Python - Filter Function

The filter() function calls the specified function which returns boolen for each item of the specified iterable (list).

Syntax:

filter(function, iterable) --> filter object

Parameters:

1. function: The function to be called for each element of the specified iterable.
2. iterables: One or more iterables separated by a comma (such as string, list, tuple, dictionary).

Return Value:

Returns an iterator object of the filter class.

The filter() function also receives two arguments, a function and a sequence (e.g. a list). Each item in the list is processed by the function which returns True or False. Only those items which return True are stored in a filter object. This can then be conveniently converted into a sequence.

The following function is_even() returns True if the passed number is an even number, otherwise it returns False. This function is used inside filter() along with the list object.

def is_even(x):
	if x%2 == 0:
		return True
	else:
		return False

>>> numbers = [1, 2, 3, 4, 5] 
>>> result = filter(is_even, numbers)
>>> next(result)
2
>>> next(result)
4

The lambda function can also be used in the filter() function, as shown below.

>>> result = filter(lambda x: x%2==0, numbers)
>>> next(result)
2
>>> next(result)
4

You can specify None as the function argument. In this case, the filter() function will return all truthy values from an iterable, as shown below.

>>> mylist = [0, 1, 2, 3, False, True, 5]
>>> result = filter(None, mylist)
>>> list(result)
[1, 2, 3, True, 5]