > Message archived at > https://mail.python.org/archives/list/python-dev@python.org/message/TYUV24XOPPWBK6HRK24A3BRDF4VGMQOT/ > Code of Conduct: http://python.org/psf/codeofconduct/ -- Night gathers, and now my watch begins. It shall not end until my death. _______________________________________________ Python-Dev mailing list -- python-dev@python.org To unsubscribe send an email to python-dev-le...@python.org https://mail.python.org/mailman3/lists/python-dev.python.org/ Message archived at https://mail.python.org/archives/list/python-dev@python.org/message/KEF6ZAPX3LX3ILNU7CSLQBT4D5G25K5B/ Code of Conduct: http://python.org/psf/codeofconduct/
