为什么len()不支持迭代器? [英] Why does len() not support iterators?

问题描述

许多Python的内置函数(例如any()，all()，sum()等)都带有可迭代对象，但为什么len()却没有呢?

Many of Python's built-in functions (any(), all(), sum() to name some) take iterables but why does len() not?

一个人总是可以将sum(1 for i in iterable)用作等效项，但是为什么len()首先不考虑可迭代项?

One could always use sum(1 for i in iterable) as an equivalent, but why is it len() does not take iterables in the first place?

推荐答案

许多可迭代对象是由生成器表达式定义的，这些生成器表达式的len定义不明确.采取以下措施，使其永远迭代:

Many iterables are defined by generator expressions which don't have a well defined len. Take the following which iterates forever:

def sequence(i=0):
    while True:
        i+=1
        yield i

基本上，要有一个明确定义的长度，您需要预先了解整个对象.将该功能与sum之类的功能进行对比.您无需一次了解整个对象就可以对其求和，只需一次获取一个元素并将其添加到已经求和的对象中即可.

Basically, to have a well defined length, you need to know the entire object up front. Contrast that to a function like sum. You don't need to know the entire object at once to sum it -- Just take one element at a time and add it to what you've already summed.

请注意诸如sum(1 for i in iterable)这样的惯用语，通常它会耗尽可迭代性，因此您将无法再使用它.或者，如果涉及大量计算，则获取第i个元素可能会很慢.可能值得自问，为什么需要知道先验长度.这可能使您对要使用哪种类型的数据结构有一定的了解(通常list和tuple可以正常工作)-或者您可以执行操作而无需调用len.

Be careful with idioms like sum(1 for i in iterable), often it will just exhaust iterable so you can't use it anymore. Or, it could be slow to get the i'th element if there is a lot of computation involved. It might be worth asking yourself why you need to know the length a-priori. This might give you some insight into what type of data-structure to use (frequently list and tuple work just fine) -- or you may be able to perform your operation without needing calling len.

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