map delaying function call

Not only the size that we already saw with the range case, but also the processing time saved by not calculating the results till you actually need it.

Let's see how it works, before getting in more explanation.

def double(n):
    print(f"double {n}")
    return 2 * n

numbers = [1, 4, 2, -1]

double_numbers = map(double, numbers)
print(double_numbers)

for num in double_numbers:
    print(num)

Output:

<map object at 0x7f90df760f98>
double 1
2
double 4
8
double 2
4
double -1
-2

Imagine a case where you apply several expensive (time consuming) transformations to some original list and then you iterate over the end-results looking for the first value that matches some condition. What if you find the value you were looking for after only a few iteration. Then making all that expensive calculations to the whole list was a waste of time.

This lazy evaluation can help you save both memory and time and you always have the option to force the immediate calculation by calling the list function.

In this example we have added a call to print in the double function in order to see when is it really executed. You can see that the first output comes from the print statement that was after the map call. Then on each iteration we see the output from inside the "double" function and then the result from the loop. In a nutshell Python does not execute the "double" function at the point where we called map. It only executes it when we iterate over the resulting object.