• map

The map function of Python applies a function to every item in an iterable and returns an iterator that can be used to iterate over the results. Wow, how many times I repeated the word iter...something. Instead of trying to untangle that sentence, let's look at the following example:

We have a list of numbers in the brilliantly named variable numbers with 1, 2, 3, 4 as the content. We could like to create a list of all the doubles (so that would be 2, 4, 6, 8 in this case) and then iterate over them printing them on the screen. Sure, you probably have some more complex operation to do on the numbers than simple double them, but in this example I did not want to complicate that part. Suffice to say that you have some computation to do in every element.

So you encapsulate your computation in a regular Python function (in our case the function is called double). Then you call map and pass to it two parameters. The first parameter is the double function itself, the second parameter is the list of the values you would like to work on. map will no go over all the values in the numbers list, call the double function with each number and provide allow you to iterate over the results. Something like this:

double_numbers = [ double(1), double(2), double(3), double(4)]

Except, that the above is not true.

When Python executes the double_numbers = map(double, numbers) line, no computation happens and no resulting list is created. Python only prepars "the possibility to do the computations". In the upcoming examples we'll see what does this sentence really mean, for now let's see what do we have in this example: double_numbers contains a **map object*, but when you iterate over it using the for num in double_numbers construct you get the expected values.

In the second half of the example you can see the same works on strings as well.

def double(n):
    return 2 * n

numbers = [1, 2, 3, 4]

double_numbers = map(double, numbers)
print(double_numbers)   # <map object at 0x7f8eb2d849e8>
#for num in double_numbers:
#    print(num)

dbl = list(double_numbers)
print(dbl)

<map object at 0x7f105fa84910>
2
4
6
8