How to find the length of a “filter” object in python

Question

>>> n = [1,2,3,4]

>>> filter(lambda x:x>3,n)


>>> len(filter(lambda x:x>3,n))
Traceback         


        

Answer 1

Converting a filter to a list will take extra memory, which may not be acceptable for large amounts of data. You can find length of the filter object without converting it to a list:

sum(1 for _ in filter(lambda x: x > 3, n))