Python-延迟加载类属性

Question

foo类有一个酒吧。除非访问，否则不会加载钢筋。进一步访问bar应该不会产生任何开销。

class Foo(object):

    def get_bar(self):
        print "initializing"
        self.bar = "12345"
        self.get_bar = self._get_bar
        return self.bar

    def _get_bar(self):
        print "accessing"
        return self.bar

是否可以使用属性或更好的属性来执行类似的操作，而不是使用getter方法？

目标是在所有后续访问中无负载地延迟加载…

Answer 1

当前的答案存在一些问题。具有属性的解决方案要求您指定其他类属性，并且在每次查找时都要检查该属性。该解决方案__getattr__的问题在于，它会隐藏该属性，直到首次访问。这不利于自省，并且__dir__不便解决。

比两个建议的解决方案更好的解决方案是直接使用描述符。werkzeug库已经有一个解决方案werkzeug.utils.cached_property。它具有一个简单的实现，因此您可以直接使用它而无需将Werkzeug作为依赖项：

_missing = object()

class cached_property(object):
    """A decorator that converts a function into a lazy property.  The
    function wrapped is called the first time to retrieve the result
    and then that calculated result is used the next time you access
    the value::

        class Foo(object):

            @cached_property
            def foo(self):
                # calculate something important here
                return 42

    The class has to have a `__dict__` in order for this property to
    work.
    """

    # implementation detail: this property is implemented as non-data
    # descriptor.  non-data descriptors are only invoked if there is
    # no entry with the same name in the instance's __dict__.
    # this allows us to completely get rid of the access function call
    # overhead.  If one choses to invoke __get__ by hand the property
    # will still work as expected because the lookup logic is replicated
    # in __get__ for manual invocation.

    def __init__(self, func, name=None, doc=None):
        self.__name__ = name or func.__name__
        self.__module__ = func.__module__
        self.__doc__ = doc or func.__doc__
        self.func = func

    def __get__(self, obj, type=None):
        if obj is None:
            return self
        value = obj.__dict__.get(self.__name__, _missing)
        if value is _missing:
            value = self.func(obj)
            obj.__dict__[self.__name__] = value
        return value