Python filter() Function: A Comprehensive Guide with Examples

Master the Python filter() function, a powerful tool for filtering lists and other iterable data structures. This comprehensive guide covers everything you need to know, from the basics to advanced topics like using lambda functions and custom functions.

The filter() function takes a function and a sequence as arguments and returns an iterable, only yielding the items in sequence for which function returns True. If None is passed instead of a function, all the items of the sequence which evaluates to False are removed. The syntax of the filter()

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 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.

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.

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']

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

Conclusion

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.

Learn More

Map, Filter, Reduce Functions in Python | Python Built-in Functions Explained

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