Python: return the index of the first element of a list which makes a passed function true

Question

The list.index(x) function returns the index in the list of the first item whose value is x.

Is there a function, list_func_index()

Answer 1

Not one single function, but you can do it pretty easily:

>>> test = lambda c: c == 'x'
>>> data = ['a', 'b', 'c', 'x', 'y', 'z', 'x']
>>> map(test, data).index(True)
3
>>>

If you don't want to evaluate the entire list at once you can use itertools, but it's not as pretty:

>>> from itertools import imap, ifilter
>>> from operator import itemgetter
>>> test = lambda c: c == 'x'
>>> data = ['a', 'b', 'c', 'x', 'y', 'z']
>>> ifilter(itemgetter(1), enumerate(imap(test, data))).next()[0]
3
>>> 

Just using a generator expression is probably more readable than itertools though.

Note in Python3, map and filter return lazy iterators and you can just use:

from operator import itemgetter
test = lambda c: c == 'x'
data = ['a', 'b', 'c', 'x', 'y', 'z']
next(filter(itemgetter(1), enumerate(map(test, data))))[0]  # 3