python any() 函数究竟是如何工作的? [英] How exactly does the python any() function work?

问题描述

在 any 的 python 文档页面中，等效的any() 函数的代码如下:

In the python docs page for any, the equivalent code for the any() function is given as:

def any(iterable):
    for element in iterable:
        if element:
            return True
    return False

如果以这种形式调用它，该函数如何知道我要测试的元素?

How does this function know what element I wanna test if call it in this form?

any(x > 0 for x in list)

从函数定义中，我只能看到我正在传递一个可迭代对象.for 循环如何知道我正在寻找的东西 >0?

From the function definition, all I can see is that I'm passing an iterable object. How does the for loop know I am looking for something > 0?

推荐答案

如果你使用 any(lst) 你会看到 lst 是可迭代的，它是一个列表的一些项目.如果它包含 [0, False, '', 0.0, [], {}, None](它们都具有 False 的布尔值)，则 any(lst) 将是 False.如果 lst 还包含以下任何一个 [-1, True, "X", 0.00001](所有这些都评估为 True)然后any(lst) 将是 True.

If you use any(lst) you see that lst is the iterable, which is a list of some items. If it contained [0, False, '', 0.0, [], {}, None] (which all have boolean values of False) then any(lst) would be False. If lst also contained any of the following [-1, True, "X", 0.00001] (all of which evaluate to True) then any(lst) would be True.

在您发布的代码中，x >0 for x in lst，这是另一种可迭代的，称为生成器表达式.在将生成器表达式添加到 Python 之前，您将创建一个 列表推导式，它看起来非常相似，但是包含了 [] 的:[x >0 表示 lst 中的 x].从包含 [-1, -2, 10, -4, 20] 的 lst 中，你会得到这个理解列表:[假，假，真，假，真].然后，这个内部值将传递给 any 函数，该函数将返回 True，因为至少有一个 True 值.

In the code you posted, x > 0 for x in lst, this is a different kind of iterable, called a generator expression. Before generator expressions were added to Python, you would have created a list comprehension, which looks very similar, but with surrounding []'s: [x > 0 for x in lst]. From the lst containing [-1, -2, 10, -4, 20], you would get this comprehended list: [False, False, True, False, True]. This internal value would then get passed to the any function, which would return True, since there is at least one True value.

但是使用生成器表达式，Python 不再需要创建 True(s) 和 False(s) 的内部列表，值将在 any 函数迭代生成器表达式一次生成一个值时生成.并且，由于any短路，它会在看到第一个True值时立即停止迭代.如果您使用诸如 lst = range(-1,int(1e9))(或 xrange，如果您正在使用 Python2.x).即使这个表达式将生成超过 10 亿个条目，any 只需要在到达 1 时到达第三个条目，它计算 True 为 x>0，所以 any 可以返回 True.

But with generator expressions, Python no longer has to create that internal list of True(s) and False(s), the values will be generated as the any function iterates through the values generated one at a time by the generator expression. And, since any short-circuits, it will stop iterating as soon as it sees the first True value. This would be especially handy if you created lst using something like lst = range(-1,int(1e9)) (or xrange if you are using Python2.x). Even though this expression will generate over a billion entries, any only has to go as far as the third entry when it gets to 1, which evaluates True for x>0, and so any can return True.

如果您创建了一个列表推导式，Python 将首先必须在内存中创建十亿个元素的列表，然后将其传递给 any.但是通过使用生成器表达式，你可以让 Python 的内置函数，如 any 和 all 尽早爆发，只要 True 或 False 值被看到.

If you had created a list comprehension, Python would first have had to create the billion-element list in memory, and then pass that to any. But by using a generator expression, you can have Python's builtin functions like any and all break out early, as soon as a True or False value is seen.

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