Question

我想在Python中做一件小事，类似于内置的property，我不知道该怎么做。

我称这个班为LazilyEvaluatedConstantProperty。它适用于应该只计算一次并且不会更改的属性，但它们应该是懒惰而不是创建对象，以提高性能。

以下是用法：

class MyObject(object):

    # ... Regular definitions here

    def _get_personality(self):
        # Time consuming process that creates a personality for this object.
        print('Calculating personality...')
        time.sleep(5)
        return 'Nice person'

    personality = LazilyEvaluatedConstantProperty(_get_personality)

你可以看到用法类似于property，除了只有一个getter，没有setter或者deleter。

目的是在第一次访问my_object.personality时，将调用_get_personality方法，然后将缓存结果，并且永远不会再为此对象调用_get_personality

实施此问题有什么问题？我想做一些有点难以提高性能的事情：我希望在第一次访问和_get_personality调用之后，personality将成为对象的数据属性，因此在后续调用中查找会更快。但我不知道它是如何可能的，因为我没有对象的引用。

有没有人有想法？

Answer 1

我实施了它：

class CachedProperty(object):
    '''
    A property that is calculated (a) lazily and (b) only once for an object.

    Usage:

        class MyObject(object):

            # ... Regular definitions here

            def _get_personality(self):
                print('Calculating personality...')
                time.sleep(5) # Time consuming process that creates personality
                return 'Nice person'

            personality = CachedProperty(_get_personality)

    '''
    def __init__(self, getter, name=None):
        '''
        Construct the cached property.

        You may optionally pass in the name that this property has in the
        class; This will save a bit of processing later.
        '''
        self.getter = getter
        self.our_name = name


    def __get__(self, obj, our_type=None):

        if obj is None:
            # We're being accessed from the class itself, not from an object
            return self

        value = self.getter(obj)

        if not self.our_name:
            if not our_type:
                our_type = type(obj)
            (self.our_name,) = (key for (key, value) in 
                                vars(our_type).iteritems()
                                if value is self)

        setattr(obj, self.our_name, value)

        return value

对于未来，可能会在此处找到维护的实施：

https://github.com/cool-RR/GarlicSim/blob/master/garlicsim/garlicsim/general_misc/caching/cached_property.py

在Python中创建一个LazilyEvaluatedConstantProperty类

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