python any（）函数究竟是如何工作的？

Question

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

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

any(x > 0 for x in list)

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 ?

Answer 1

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 .

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.

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 .

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.

Answer 2

>>> names = ['King', 'Queen', 'Joker']
>>> any(n in 'King and john' for n in names)
True

>>> all(n in 'King and Queen' for n in names)
False

It just reduce several line of code into one. You don't have to write lengthy code like:

for n in names:
    if n in 'King and john':
       print True
    else:
       print False

Answer 3

(x > 0 for x in list) in that function call creates a generator expression eg.

>>> nums = [1, 2, -1, 9, -5]
>>> genexp = (x > 0 for x in nums)
>>> for x in genexp:
        print x


True
True
False
True
False

Which any uses, and shortcircuits on encountering the first object that evaluates True

Answer 4

It's because the iterable is

(x > 0 for x in list)

Note that x > 0 returns either True or False and thus you have an iterable of booleans.

Answer 5

Simply saying, any() does this work : according to the condition even if it encounters one fulfilling value in the list, it returns true, else it returns false.

list = [2,-3,-4,5,6]

a = any(x>0 for x in lst)

print a:
True


list = [2,3,4,5,6,7]

a = any(x<0 for x in lst)

print a:
False