How does this input work with the Python 'any' function? [duplicate]

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?

Monoclinous

This question, as it turns out, wasn't actually about any, but about the argument to any in the example code, which is a generator expression. I thus closed it as a duplicate of the reference question for generator expressions. If you are actually looking for information about any, please see stackoverflow.com/questions/19389490 for the canonical. – Showroom

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.

Thais

It would be also worth mentioning that any(x > 0 for x in list) is just a syntactic sugar for any((x > 0 for x in list)). – Luanneluanni

You ought to add None to the list of elements which have boolean values of False – Inamorata

Adding to @georg, the syntactic sugar is not special to any. def b(x): return x; print b(x > 1 for x in xs) # prints <generator object .. – Antimicrobial

@Luanneluanni Thank you for this clarification. This is a really important point that had me confused when testing the code with any brackets. – Squirt

not set() also seems to evaluate to True so I would also add that to the list. Actually this is not a comprehensive list, but pointless to add it here, better link to it - #43693016 – Vigen

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

Hillie

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

Georgetown

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.

Harmon

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

Maley