On Sun, Apr 26, 2020 at 03:03:16PM +0100, Tom Forbes <tom@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/ phd@phdru.name Programmers don't die, they just GOSUB without RETURN.