filter() in python

The filter() function in Python is used to filter a sequence (like a list, tuple, or string) based on a condition provided by a function. It takes a function and an iterable as input and returns a filter object, which is an iterator containing only those elements of the iterable for which the function returns True.

Syntax:

• function: A function that takes one argument and returns a boolean value (True or False). The function is applied to each item in the iterable.
  • If the function returns True, the item is included in the result.
  • If the function returns False, the item is excluded.
• iterable: The iterable (like a list, tuple, or set) that you want to filter.

The filter() function does not modify the original iterable; it simply returns a new filter object containing the filtered elements.

Example Usage of filter():

1. Filtering even numbers from a list:

2. Using a lambda function to filter odd numbers:

You can use a lambda function (a small anonymous function) directly within filter() to avoid defining a separate function.

3. Filtering out empty strings from a list:

4. Filtering based on multiple conditions:

You can also filter based on multiple conditions. For example, filtering numbers that are both positive and even:

Important Notes:

• The result of filter() is an iterator, so if you want to use the filtered result multiple times, you need to convert it into a list (or another iterable type) using list(), tuple(), or similar.
• If the function argument is None, filter() simply removes items that are “falsy” (i.e., items that evaluate to False like None, 0, False, "", and []).

Example with None:

Summary:

• filter() is used to filter out elements from an iterable based on a given condition (function).
• It returns an iterator, so converting it to a list or another iterable is often useful.
• The function passed to filter() should return True or False for each item in the iterable.