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

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?

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 https://stackoverflow.com/questions/19389490 for the canonical. — Karl Knechtel, Aug 02 '22 at 21:33

PaulMcG · Accepted Answer · 2017-06-20T14:48:20.440

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.

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))`. — georg, May 12 '13 at 09:53
You ought to add `None` to the list of elements which have boolean values of `False` — Alok Mysore, Oct 26 '16 at 16:55
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 — industryworker3595112, Jan 17 '18 at 07:50
@georg Thank you for this clarification. This is a really important point that had me confused when testing the code with any brackets. — MasayoMusic, Jun 24 '20 at 22:09
`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 - https://stackoverflow.com/questions/43692516/what-are-the-true-and-false-criteria-for-a-python-object — Szymon, Dec 26 '22 at 12:03

score 50 · Answer 2 · edited Jun 08 '16 at 08:33

50

>>> 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

edited Jun 08 '16 at 08:33

Håken Lid

22,318
9
52
67

answered Jun 08 '16 at 07:29

Pranjay Kaparuwan

881
1
11
17

jamylak · Answer 3 · 2013-05-12T08:25:51.403

23

(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

edited May 12 '13 at 08:25

answered May 12 '13 at 08:21

jamylak

128,818
30
231
230

score 9 · Answer 4 · edited May 12 '13 at 08:21

9

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.

edited May 12 '13 at 08:21

Lev Levitsky

63,701
20
147
175

answered May 12 '13 at 08:20

freakish

54,167
9
132
169

score 7 · Answer 5 · edited May 05 '16 at 09:54

7

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

edited May 05 '16 at 09:54

thor

21,418
31
87
173

answered May 05 '16 at 09:37

Alisha

71
1
1

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

5 Answers5

Linked

Related