The Python filter() function constructs an iterator from those elements of iterable for which function returns true. In simple words, filter() returns a sequence from those elements of iterable for which the function returns true.

Python filter() Syntax

filter(function, iterable)

As you can see in syntax, filter() method takes two arguments.

• function (required) – It determines whether or not the items in iterable returns true. If set to None, it removes all false elements and returns true elements only.
• iterable (required) – It can be any iterable (sets, tuples, lists or any).

If the function is defined, then filter(function, iterable) is equivalent to:

item for item in iterable if function(item)

And when the function is not defined (i.e is None), filter(function, iterable) is equivalent to:

item for item in iterable if item

Python filter() Function Example

#defining a filter function
def func(x):
 if(x>0):
   return True
 else:
   return False

#Using the function to filter the list
a = filter(func, [-2, -1, 0, 1, 2])

for i in a:
  print(i)

This above script will generate following output.

1
2

Python filter() Function Example Without filter() Function

>>> f = filter((0, 1, True, False, None))
>>> for i in f:
       print(i)
1
True

Here in above example as you can see, when filter function is not defined, filter() method returns only True values.