How to Solve Reduce Is Not Defined in Python

From time to time, we often face Not Defined error messages and, more broadly, NameError. These error messages are typical and easy to solve if a simple understanding of what has been named within your Python code exists.

The NameError and Not Defined errors stem from a non-existent variable, binding, function, or library. For this context, the reduce function does not exist within your code.

This article will discuss solving the reduce is not defined error within your Python code.

Use functools to Solve NameError: name 'reduce' is not defined in Python

The reduce() function helps compute a single value from a list using a lambda function on each element within the list. The reduce() function builds a value by taking the current element from the array and combining or comparing it to the current value until it goes through all the elements within the list.

Pre-Python 3, the reduce() function worked as a built-in function; therefore, the below code snippet to sum up all the elements within a list would have worked.

def anon(x, y):
    return x + y


sum = reduce(anon, [1, 2, 3, 4])
print(sum)

However, if you run the above code, it would give the below error:

Traceback (most recent call last):
  File "c:\Users\akinl\Documents\HTML\python\txt.py", line 2, in <module>
    l = reduce(anon, [1,2,3,4])
NameError: name 'reduce' is not defined

That is because reduce() is no longer a built-in function but a function within a built-in library called functools, which contains higher-order functions and operations on callable objects. As reduce() is a higher-order function, it makes sense to be present.

reduce() is a higher-order function because it takes as its argument another function. Now to make use of reduce(), we need to import functools:

import functools


def anon(x, y):
    return x + y


sum = functools.reduce(anon, [1, 2, 3, 4])
print(sum)

The output of the code:

10

We can make it easier within our code to use the from keyword to import reduce:

from functools import reduce


def anon(x, y):
    return x + y


sum = reduce(anon, [1, 2, 3, 4])
print(sum)

The output of the code:

10

We no longer have a reduce is not defined error within our code and can now understand what to look for when we see such.