Question

i’m trying to find the docs for the function

in

Inside python. It is very hard because the search engine can’t function with a word like in.

Answer 1

“in” is not a function. Its a built-in keyword. Using it can trigger the target objects __contains__()

http://docs.python.org/reference/datamodel.html#object.contains

>>> l = ['a']
>>> l.__contains__('a')
True
>>> 'a' in l
True

I say “can trigger” because the logic has different ways it might resolve under the hood:

For objects that don’t define __contains__(), the membership test
first tries iteration via __iter__(), then the old sequence iteration
protocol via __getitem__()

Note that this example only covers a membership test, as opposed to using it to loop over an iterable.

To illustrate how in is resolved during a loop operation, an iterator is first looked for using __iter__() :

l = range(5)
l.__iter__()
# <listiterator object at 0x1004e4fd0>
it = l.__iter__()
it.next()
# 0
it.next()
# 1
it.next()
# 2

In the event that the object does not have an __iter__() defined, it will try using the older __getitem__(). Take a string, for instance, which does not have an __iter__(). This is just an illustration…

>>> i = 0
>>> s = "string"
>>> while i < len(s):
...  print s.__getitem__(i)
...  i+=1
... 
s
t
r
i
n
g

And to show a similar iterator (generator here):

gen = (s.__getitem__(i) for i in xrange(len(s)))
# <generator object <genexpr> at 0x1004e8780>
gen.next()
# 's'
gen.next()
# 't'
gen.next()
# 'r'