python resettable实例方法备忘录装饰器 [英] python resettable instance method memoization decorator

问题描述

我正在尝试为将记住结果的类的实例方法构建一个装饰器. (这已经完成了一百万次了)但是，我希望可以在任何时候重置已记忆的缓存(例如，如果实例状态中的某些内容发生了更改，这可能会更改没有任何内容的方法的结果)与它的args有关).因此，我尝试将装饰器构建为类而不是函数，以使我可以作为类成员访问缓存.这使我走下了学习描述符的道路，特别是__get__方法，这是我真正遇到的问题.我的代码如下所示:

I'm attempting to build a decorator for an instance method of a class that will memoize the result. (This has been done a million times before) However, I'd like the option of being able to reset the memoized cache at any point (say, if something in the instance state changes, which might change the result of the method having nothing to do with its args). So, I attempted to build a decorator as a class instead of a function, so that I might have access to the cache as a class member. This led me down the path of learning about descriptors, specifically the __get__ method, which is where I'm actually stuck. My code looks like so:

import time

class memoized(object):

    def __init__(self, func):
        self.func = func
        self.cache = {}

    def __call__(self, *args, **kwargs):

        key = (self.func, args, frozenset(kwargs.iteritems()))

        try:
            return self.cache[key]
        except KeyError:
            self.cache[key] = self.func(*args, **kwargs)
            return self.cache[key]
        except TypeError:
            # uncacheable, so just return calculated value without caching
            return self.func(*args, **kwargs)

    # self == instance of memoized
    # obj == instance of my_class
    # objtype == class object of __main__.my_class
    def __get__(self, obj, objtype=None):
        """Support instance methods"""
        if obj is None:
            return self

        # new_func is the bound method my_func of my_class instance
        new_func = self.func.__get__(obj, objtype)

        # instantiates a brand new class...this is not helping us, because it's a 
        # new class each time, which starts with a fresh cache
        return self.__class__(new_func)

    # new method that will allow me to reset the memoized cache
    def reset(self):
        print "IN RESET"
        self.cache = {}

class my_class:
    @memoized
    def my_func(self, val):
        print "in my_func"
        time.sleep(2)
        return val


c = my_class()

print "should take time"
print c.my_func(55)
print

print "should be instant"
print c.my_func(55)
print

c.my_func.reset()

print "should take time"
print c.my_func(55)

这是明确的和/或可能的吗?每次调用__get__时，我都会得到该备注类的全新实例，该实例将丢失其中包含实际数据的缓存.我一直在努力__get__，但没有取得太大进展.

Is this clear and/or possible? Each time __get__ is called, I get a brand new instance of the memoized class, which loses me the cache with actual data in it. I've been working hard with __get__, but am not making much progress.

我是否完全缺少针对此问题的完全独立的方法?并欢迎所有建议/建议.谢谢.

Is there a completely separate approach to this problem that I'm completely missing? And and all advice/suggestions are welcome and appreciated. Thanks.

推荐答案

我没有尝试弄清实现的机制，而是从

Rather than trying to work out the mechanics of your implementation, I've taken the memoized decorator class from PythonDecoratorLibrary, and have modified it to add reset. Below is the result; the trick I've used is to add a callable reset attribute to the decorated function itself.

    class memoized2(object):
       """Decorator that caches a function's return value each time it is called.
       If called later with the same arguments, the cached value is returned, and
       not re-evaluated.
       """
       def __init__(self, func):
          self.func = func
          self.cache = {}
       def __call__(self, *args):
          try:
             return self.cache[args]
          except KeyError:
             value = self.func(*args)
             self.cache[args] = value
             return value
          except TypeError:
             # uncachable -- for instance, passing a list as an argument.
             # Better to not cache than to blow up entirely.
             return self.func(*args)
       def __repr__(self):
          """Return the function's docstring."""
          return self.func.__doc__
       def __get__(self, obj, objtype):
          """Support instance methods."""
          fn = functools.partial(self.__call__, obj)
          fn.reset = self._reset
          return fn
       def _reset(self):
          self.cache = {}


    class my_class:
        @memoized2
        def my_func(self, val):
            print "in my_func"
            time.sleep(2)
            return val


    c = my_class()

    print "should take time"
    print c.my_func(55)
    print

    print "should be instant"
    print c.my_func(55)
    print

    c.my_func.reset()

    print "should take time"
    print c.my_func(55)

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