python - 动态添加len方法？

Question

如何以 len(...) 喜欢的方式将 __len__(...) 添加到现有对象？

用例是在第一次迭代后以不需要在迭代站点之间编写/更改代码的方式记住生成器的长度。

这是我目前所拥有的:

def generator2iterator(generator):
    class Iterator(object):
        def __iter__(self):
            i = 0
            for e in generator():
                i += 1
                yield e
            #self.__len__ = types.MethodType(lambda self: i, self)
            self.__len__ = (lambda self: i).__get__(self)
    return Iterator()

def test_generator():
    yield 3

iterator = generator2iterator(test_generator)

print(list(iterator))
# [3]

print(iterator.__len__())
# 1

print(len(iterator))
# TypeError: object of type 'Foo' has no len()

Answer 1

定义一个保存长度的属性并为 __len__ 返回它。

def generator2iterator(generator, length=None):
    class Iterator(object):
        def __iter__(self):
            for i, e in enumerate(generator(), 1):
                self._len = i
                yield e
            #self.__len__ = types.MethodType(lambda self: i, self)

        def __len__(self):
            if not hasattr(self, '_len'):
                raise TypeError("object of type 'Iterator' has no defined len() yet.")
            return self._len
    return Iterator()

# examples:
## define a generator with 10 elements
def g():
    for i in range(10):
        yield i

# pass it to the iterator
it = generator2iterator(g)

# check len
len(it)
TypeError: object of type 'Iterator' has no defined len() yet.

# run the iterator
for _ in it:
    pass

# check the length
len(it)
# returns:
10