The Python **reduce()** function works such that it:

**Applies**a function to the first and the second element of an iterable.**Stores**the result.**Applies**the function to the third element and the result.**Continues**this process until no values left.

In other words, the **reduce()** reduces an iterable into a single cumulative value.

For example, let’s calculate a sum of a list of numbers with the **reduce()** function:

from functools import reduce numbers = [1, 2, 3, 4] sum = reduce(lambda x, y: x + y, numbers) # returns 10

Here is an illustration of how it works:

But as it turns out, using `reduce()`

is useless most of the time. Writing a for loop is a better option 99% of the time.

In this guide, you will learn how to:

**Use**the**reduce()**function.**Implement**a custom reduce function to support understanding the concept.**Call****reduce()**function with lambda expressions.**Solve**common tasks with**reduce()**.

## Introduction to reduce() in Python

Python’s **reduce()** function implements what is known as folding in mathematics. Folding means you reduce a list of numbers into a single value.

For example, you can fold a list of numbers to obtain the sum of it.

The `reduce()`

function works for any iterable in Python—not only for lists!

Python’s **reduce()** function is part of the built-in **functools** library. Remember to import the library before reducing!

The reduction generally follows this procedure:

**Call**a function on the first two elements of an iterable to calculate a partial result.**Call**the function on this partial result and the third element of the iterable to create a new partial result.**Repeat**this process until there are no values left in the iterable.**Return**the result.

The whole point of `reduce()`

function is to replace loops with a more concise shorthand notation.

### Why reduce() Is a Part of Functools

Initially, the `reduce()`

function was a part of the built-in functions of Python 2.x.

Later on, the `reduce()`

was moved to `functools`

module for Python 3.x.

This move “made some space” for new useful built-in functions, such as `max()`

, `min()`

, `sum()`

, `len()`

, `all()`

, and so on.

These functions are more efficient, readable, and Pythonic than solving the related task with `reduce()`

.

### Reduce() Function Syntax in Python

The reduce function follows this syntax:

functools.reduce(function, iterable[, initializer])

Where

**Function**is a mandatory argument.**Iterable**is a mandatory argument.**Initializer**is an optional argument.

Let’s take a closer look at what these arguments do.

### Required Arguments: Function and Iterable

The first argument is called **function**.

This is the function that is cumulatively applied for each element in the iterable to fold the iterable into a single final value.

The second argument is called **iterable**.

This can be any python object that can be iterated over. These are for example lists, tuples, sets, dictionaries, and so on.

### Optional Arguments: Initializer

The third argument is an optional argument **initializer**.

This is the initial value where the reduction starts from. The final value is accumulated on top of the initial value.

### Example—How to Caluclate the Sum of a List

Let’s see a simple example of how to use the `reduce()`

function in Python.

Let’s sum a list of numbers using the `reduce()`

function.

To do this we need to:

**Import**the`reduce()`

function from the`functools`

library.**Have**a list of numbers.**Define**a function that calculates the sum of two numbers.**Reduce**the list of numbers by calling the sum() function for each element cumulatively.**Print**the result.

Here is the code:

from functools import reduce numbers = [1, 2, 3, 4] def sum(a, b): return a + b # Call sum() function on each element accumulatively total = reduce(sum, numbers) print(total)

Output:

10

Now you have a basic understanding of how the `reduce()`

function works in Python.

Next, let’s implement our own naive version of it to support the understanding.

## How to Implement Custom reduce() Function in Python

A great way to understand how `reduce()`

function works is by implementing one.

The custom reducer function needs to take three parameters:

- A function that is cumulatively applied for each element of the iterable.
- An iterable, such as a list.
- An optional initial starting value.

As described earlier, the reducer accumulates a result over the iterable from left to right, eventually producing a single value.

Here is the code:

def my_reduce(function, iterable, start_value=None): index = 0 if start_value is None: value = iterable[index] else: value = start_value while index < len(iterable) - 1: next_value = iterable[index + 1] value = function(value, next_value) index += 1 return value

Here, the while loop takes care of accumulating the result value. It does this by calling the `function`

on the previous value and the next element.

Now we can use this custom reducer.

For example, let’s calculate the sum of a list of numbers:

numbers = [1, 2, 3, 4] # A function that calculates sum of two values def sum(a, b): return a + b # The reducer calls sum for each (result, element) pair accumulatively total = my_reduce(sum, numbers) print(total)

This produces the following result:

10

## The reduce() Function and Lambda Expressions

Now that you understand how reducing works in Python, you are almost ready to see it in action.

In this guide, we are going to use lambda expressions inside the `reduce()`

function calls.

If you don’t know what lambda expressions are, here is a quick primer.

A Lambda expression is a nameless function. It is used as a shorthand when we don’t want to define a separate function for a simple task.

To demonstrate, let’s use a reducer to get the sum of a list of numbers.

First, let’s define a function that calculates the sum of two numbers. Then let’s pass this function into the `reduce()`

function call:

from functools import reduce numbers = [1, 2, 3, 4] def sum(a, b): return a + b # Call sum() function on each element accumulatively total = reduce(sum, numbers) print(total)

Output:

10

This works.

But it is useless to have a separate definition for `sum()`

. This is because it is such a simple function and we only use it once.

Here is where lambda expressions come in handy.

Instead of defining a separate function `sum()`

, we can pass a nameless shorthand version of it into the `reduce()`

call.

Here is how:

from functools import reduce numbers = [1, 2, 3, 4] total = reduce(lambda a, b: a + b, numbers) print(total)

Here `lambda a, b: a + b`

does the exact same as the `sum()`

function earlier. It is a nameless function that takes two parameters and sums them up.

Output:

10

Learn more about lambda expressions in Python by reading this article.

## Python reduce() Function in Action

Let’s see some common tasks that can be solved using reducers in Python.

But as you know, reducing is usually not the most optimal way to solve problems in Python. Thus, I’ve included better alternatives to solve the problems in each example.

### Sum

As you already saw, you can use the `reduce()`

function to calculate the sum of a list.

Here is the code:

from functools import reduce numbers = [1, 2, 3, 4] sum = reduce(lambda x, y: x + y, numbers) print(sum)

Output:

10

Keep in mind counting the sum this way is not the most Pythonic nor readable.

There is a built-in function called `sum()`

. You can use it to calculate the sum of a list of numbers.

For example:

sum([1, 2, 3, 4]) # returns 10

### Product

Just like calculating a sum with reduce, you can compute an accumulated product with it.

For instance:

from functools import reduce numbers = [1, 2, 3, 4] product = reduce(lambda x, y: x * y, numbers) print(product)

Output:

24

Meanwhile, this approach works, it may not be the most Pythonic way to tackle the problem.

As of Python 3.8, the math module has included a `prod()`

function.

from math import prod numbers = [1, 2, 3, 4] product = prod(numbers) print(product)

Output:

24

### Max Value of a List

So far you have seen examples of how to do some arithmetic operations with the `reduce()`

function.

But the reduce function accepts any function as an argument that takes two arguments and returns a result.

In Python, there is a built-in function `max()`

. You can use it to figure out the greatest value.

For example, let’s figure out which number is greater with the `max()`

function:

max(10, 1000) # Returns 1000

Now, we can use `reduce()`

function in conjunction with the `max()`

function to figure out the largest number in a list.

Here is the code:

from functools import reduce numbers = [3, 99, 12, 3000, 2] greatest = reduce(max, numbers) print(greatest)

Just like the previous examples you’ve seen, it:

- Calls the
`max()`

function on the first two elements. - Stores the greater number as a partial result.
- Applies the max function on the next number in the list and the partial result.
- It repeats this process until the list has no values.
- Then it returns the greatest value.

As a result, it returns the greatest number in the list, which in this case is:

3000

Meanwhile, this approach works, it is recommended to use the built-in max() function to solve this task.

For example:

numbers = [3, 99, 12, 3000, 2] greatest = max(numbers) print(greatest)

Output:

3000

### Min Value of a List

To find the smallest value of a list using `reduce()`

, follow the logic of finding the maximum in the previous section.

For example:

from functools import reduce numbers = [3, 99, 12, 3000, 2] smallest = reduce(min, numbers) print(smallest)

Output:

2

Just like it is not recommended to use `reduce()`

to find the maximum, you should not use it to find the minimum of a list either.

Instead, use the built-in `min()`

function. This makes the code more readable and Pythonic.

For example:

numbers = [3, 99, 12, 3000, 2] smallest = min(numbers) print(smallest)

Output:

2

### All Values True/False?

You can use `reduce()`

to find out if a list of booleans only contains True/False values.

To do this:

- Create a function that takes two boolean values and checks if both are
`True`

. You can use a lambda function that looks like this:`lambda a, b: a and b`

. - Then input this function the reducing function in the
`reduce()`

call. - The
`reduce()`

function then loops through the list and accumulates the boolean value result by applying and operations between the booleans.

Here is the code:

from functools import reduce bools = [True, True, True] all_true = reduce(lambda a, b: a and b, bools) print(all_true)

Output:

True

To check if all booleans in a list are `False`

, follow similar logic except but add `not`

in the lambda function.

Here is the code

from functools import reduce bools = [False, True, False] all_false = reduce(lambda a, b: not a and b, bools) print(all_false)

Output:

False

In reality, you should use the built-in functions `all()`

and `any()`

to check if all values are `True`

or `False`

respectively.

For instance:

bools = [False, False, False] # All Trues? print(all(bools)) # False # All Falses? print(not any(bools)) # True

## When Use reduce() in Python

Use functools.reduce() if you really need it; however, 99 percent of the time an explicit for loop is more readable.

Source: What’s new in Python 3.0

Now that you understand how Python reduce() function works, it is a good time to discuss when you should or should not use it.

As suggested earlier, `reduce()`

is something you usually want to avoid using.

For this exact reason, I showed you better alternatives in the examples of reducers in action.

But why is reducing bad?

Python’s `reduce()`

function has a bad performance. This is because it calls the function multiple times. Naturally, this could lead to slow and inefficient code.

Also, `reduce()`

compromises code readability.

For example, which approach do you understand better?

from functools import reduce numbers = [3, 1, 22] # Non-Pythonic reduce() + lambda approach: greatest_num = reduce(lambda x, y: x if x > y else y, numbers) # Pythonic max() function approach: greatest_num = max(numbers)

### Reduce() Function Performance Comparison

Let’s go back to the earlier examples and compare the performance of using `reduce()`

vs. using a dedicated built-in function.

More specifically, let’s use reduce in three ways to compute a sum of a range of numbers:

- Reduce with a custom sum function.
- Reduce with a lambda expression
- Reduce with the operator.add

And let’s compare these to the built-in `sum()`

function.

Here is the code:

>>> from functools import reduce >>> from timeit import timeit >>> >>> def add(a, b): ... return a + b ... >>> # 1. Reduce() with a user-defined function >>> call_add = "functools.reduce(add, range(50))" >>> timeit(call_add, "import functools", globals={"add": add}) # 2. Reduce() with a Lambda expression call_lambda = "functools.reduce(lambda x, y: x + y, range(50))" timeit(call_lambda, "import functools") # 3. Reduce() with Operator.add call_operator_add = "functools.reduce(operator.add, range(50))" timeit(call_operator_add, "import functools, operator") # 4. Built-in sum() function timeit("sum(range(50))", globals={"sum": sum}) 5.3198932689999765 >>> >>> # 2. Reduce() with a Lambda expression >>> call_lambda = "functools.reduce(lambda x, y: x + y, range(50))" >>> timeit(call_lambda, "import functools") 5.042331120999961 >>> >>> # 3. Reduce() with Operator.add >>> call_operator_add = "functools.reduce(operator.add, range(50))" >>> timeit(call_operator_add, "import functools, operator") 2.154597568999975 >>> >>> # 4. Built-in sum() function >>> timeit("sum(range(50))", globals={"sum": sum}) 0.7353007320000415

As you can see, the time it takes to reduce the sum is significantly more than using the `sum()`

function.

Here is a similar performance experiment with multiplication and reduce().

>>> from functools import reduce >>> from timeit import timeit >>> >>> def prod(a, b): ... return a * b ... >>> # 1. Reduce() with a custom product function >>> call_prod = "functools.reduce(prod, range(50))" >>> timeit(call_prod, "import functools", globals={"prod": prod}) # 2. Reduce() with a Lambda expression call_lambda = "functools.reduce(lambda x, y: x + y, range(50))" timeit(call_lambda, "import functools") # 3. Reduce() with Operator.prod call_operator_mul = "functools.reduce(operator.mul, range(50))" timeit(call_operator_mul, "import functools, operator") # 4. Built-in prod() function call_math_prod = "math.prod(range(50))" timeit(call_math_prod, "import math") 4.836152304000052 >>> >>> # 2. Reduce() with a Lambda expression >>> call_lambda = "functools.reduce(lambda x, y: x + y, range(50))" >>> timeit(call_lambda, "import functools") 4.990506494000101 >>> >>> # 3. Reduce() with Operator.prod >>> call_operator_mul = "functools.reduce(operator.mul, range(50))" >>> timeit(call_operator_mul, "import functools, operator") 1.7283722960000887 >>> >>> # 4. Built-in prod() function >>> call_math_prod = "math.prod(range(50))" >>> timeit(call_math_prod, "import math") 0.7617921809999189

Here you can also see how much better the dedicated `math.prod()`

function is than any other reduction trick.

Long story short, do not use `reduce`

.

Use the dedicated functionality for calculating results like this.

For example, use the built-in function `sum()`

to get a sum of a list of numbers. And use `math.prod()`

to get a total product of a list of numbers.

This is a more efficient and readable approach to tackle these problems.

### Reduce() Function Readability

Let’s tackle the code readability aspect of reducing with one more coding example.

Say you want to figure out the product of all the odd numbers in a list.

The `reduce()`

approach to this problem would look like this:

from functools import reduce numbers = [1, 2, 3, 4, 5] odd_prod = reduce(lambda x, y: x * y if y % 2 != 0 else x, numbers) # Returns 15 (1 * 3 * 5)

But is this code readable?

I would say no.

It takes a while to wrap your head around the lambda expression as well as the whole reduce thing.

What would be a more suitable approach?

For example, a regular for loop:

numbers = [1, 2, 3, 4, 5] odd_prod = 1 for number in numbers: if number % 2 != 0: odd_prod *= number # odd_prod is 15 (1 * 3 * 5)

Or another option is to use a generator expression like this:

from math import prod numbers = [1, 2, 3, 4, 5] def odd_product(numbers): return prod(num for num in numbers if num % 2 != 0) odd_product(numbers) # Returns 15 (1 * 3 * 5)

These approaches are more Pythonic. Also, they don’t sacrifice code readability.

## How Does Python Implement the Reduce() Function

Earlier in this guide, you implemented a version of reduce function yourself to support understanding. That implementation followed the same idea as Python’s own implementation of `reduce()`

.

But in case you are interested, the real implementation of `reduce()`

uses iterables and iterators instead of while loops.

In case you are not interested in the implementation details, feel free to skip this part.

The “real implementation” of reduce() function can be found from Python’s open-source codebase on Github.

The real implementation of `reduce()`

looks like this:

def reduce(function, sequence, initial=_initial_missing): """ reduce(function, iterable[, initial]) -> value Apply a function of two arguments cumulatively to the items of a sequence or iterable, from left to right, so as to reduce the iterable to a single value. For example, reduce(lambda x, y: x+y, [1, 2, 3, 4, 5]) calculates ((((1+2)+3)+4)+5). If initial is present, it is placed before the items of the iterable in the calculation, and serves as a default when the iterable is empty. """ it = iter(sequence) if initial is _initial_missing: try: value = next(it) except StopIteration: raise TypeError( "reduce() of empty iterable with no initial value") from None else: value = initial for element in it: value = function(value, element) return value

For simplicity, we can take out the “unnecessary” parts of the implementation to inspect how the iterators and iterables relate to `reduce()`

.

This simplifies the implementation roughly to something like this:

def reduce(function, iterable, initializer=None): # 1. Grab the iterator it = iter(iterable) # 2. Notice the possible initial value if initializer is None: value = next(it) else: value = initializer # 3. Loop through the iterable and accumulate a result for element in it: value = function(value, element) # 4. Return the result return value

As you can see, instead of using while loops, the `reduce()`

function uses iterators to loop through the iterables.

In short, each iterable object in Python implements an iterator that can be used to loop through the iterable.

To understand iterators, consider this example.

When you do:

numbers = [1, 2, 3] for number in numbers: print(number)

The for loop looks like this under the hood of Python:

# Mimicing for in loop in Python using iterators numbers = [1, 2, 3] # Grab the iterator from numbers that is used to loop through the numbers it = iter(numbers) while True: # If there are number left, get the next number try: next_num = next(it) print(next_num) # If no numbers left, stop iteration except StopIteration: break

This means the for loop:

- The loop grabs the iterator object from the iterable.
- It then enters an endless loop that terminates when there are no values left in the iterable.

Now if you look at the rough implementation of the `reduce()`

function, you start to see more clearly how it works.

Iterators and iterables are big concepts in Python. To truly understand how they work, check out this article.

## Conclusion

Today you learned how Python’s **reduce()** function works.

The `reduce()`

function lets you perform actions on Python iterables that you would normally do with for loops.

To take home, `reduce()`

is an inefficient way to solve problems. Most of the time, there is a built-in function that solves the problem way more efficiently. These include `max()`

, `min()`

, `sum()`

, `all()`

, `any()`

and more.

99% of the time, you should **not** use `reduce()`

.

Today you learned how to solve problems with `reduce()`

. In each case, you also learned that you can replace `reduce()`

with more suitable built-in functions.

You learned how to implement a custom version of `reduce()`

. You also saw how it is really implemented behind the scenes using iterators.

Thanks for reading. I hope you find it useful.

Happy coding!

## Further Reading

50 Python Interview Questions with Answers

#### About the Author

- I'm an entrepreneur and a blogger from Finland. My goal is to make coding and tech easier for you with comprehensive guides and reviews.

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