Python filter() function applies another function on a given iterable (List/String/Dictionary, etc.) to test which of its item to keep or discard. In simple words, it filters the ones that don’t pass the test and returns the rest as a filter object.

The filter object is of the iterable type. It retains those elements which the function passed by returning True. We can also convert it to List or Tuple or other types using their factory functions.

In this tutorial, you’ll learn how to use the filter() function with different types of sequences. Also, you can refer to the examples that we’ve added to bring clarity.

Python Map() Function

Python Filter() Function Explained

Python Filter() Function

The filter() function accepts only two parameters. The first argument is the name of a user-defined function, and second is iterable like a list, string, set, tuple, etc.

It calls the given function for every element of iterable, just like in a loop. It has the following syntax:

# Python filter() syntax filter(in_function|None, iterable) |__filter object

The first parameter is a function which has a condition to filter the input. It returns True on success or False otherwise. However, if you provide a None, then it removes all items except those evaluate to True.

Next parameter is iterable, i.e., a sequence of elements to test against a condition. Each function call carries one item from the seq for testing.

The return value is a filter object a sequence having elements that passed the function check.

Filter() Function Examples

Here are some examples to explain how to use filter() function.

Filter odd numbers from the list

In this example, we have an iterable list of numeric values out of which some are even, and few are odd.

# list of numbers numbers = [1, 2, 4, 5, 7, 8, 10, 11]

Now, here is a function that filters out the odd number from the given list. We’ll be passing it as the first argument to the filter() call.

# function that filters odd numbers def filterOddNum(in_num): if(in_num % 2) == 0: return True else: return False

Let’s now join the bricks and see the full working code:

""" Desc: Python program to filter odd numbers from the list using filter() function """ # list of numbers numbers = [1, 2, 4, 5, 7, 8, 10, 11] # function that filters vowels def filterOddNum(in_num): if(in_num % 2) == 0: return True else: return False # Demonstrating filter() to remove odd numbers out_filter = filter(filterOddNum, numbers) print("Type of filter object: ", type(out_filter)) print("Filtered seq. is as follows: ", list(out_filter))

A couple of points to notice in the example are:

The filter() function is returning out_filter, and we used type() to check its data type.

We called the list() constructor to convert the filter object to a Python list.

After running the example, you should see the following outcome:

Type of filter object: <class 'filter'> Filtered seq. is as follows: [2, 4, 8, 10]

It printed only the even numbers filtering out the odd ones.

Filter duplicates from two lists

We can use filter() function to get the difference between two sequences. For this, we’ve to filter out duplicate items.

So, let’s assume the following two list of strings.

# List of strings having similar items list1 = ["Python", "CSharp", "Java", "Go"] list2 = ["Python", "Scala", "JavaScript", "Go", "PHP", "CSharp"]

You can check that given lists have common entries for some of the programming languages. So, we need to write a function that checks for duplicates names.

# function that filters duplicate string def filterDuplicate(string_to_check): if(string_to_check in ll): return False else: return True

Let’s now get all the bits and pieces together.

""" Desc: Python program to find the diff. between two lists using filter() function """ # List of strings having similar items list1 = ["Python", "CSharp", "Java", "Go"] list2 = ["Python", "Scala", "JavaScript", "Go", "PHP", "CSharp"] # function that filters duplicate string def filterDuplicate(string_to_check): if(string_to_check in ll): return False else: return True # Demonstrating filter() to remove duplicate strings ll = list2 out_filter = list(filter(filterDuplicate, list1)) ll = list1 out_filter += list(filter(filterDuplicate, list2)) print("Filtered seq. is as follows: ", out_filter)

After executing the example, it produces the following result:

Filtered seq. is as follows: ['Java', 'Scala', 'JavaScript', 'PHP']

As desired, our code printed the difference between the two given lists. However, it was merely an illustration for learning how Python filter() function works.

Difference Between Two Lists in Python

Using lambda expression with filter()

Python lambda expression also works as an inline function. Hence, we can specify it instead of a function argument in the filter() call.

In this way, we can get away from writing a dedicated function for the filtering purpose.

Let’s consider some examples to see how to use lambda with filter().

Filter stop words from a string

In this example, we are going to remove stop words from a given string. We’ve mentioned them in the below list.

list_of_stop_words = ["in", "of", "a", "and"]

Below is the string that contains the stop words.

string_to_process = "A citizen of New York city fought and won in the election."

Now, we’ll see the complete code to filter stop words.

""" Desc: Python program to remove stop words from string using filter() function """ # List of stop words list_of_stop_words = ["in", "of", "a", "and"] # String containing stop words string_to_process = "a citizen of New York city fought and won in the election." # Lambda expression that filters stop words split_str = string_to_process.split() filtered_str = ' '.join((filter(lambda s: s not in list_of_stop_words, split_str))) print("Filtered seq. is as follows: ", filtered_str)

Since we had to remove a whole word, so we split the string into words. After that, we filtered the stop words and joined the rest.

You should get the following outcome after execution:

Filtered seq. is as follows: citizen New York city fought won the election.

Python Lambda

The Intersection of two arrays

In this example, we’ll create a lambda expression and apply filter() function to find the common elements in two arrays.

Below is the input data for our test.

# Defining two arrays having some common elements arr1 = ['p','y','t','h','o','n',' ','3','.','0'] arr2 = ['p','y','d','e','v',' ','2','.','0']

Let’s create the lambda expression that will filter the difference and return common elements.

# Lambda expression using filter() to find common values out = list(filter(lambda it: it in arr1, arr2))

Now, we’ll see the full implementation:

""" Desc: Python program to find common items in two arrays using lambda and filter() function """ # Defining two arrays having some common elements arr1 = ['p','y','t','h','o','n',' ','3','.','0'] arr2 = ['p','y','d','e','v',' ','2','.','0'] def interSection(arr1, arr2): # find identical elements # Lambda expression using filter() to find common values out = list(filter(lambda it: it in arr1, arr2)) return out # Main program if __name__ == "__main__": out = interSection(arr1, arr2) print("Filtered seq. is as follows: ", out)

You should get the following outcome after execution:

Filtered seq. is as follows: ['p', 'y', ' ', '.', '0']

Python Set

Filter function without a function

Yes, you can call filter() without passing an actual function as the first argument. Instead, you can specify it as None.

When None is specified in the filter(), then it pops out all elements that evaluate to False. Let’s consider the following list for illustration:

# List of values that could be True or False bools = ['bool', 0, None, True, False, 1-1, 2%2]

Here is the full code to analyze the behavior of filter() with None as the function argument.

""" Desc: Calling filter() function without a function """ # List of values that could True or False bools = ['bool', 0, None, True, False, 1, 1-1, 2%2] # Pass None instead of a function in filter() out = filter(None, bools) # Print the result for iter in out: print(iter)

The following is the result after execution:

bool True 1

We hope that after wrapping up this tutorial, you should feel comfortable in using the Python filter() function. However, you may practice more with examples to gain confidence.

Also, to learn Python from scratch to depth, do read our step by step Python tutorial.