Filter

Syntax

  • filter(function, iterable)
  • itertools.ifilter(function, iterable)
  • future_builtins.filter(function, iterable)
  • itertools.ifilterfalse(function, iterable)
  • itertools.filterfalse(function, iterable)

Parameters

ParameterDetails
functioncallable that determines the condition or None then use the identity function for filtering (positional-only)
iterableiterable that will be filtered (positional-only)

Remarks

In most cases a comprehension or generator expression is more readable, more powerful and more efficient than filter() or ifilter().

Basic use of filter

To filter discards elements of a sequence based on some criteria:

names = ['Fred', 'Wilma', 'Barney']

def long_name(name):
    return len(name) > 5
Python 2.x2.0
filter(long_name, names)
# Out: ['Barney']

[name for name in names if len(name) > 5] # equivalent list comprehension
# Out: ['Barney']


from itertools import ifilter
ifilter(long_name, names)       # as generator (similar to python 3.x filter builtin)
# Out: <itertools.ifilter at 0x4197e10>
list(ifilter(long_name, names)) # equivalent to filter with lists
# Out: ['Barney']

(name for name in names if len(name) > 5) # equivalent generator expression
# Out: <generator object <genexpr> at 0x0000000003FD5D38>
Python 2.x2.6
# Besides the options for older python 2.x versions there is a future_builtin function:
from future_builtins import filter
filter(long_name, names)       # identical to itertools.ifilter
# Out: <itertools.ifilter at 0x3eb0ba8>
Python 3.x3.0
filter(long_name, names)        # returns a generator
# Out: <filter at 0x1fc6e443470>
list(filter(long_name, names))  # cast to list
# Out: ['Barney']

(name for name in names if len(name) > 5) # equivalent generator expression
# Out: <generator object <genexpr> at 0x000001C6F49BF4C0>

Complementary function: filterfalse, ifilterfalse

There is a complementary function for filter in the itertools-module:

Python 2.x2.0.1
 # not recommended in real use but keeps the example valid for python 2.x and python 3.x
from itertools import ifilterfalse as filterfalse
Python 3.x3.0.0
from itertools import filterfalse

which works exactly like the generator filter but keeps only the elements that are False:

# Usage without function (None):
list(filterfalse(None, [1, 0, 2, [], '', 'a']))  # discards 1, 2, 'a' 
# Out: [0, [], '']

# Usage with function
names = ['Fred', 'Wilma', 'Barney']

def long_name(name):
    return len(name) > 5

list(filterfalse(long_name, names))
# Out: ['Fred', 'Wilma']

# Short-circuit useage with next:
car_shop = [('Toyota', 1000), ('rectangular tire', 80), ('Porsche', 5000)]
def find_something_smaller_than(name_value_tuple):
    print('Check {0}, {1}$'.format(*name_value_tuple)
    return name_value_tuple[1] < 100
next(filterfalse(find_something_smaller_than, car_shop))
# Print: Check Toyota, 1000$
# Out: ('Toyota', 1000)

# Using an equivalent generator:
car_shop = [('Toyota', 1000), ('rectangular tire', 80), ('Porsche', 5000)]
generator = (car for car in car_shop if not car[1] < 100)
next(generator)

Filter as short-circuit check

filter (python 3.x) and ifilter (python 2.x) return a generator so they can be very handy when creating a short-circuit test like or or and:

Python 2.x2.0.1
 # not recommended in real use but keeps the example short:
from itertools import ifilter as filter
Python 2.x2.6.1
 from future_builtins import filter

To find the first element that is smaller than 100:

car_shop = [('Toyota', 1000), ('rectangular tire', 80), ('Porsche', 5000)]
def find_something_smaller_than(name_value_tuple):
    print('Check {0}, {1}$'.format(*name_value_tuple)
    return name_value_tuple[1] < 100
next(filter(find_something_smaller_than, car_shop))
# Print: Check Toyota, 1000$
#        Check rectangular tire, 80$
# Out: ('rectangular tire', 80)

The next-function gives the next (in this case first) element of and is therefore the reason why it's short-circuit.

Filter without function

If the function parameter is None, then the identity function will be used:

list(filter(None, [1, 0, 2, [], '', 'a']))  # discards 0, [] and ''   
# Out: [1, 2, 'a']
Python 2.x2.0.1
[i for i in [1, 0, 2, [], '', 'a'] if i] # equivalent list comprehension
Python 3.x3.0.0
(i for i in [1, 0, 2, [], '', 'a'] if i) # equivalent generator expression