How exactly does the python any() function work?
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.
>>> 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
(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