filter() is a built-in function that allows you to process an iterable and extract those items that satisfy a given condition. This process is commonly known as a filtering operation. With
filter(), you can apply a filtering function to an iterable and produce a new iterable with the items that satisfy the condition at hand. In Python,
filter() is one of the tools you can use for functional programming.
In this tutorial, you’ll learn how to:
- Use Python’s
filter()in your code
- Extract needed values from your iterables
filter()with other functional tools
filter()with more Pythonic tools
With this knowledge, you’ll be able to use
filter() effectively in your code. Alternatively, you have the choice of using list comprehensions or generator expressions to write more Pythonic and readable code.
To better understand
filter(), it would be helpful for you to have some previous knowledge on iterables,
for loops, functions, and
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Coding With Functional Style in Python
Functional programming is a paradigm that promotes using functions to perform almost every task in a program. A pure functional style relies on functions that don’t modify their input arguments and don’t change the program’s state. They just take a specific set of arguments and return the same result every time. These kinds of functions are known as pure functions.
In functional programming, functions often operate on arrays of data, transform them, and produce new arrays with added features. There are three fundamental operations in functional programming:
- Mapping applies a transformation function to an iterable and produces a new iterable of transformed items.
- Filtering applies a predicate, or Boolean-valued, function to an iterable and generates a new iterable containing the items that satisfy the Boolean condition.
- Reducing applies a reduction function to an iterable and returns a single cumulative value.
Python isn’t heavily influenced by functional languages but by imperative ones. However, it provides several features that allow you to use a functional style:
- Anonymous functions
Functions in Python are first-class objects, which means that you can pass them around as you’d do with any other object. You can also use them as arguments and return values of other functions. Functions that accept other functions as arguments or that return functions (or both) are known as higher-order functions, which are also a desirable feature in functional programming.
In this tutorial, you’ll learn about
filter(). This built-in function is one of the more popular functional tools of Python.
Understanding the Filtering Problem
Say you need to process a list of numbers and return a new list containing only those numbers greater than
0. A quick way to approach this problem is to use a
for loop like this:
>>> numbers = [-2, -1, 0, 1, 2] >>> def extract_positive(numbers): ... positive_numbers =  ... for number in numbers: ... if number > 0: # Filtering condition ... positive_numbers.append(number) ... return positive_numbers ... >>> extract_positive(numbers) [1, 2]
The loop in
extract_positive() iterates through
numbers and stores every number greater than
positive_numbers. The conditional statement filters out the negative numbers and
0. This kind of functionality is known as a filtering.
Filtering operations consist of testing each value in an iterable with a predicate function and retaining only those values for which the function produces a true result. Filtering operations are fairly common in programming, so most programming languages provide tools to approach them. In the next section, you’ll learn about Python’s way to filter iterables.
Getting Started With Python’s
Python provides a convenient built-in function,
filter(), that abstracts out the logic behind filtering operations. Here’s its signature:
The first argument,
function, must be a single-argument function. Typically, you provide a predicate (Boolean-valued) function to this argument. In other words, you provide a function that returns either
False according to a specific condition.
function plays the role of a decision function, also known as a filtering function, because it provides the criteria to filter out unwanted values from the input iterable and to keep those values that you want in the resulting iterable. Note that the term unwanted values refers to those values that evaluate to false when
filter() processes them using
Note: The first argument to
filter() is a function object, which means that you need to pass a function without calling it with a pair of parentheses.
The second argument,
iterable, can hold any Python iterable, such as a list, tuple, or set. It can also hold generator and iterator objects. An important point regarding
filter() is that it accepts only one
To perform the filtering process,
function to every item of
iterable in a loop. The result is an iterator that yields the values of
iterable for which
function returns a true value. The process doesn’t modify the original input iterable.
filter() is written in C and is highly optimized, its internal implicit loop can be more efficient than a regular
for loop regarding execution time. This efficiency is arguably the most important advantage of using the function in Python.
A second advantage of using
filter() over a loop is that it returns a
filter object, which is an iterator that yields values on demand, promoting a lazy evaluation strategy. Returning an iterator makes
filter() more memory efficient than an equivalent
Note: In Python 2.x,
list objects. This behavior changed in Python 3.x. Now the function returns a
filter object, which is an iterator that yields items on demand. Python iterators are well known to be memory efficient.
In your example about positive numbers, you can use
filter() along with a convenient predicate function to extract the desired numbers. To code the predicate, you can use either a
lambda or a user-defined function:
>>> numbers = [-2, -1, 0, 1, 2] >>> # Using a lambda function >>> positive_numbers = filter(lambda n: n > 0, numbers) >>> positive_numbers <filter object at 0x7f3632683610> >>> list(positive_numbers) [1, 2] >>> # Using a user-defined function >>> def is_positive(n): ... return n > 0 ... >>> list(filter(is_positive, numbers)) [1, 2]
In the first example, you use a
lambda function that provides the filtering functionality. The call to
filter() applies that
lambda function to every value in
numbers and filters out the negative numbers and
filter() returns an iterator, you need to call
list() to consume the iterator and create the final list.
filter() is a built-in function, you don’t have to import anything to be able to use it in your code.
In the second example, you write
is_positive() to take a number as an argument and return
True if the number is greater than
0. Otherwise, it returns
False. The call to
is_positive() to every value in
numbers, filtering out the negative numbers. This solution is way more readable than its
filter() isn’t limited to Boolean functions such as those in the examples above. You can use other types of functions, and
filter() will evaluate their return value for truthiness:
>>> def identity(x): ... return x ... >>> identity(42) 42 >>> objects = [0, 1, , 4, 5, "", None, 8] >>> list(filter(identity, objects)) [1, 4, 5, 8]
In this example, the filtering function,
identity(), doesn’t return
False explicitly but the same argument it takes. Since
None are falsy,
filter() uses their truth value to filter them out. The final list contains only those values that are truthy in Python.
Note: Python follows a set of rules to determine an object’s truth value.
For example, the following objects are falsy:
- Constants like
- Numeric types with a zero value, like
- Empty sequences and collections, like
- Objects that implement
__bool__()with a return value of
__len__()with a return value of
Any other object will be considered truthy.
Finally, if you pass
filter() uses the identity function and yields all the elements of
iterable that evaluate to
>>> objects = [0, 1, , 4, 5, "", None, 8] >>> list(filter(None, objects)) [1, 4, 5, 8]
In this case,
filter() tests every item in the input iterable using the Python rules you saw before. Then it yields those items that evaluate to
So far, you’ve learned the basics of
filter() and how it works. In the following sections, you’ll learn how to use
filter() to process iterables and throw away unwanted values without a loop.
Filtering Iterables With
The job of
filter() is to apply a decision function to each value in an input iterable and return a new iterable with those items that pass the test. The following sections provide some practical examples so you can get up and running with
Extracting Even Numbers
As a first example, say you need to process a list of integer numbers and build a new list containing the even numbers. Your first approach to this problem might be to use a
for loop like this:
>>> numbers = [1, 3, 10, 45, 6, 50] >>> def extract_even(numbers): ... even_numbers =  ... for number in numbers: ... if number % 2 == 0: # Filtering condition ... even_numbers.append(number) ... return even_numbers ... >>> extract_even(numbers) [10, 6, 50]
extract_even() takes an iterable of integer numbers and returns a list containing only those that are even. The conditional statement plays the role of a filter that tests every number to find out if it’s even or not.
When you run into code like this, you can extract the filtering logic into a small predicate function and use it with
filter(). This way, you can perform the same computation without using an explicit loop:
>>> numbers = [1, 3, 10, 45, 6, 50] >>> def is_even(number): ... return number % 2 == 0 # Filtering condition ... >>> list(filter(is_even, numbers)) [10, 6, 50]
is_even() takes an integer and returns
True if it’s even and
False otherwise. The call to
filter() does the hard work and filters out the odd numbers. As a result, you get a list of the even numbers. This code is shorter and more efficient than its equivalent
Finding Prime Numbers
Another interesting example might be to extract all the prime numbers in a given interval. To do that, you can start by coding a predicate function that takes an integer as an argument and returns
True if the number is prime and
False otherwise. Here’s how you can do that:
>>> import math >>> def is_prime(n): ... if n <= 1: ... return False ... for i in range(2, int(math.sqrt(n)) + 1): ... if n % i == 0: ... return False ... return True ... >>> is_prime(5) True >>> is_prime(12) False
The filtering logic is now in
is_prime(). The function iterates through the integers between
2 and the square root of
n. Inside the loop, the conditional statement checks if the current number is divisible by any other in the interval. If so, then the function returns
False because the number isn’t prime. Otherwise, it returns
True to signal that the input number is prime.
is_prime() in place and tested, you can use
filter() to extract prime numbers from an interval like this:
>>> list(filter(is_prime, range(1, 51))) [2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47]
This call to
filter() extracts all the prime numbers in the range between
50. The algorithm used in
is_prime() comes from Wikipedia’s article about primality tests. You can check out that article if you need more efficient approaches.
Removing Outliers in a Sample
When you’re trying to describe and summarize a sample of data, you probably start by finding its mean, or average. The mean is a quite popular central tendency measurement and is often the first approach to analyzing a dataset. It gives you a quick idea of the center, or location, of the data.
In some cases, the mean isn’t a good enough central tendency measure for a given sample. Outliers are one of the elements that affect how accurate the mean is. Outliers are data points that differ significantly from other observations in a sample or population. Other than that, there is no unique mathematical definition for them in statistics.
However, in normally distributed samples, outliers are often defined as data points that lie more than two standard deviations from the sample mean.
Now suppose you have a normally distributed sample with some outliers that are affecting the mean accuracy. You’ve studied the outliers, and you know they’re incorrect data points. Here’s how you can use a couple of functions from the
statistics module along with
filter() to clean up your data:
>>> import statistics as st >>> sample = [10, 8, 10, 8, 2, 7, 9, 3, 34, 9, 5, 9, 25] >>> # The mean before removing outliers >>> mean = st.mean(sample) >>> mean 10.692307692307692 >>> stdev = st.stdev(sample) >>> low = mean - 2 * stdev >>> high = mean + 2 * stdev >>> clean_sample = list(filter(lambda x: low <= x <= high, sample)) >>> clean_sample [10, 8, 10, 8, 2, 7, 9, 3, 9, 5, 9, 25] >>> # The mean after removing outliers >>> st.mean(clean_sample) 8.75
In the highlighted line, the
lambda function returns
True if a given data point lies between the mean and two standard deviations. Otherwise, it returns
False. When you filter the
sample with this function,
34 is excluded. After this cleanup, the mean of the sample has a significantly different value.
Validating Python Identifiers
You can also use
filter() with iterables containing nonnumeric data. For example, say you need to process a list of strings and extract those that are valid Python identifiers. After doing some research, you find out that Python’s
str provides a method called
.isidentifier() that can help you out with that validation.
Here’s how you can use
filter() along with
str.isidentifier() to quickly validate identifiers:
>>> words = ["variable", "file#", "header", "_non_public", "123Class"] >>> list(filter(str.isidentifier, words)) ['variable', 'header', '_non_public']
In this case,
.isidentifier() on every string in
words. If the string is a valid Python identifier, then it’s included in the final result. Otherwise, the word is filtered out. Note that you need to use
str to access
.isidentifier() in the call to
str provides a rich set of
.is*() methods that can be useful for filtering iterables of strings.
Finally, an interesting exercise might be to take the example further and check if the identifier is also a keyword. Go ahead and give it a try! Hint: you can use
.kwlist from the
Finding Palindrome Words
An exercise that often arises when you’re getting familiar with Python strings is to find palindrome words in a list of strings. A palindrome word reads the same backward as forward. Typical examples are “madam” and “racecar.”
To solve this problem, you’ll start by coding a predicate function that takes a string and checks if it reads the same in both directions, backward and forward. Here’s a possible implementation:
>>> def is_palindrome(word): ... reversed_word = "".join(reversed(word)) ... return word.lower() == reversed_word.lower() ... >>> is_palindrome("Racecar") True >>> is_palindrome("Python") False
is_palindrome(), you first reverse the original
word and store it in
reversed_word. Then you return the result of comparing both words for equality. In this case, you use
.lower() to prevent case-related differences. If you call the function with a palindrome word, then you get
True. Otherwise, you get
You already have a working predicate function to identify palindrome words. Here’s how you can use
filter() to do the hard work:
>>> words = ("filter", "Ana", "hello", "world", "madam", "racecar") >>> list(filter(is_palindrome, words)) ['Ana', 'madam', 'racecar']
Cool! Your combination of
is_palindrome() works properly. It’s also concise, readable, and efficient. Good job!
filter() With Other Functional Tools
So far, you’ve learned how to use
filter() to run different filtering operations on iterables. In practice, you can combine
filter() with other functional tools to perform many different tasks on iterables without using explicit loops. In the next two sections, you’ll learn the basics of using
filter() along with
The Square of Even Numbers:
Sometimes you need to take an iterable, process each of its items with a transformation function, and produce a new iterable with the resulting items. In that case, you can use
map(). The function has the following signature:
map(function, iterable[, iterable1, ..., iterableN])
The arguments work like this:
functionholds the transformation function. This function should take as many arguments as iterables you pass into
iterableholds a Python iterable. Note that you can provide several iterables to
map(), but that’s optional.
function to each item in
iterable to transform it into a different value with additional features. Then
map() yields each transformed item on demand.
To illustrate how you can use
filter() along with
map(), say you need to compute the square value of all the even numbers in a given list. In that case, you can use
filter() to extract the even numbers and then
map() to calculate the square values:
>>> numbers = [1, 3, 10, 45, 6, 50] >>> def is_even(number): ... return number % 2 == 0 ... >>> even_numbers = list(filter(is_even, numbers)) >>> even_numbers [10, 6, 50] >>> list(map(lambda n: n ** 2, even_numbers)) [100, 36, 2500] >>> list(map(lambda n: n ** 2, filter(is_even, numbers))) [100, 36, 2500]
First, you get the even numbers using
is_even() just like you’ve done so far. Then you call
map() with a
lambda function that takes a number and returns its square value. The call to
map() applies the
lambda function to each number in
even_numbers, so you get a list of square even numbers. The final example shows how to combine
map() in a single expression.
The Sum of Even Numbers:
Another functional programming tool in Python is
map(), which are still built-in functions,
reduce() was moved to the
functools module. This function is useful when you need to apply a function to an iterable and reduce it to a single cumulative value. This kind of operation is commonly known as a reduction or folding.
The signature of
reduce() is like this:
reduce(function, iterable, initial)
Here’s what the arguments mean:
functionholds any Python callable that accepts two arguments and returns a single value.
iterableholds any Python iterable.
initialholds a value that serves as a starting point for the first partial computation or reduction. It’s an optional argument.
A call to
reduce() starts by applying
function to the first two items in
iterable. This way, it computes the first cumulative result, called an accumulator. Then
reduce() uses the accumulator and the third item in
iterable to compute the next cumulative result. The process continues until the function returns with a single value.
If you supply a value to
reduce() runs the first partial computation using
initial and the first item of
Here’s an example that combines
reduce() to cumulatively calculate the total sum of all the even numbers in a list:
>>> from functools import reduce >>> numbers = [1, 3, 10, 45, 6, 50] >>> def is_even(number): ... return number % 2 == 0 ... >>> even_numbers = list(filter(is_even, numbers)) >>> reduce(lambda a, b: a + b, even_numbers) 66 >>> reduce(lambda a, b: a + b, filter(is_even, numbers)) 66
Here, the first call to
reduce() computes the sum of all the even numbers that
filter() provides. To do that,
reduce() uses a
lambda function that adds two numbers at a time.
The final example shows how to chain
reduce() to produce the same result you got before.
Filtering Iterables With
itertools, you’ll find a function called
filterfalse() that does the inverse of
filter(). It takes an iterable as argument and returns a new iterator that yields the items for which the decision function returns a false result. If you use
None as the first argument to
filterfalse(), then you get the items that are falsy.
The point of having the
filterfalse() function is to promote code reuse. If you already have a decision function in place, then you can use it with
filterfalse() to get the rejected items. This saves you from coding an inverse decision function.
In the following sections, you’ll code some examples that show how you can take advantage of
filterfalse() to reuse existing decision functions and continue doing some filtering.
Extracting Odd Numbers
You already coded a predicate function called
is_even() to check if a number is even or not. With that function and the help of
filterfalse(), you can build an iterator that yields odd numbers without having to code an
>>> from itertools import filterfalse >>> numbers = [1, 3, 10, 45, 6, 50] >>> def is_even(number): ... return number % 2 == 0 ... >>> list(filterfalse(is_even, numbers)) [1, 3, 45]
In this example,
filterfalse() returns an iterator that yields the odd numbers from the input iterator. Note that the call to
filterfalse() is straightforward and readable.
Filtering Out NaN Values
Sometimes when you’re working with floating-point arithmetic, you can face the issue of having NaN (not a number) values. For example, say you’re calculating the mean of a sample of data that contains NaN values. If you use Python’s
statistics module for this computation, then you get the following result:
>>> import statistics as st >>> sample = [10.1, 8.3, 10.4, 8.8, float("nan"), 7.2, float("nan")] >>> st.mean(sample) nan
In this example, the call to
nan, which isn’t the most informative value you can get. NaN values can have different origins. They can be due to invalid inputs, corrupted data, and so on. You should find the right strategy to deal with them in your applications. One alternative might be to remove them from your data.
math module provides a convenient function called
isnan() that can help you out with this problem. The function takes a number
x as an argument and returns
x is a NaN and
False otherwise. You can use this function to provide the filtering criteria in a
>>> import math >>> import statistics as st >>> from itertools import filterfalse >>> sample = [10.1, 8.3, 10.4, 8.8, float("nan"), 7.2, float("nan")] >>> st.mean(filterfalse(math.isnan, sample)) 8.96
math.isnan() along with
filterfalse() allows you to exclude all the NaN values from the mean computation. Note that after the filtering, the call to
mean() returns a value that provides a better description of your sample data.
Coding With Pythonic Style
reduce() have been around for a long time in the Python ecosystem, list comprehensions and generator expressions have become strong and Pythonic competitors in almost every use case.
The functionality these functions provide is almost always more explicitly expressed using a generator expression or a list comprehension. In the following two sections, you’ll learn how to replace a call to
filter() with a list comprehension or a generator expression. This replacement will make your code more Pythonic.
filter() With a List Comprehension
You can use the following pattern to quickly replace a call to
filter() with an equivalent list comprehension:
# Generating a list with filter() list(filter(function, iterable)) # Generating a list with a list comprehension [item for item in iterable if function(item)]
In both cases, the final purpose is to create a list object. The list comprehension approach is more explicit than its equivalent
filter() construct. A quick read through the comprehension reveals the iteration and also the filtering functionality in the
Using list comprehensions instead of
filter() is probably the path most Python developers take nowadays. However, list comprehensions have some drawbacks compared to
filter(). The most notable one is the lack of lazy evaluation. Also, when developers start reading code that uses
filter(), they immediately know that the code is performing filtering operations. However, that’s not so evident in code that uses list comprehensions with the same goal.
A detail to notice when turning a
filter() construct into a list comprehension is that if you pass
None to the first argument of
filter(), then the equivalent list comprehension looks like this:
# Generating a list with filter() and None list(filter(None, iterable)) # Equivalent list comprehension [item for item in iterable if item]
In this case, the
if clause in the list comprehension tests
item for its truth value. This test follows the standard Python rules about truth values you already saw.
Here’s an example of replacing
filter() with a list comprehension to build a list of even numbers:
>>> numbers = [1, 3, 10, 45, 6, 50] >>> # Filtering function >>> def is_even(x): ... return x % 2 == 0 ... >>> # Use filter() >>> list(filter(is_even, numbers)) [10, 6, 50] >>> # Use a list comprehension >>> [number for number in numbers if is_even(number)] [10, 6, 50]
In this example, you can see that the list comprehension variant is more explicit. It reads almost like plain English. The list comprehension solution also avoids having to call
list() to build the final list.
filter() With a Generator Expression
The natural replacement for
filter() is a generator expression. That’s because
filter() returns an iterator that yields items on demand just like a generator expression does. Python iterators are known to be memory efficient. That’s why
filter() now returns an iterator instead of a list.
Here’s how you can use generator expressions to write the example in the above section:
>>> numbers = [1, 3, 10, 45, 6, 50] >>> # Filtering function >>> def is_even(x): ... return x % 2 == 0 ... >>> # Use filter() >>> even_numbers = filter(is_even, numbers) >>> even_numbers <filter object at 0x7f58691de4c0> >>> list(even_numbers) [10, 6, 50] >>> # Use a generator expression >>> even_numbers = (number for number in numbers if is_even(number)) >>> even_numbers <generator object <genexpr> at 0x7f586ade04a0> >>> list(even_numbers) [10, 6, 50]
A generator expression is as efficient as a call to
filter() in terms of memory consumption. Both tools return iterators that yield items on demand. Using either one might be a question of taste, convenience, or style. So, you’re in charge!
filter() allows you to perform filtering operations on iterables. This kind of operation consists of applying a Boolean function to the items in an iterable and keeping only those values for which the function returns a true result. In general, you can use
filter() to process existing iterables and produce new iterables containing the values that you currently need.
In this tutorial, you learned how to:
- Work with Python’s
filter()to process iterables and keep the values you need
reduce()to approach different problems
filter()with list comprehensions and generator expressions
With this new knowledge, you can now use
filter() in your code to give it a functional style. You can also switch to a more Pythonic style and replace
filter() with list comprehensions or generator expressions.