Think Python - Denison University

10.7. Map, filter and reduce 93List methods are all void; they modify the list and return None. If you accidentally write t =t.sort(),you willbe disappointed withthe result.10.7 Map,filterand reduceTo add up all thenumbers inalist,you can usealoop likethis:def add_all(t):total = 0for x in t:total += xreturn totaltotal is initialized to 0. Each time through the loop, x gets one element from the list. The +=operator provides ashortway toupdate avariable. This augmented assignment statement:total += xisequivalent to:total = total + xAs the loop executes, total accumulates the sum of the elements; a variable used this way issometimes called an accumulator.Adding up the elements of a list is such a common operation that Python provides it as a built-infunction,sum:>>> t = [1, 2, 3]>>> sum(t)6An operation like this that combines a sequence of elements into a single value is sometimes calledreduce.Sometimesyouwanttotraverseonelistwhilebuildinganother. Forexample,thefollowingfunctiontakes a listof stringsand returns anew listthat contains capitalized strings:def capitalize_all(t):res = []for s in t:res.append(s.capitalize())return resres is initialized with an empty list; each time through the loop, we append the next element. Soresisanother kind of accumulator.Anoperationlikecapitalize_allissometimescalledamapbecauseit“maps”afunction(inthiscase themethodcapitalize)onto each of theelements inasequence.Anothercommonoperationistoselectsomeoftheelementsfromalistandreturnasublist. Forexample,thefollowingfunctiontakesalistofstringsandreturnsalistthatcontainsonlytheuppercasestrings: