Get number of items from list (or other iterable) with certain condition

You can use a generator expression:

>>> l = [1, 3, 7, 2, 6, 8, 10]
>>> sum(1 for i in l if i % 4 == 3)
2

or even

>>> sum(i % 4 == 3 for i in l)
2

which uses the fact that True == 1 and False == 0.

Alternatively, you could use itertools.imap (python 2) or simply map (python 3):

>>> def my_condition(x):
...     return x % 4 == 3
... 
>>> sum(map(my_condition, l))
2