如何len(generator())

问题描述

Python生成器非常有用.与返回列表的函数相比，它们具有优势.但是，您可以len(list_returning_function()).有没有办法len(generator_function())?

Python generators are very useful. They have advantages over functions that return lists. However, you could len(list_returning_function()). Is there a way to len(generator_function())?

更新:
当然len(list(generator_function()))可以工作.....
我正在尝试使用自己在正在创建的新生成器中创建的生成器.作为新生成器中计算的一部分，它需要知道旧生成器的长度.但是，我想将它们和生成器的属性保持在一起，特别是-不要将整个列表保存在内存中，因为它可能非常很长.

UPDATE:
Of course len(list(generator_function())) would work.....
I'm trying to use a generator I've created inside a new generator I'm creating. As part of the calculation in the new generator it needs to know the length of the old one. However I would like to keep both of them together with the same properties as a generator, specifically - not maintain the entire list in memory as it may be very long.

更新2:
假设生成器知道，即使从第一步开始，它就是目标长度.另外，没有理由维持len()语法.示例-如果Python中的函数是对象，那么我不能将长度分配给该对象的变量，新生成器可以访问该变量吗?

UPDATE 2:
Assume the generator knows it's target length even from the first step. Also, there's no reason to maintain the len() syntax. Example - if functions in Python are objects, couldn't I assign the length to a variable of this object that would be accessible to the new generator?

推荐答案

生成器没有长度，毕竟不是集合.

Generators have no length, they aren't collections after all.

生成器是具有内部状态的函数(和精美的语法).您可以反复调用它们以获取值的序列，因此可以在循环中使用它们.但是它们不包含任何元素，因此要求生成器的长度就像要求函数的长度一样.

Generators are functions with a internal state (and fancy syntax). You can repeatedly call them to get a sequence of values, so you can use them in loop. But they don't contain any elements, so asking for the length of a generator is like asking for the length of a function.

函数是对象，但是不能为它们分配新的属性.原因可能是使这样的基本对象尽可能高效.

Functions are objects, but you cannot assign new attributes to them. The reason is probably to keep such a basic object as efficient as possible.

不过，您可以从函数中简单地返回(generator, length)对，或者将生成器包装在一个简单的对象中，如下所示:

You can however simply return (generator, length) pairs from your functions or wrap the generator in a simple object like this:

class GeneratorLen(object):
    def __init__(self, gen, length):
        self.gen = gen
        self.length = length

    def __len__(self): 
        return self.length

    def __iter__(self):
        return self.gen

g = some_generator()
h = GeneratorLen(g, 1)
print len(h), list(h)

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