{"id":381,"date":"2020-10-08T09:04:20","date_gmt":"2020-10-08T09:04:20","guid":{"rendered":"https:\/\/www.pythontutorial.net\/?page_id=381"},"modified":"2025-03-31T02:55:25","modified_gmt":"2025-03-31T02:55:25","slug":"python-reduce-list","status":"publish","type":"page","link":"https:\/\/www.pythontutorial.net\/python-basics\/python-reduce-list\/","title":{"rendered":"How to Use the Python Reduce() function to Reduce a List into a Single Value"},"content":{"rendered":"\n

Summary<\/strong>: in this tutorial, you’ll learn how to use the Python reduce() function to reduce a list into a single value.<\/p>\n\n\n\n

Reducing a list #<\/a><\/h2>\n\n\n\n

Sometimes, you want to reduce a list<\/a> to a single value. For example, suppose that you have a list of numbers<\/a>:<\/p>\n\n\n

scores = [75<\/span>, 65<\/span>, 80<\/span>, 95<\/span>, 50<\/span>]<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
scores = [75<\/span>, 65<\/span>, 80<\/span>, 95<\/span>, 50<\/span>]\n\ntotal = 0<\/span>\n\nfor<\/span> score in<\/span> scores:\n    total += score\n\nprint(total)\n<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
365<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
In this example, we have reduced the whole list into a single value, which is the sum of all elements from the list.<\/p>\n\n\n\n
Introduction the Python reduce() function #<\/a><\/h2>\n\n\n\n
Python offers a function called reduce()<\/code> that allows you to reduce a list in a more concise way.<\/p>\n\n\n\n
Here is the syntax of the reduce()<\/code> function:<\/p>\n\n\n
reduce(fn,list)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
The reduce()<\/code> function applies the fn<\/code>  function of two arguments cumulatively to the items of the list, from left to right, to reduce the list into a single value.<\/p>\n\n\n\n
To use the reduce()<\/code> function, you need to import it from the functools<\/code> module using the following statement at the top of the file:<\/p>\n\n\n
from<\/span> functools import<\/span> reduce<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Note that you’ll learn more about modules and how to use them in the later tutorial.<\/p>\n\n\n\n
The following illustrates how to use the reduce()<\/code> function to calculate the sum of elements of the scores<\/code> list:<\/p>\n\n\n
from<\/span> functools import<\/span> reduce\n\ndef<\/span> sum<\/span>(a, b)<\/span>:<\/span>\n    print(f\"a={a}<\/span>, b={b}<\/span>, {a}<\/span> + {b}<\/span> ={a+b}<\/span>\"<\/span>)\n    return<\/span> a + b\n\n\nscores = [75<\/span>, 65<\/span>, 80<\/span>, 95<\/span>, 50<\/span>]\ntotal = reduce(sum, scores)\nprint(total)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
a=75<\/span>, b=65<\/span>, 75<\/span> + 65<\/span> = 140<\/span>\na=140<\/span>, b=80<\/span>, 140<\/span> + 80<\/span> = 220<\/span>\na=220<\/span>, b=95<\/span>, 220<\/span> + 95<\/span> = 315<\/span>\na=315<\/span>, b=50<\/span>, 315<\/span> + 50<\/span> = 365<\/span>\n365<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
As you can see clearly from the output, the reduce()<\/code> function cumulatively adds two elements of the list from left to right and reduces the whole list into a single value.<\/p>\n\n\n\n
from<\/span> functools import<\/span> reduce\n\nscores = [75<\/span>, 65<\/span>, 80<\/span>, 95<\/span>, 50<\/span>]\ntotal = reduce(lambda<\/span> a, b: a + b, scores)\n\nprint(total)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Summary #<\/a><\/h2>\n\n\n\n
Try it<\/a><\/p>\n\n\n\n
To make the code more concise, you can use a lambda expression<\/a> instead of defining the sum()<\/code> function:<\/p>\n\n\n
Try it<\/a><\/p>\n\n\n\n
Unlike the map()<\/a><\/code> and filter()<\/a><\/code> functions, the reduce()<\/code> isn’t a built-in function in Python. In fact, the reduce()<\/code> function belongs to the functools<\/code> module.<\/p>\n\n\n\n
Try it<\/a><\/p>\n\n\n\n
And to calculate the sum of all elements in the scores<\/code> list, you can use a for<\/code> loop<\/a> like this:<\/p>\n\n\n