23.2. Map

You previously were introduced to accumulating a list by transforming each of the elements. Here we revisit that pattern.

The following function produces a new list with each item in the original list doubled. It is an example of a mapping, from the original list to a new list of the same length, where each element is doubled.

The doubleStuff function is an example of the accumulator pattern, in particular the mapping pattern. On line 3, new_list is initialized. On line 5, the doubled value for the current item is produced and on line 6 it is appended to the list we’re accumulating. Line 7 executes after we’ve processed all the items in the original list: it returns the new_list. Once again, codelens helps us to see the actual references and objects as they are passed and returned.


This pattern of computation is so common that python offers a more general way to do mappings, the map function, that makes it more clear what the overall structure of the computation is. map takes two arguments, a function and a sequence. The function is the mapper that transforms items. It is automatically applied to each item in the sequence. You don’t have to initialize an accumulator or iterate with a for loop at all.

As we did when passing a function as a parameter to the sorted function, we can specify a function to pass to map either by referring to a function by name, or by providing a lambda expression.

Of course, once we get used to using the map function, it’s no longer necessary to define functions like tripleStuff and quadrupleStuff.


Technically, in a proper Python 3 interpreter, the map function produces an “iterator”, which is like a list but produces the items as they are needed. Most places in Python where you can use a list (e.g., in a for loop) you can use an “iterator” as if it was actually a list. So you probably won’t ever notice the difference. If you ever really need a list, you can explicitly turn the output of map into a list: list(map(...)).

Check Your Understanding

1. Using map, create a list assigned to the variable greeting_doubled that doubles each element in the list lst.
2. Below, we have provided a list of strings called abbrevs. Use map to produce a new list called abbrevs_upper that contains all the same strings in upper case.
Next Section - 23.3. Filter