3.7.5. Looping a list using for and enumerate

We learned two methods to loop through a list): looping through the list elements, which is the simplest, and looping through the list indexes, which allows modifying the list.

The convenient enumerate function offers both advantages. When used to loop a list, it returns both the element and its index:

the_list = [10, 20, 30, 40]

for i_element, element in enumerate(the_list):
    print("The element index is", i_element)
    print("The element is", element)

The element index is 0
The element is 10
The element index is 1
The element is 20
The element index is 2
The element is 30
The element index is 3
The element is 40

Note

This is the first time we see a function that has two return values. In reality, enumerate returns the tuple (i_element, element). The following code is equivalent:

the_list = [10, 20, 30, 40]

for one_element_tuple in enumerate(the_list):
    i_element = one_element_tuple[0]
    element = one_element_tuple[1]
    
    print("The element index is", i_element)
    print("The element is", element)

However, Python allows decomposing a tuple’s contents into multiple variables simultaneously:

example_tuple = (1, 2, 3)
value1, value2, value3 = example_tuple

This is why the line

for i_element, element in enumerate(the_list):

works, and is much cleaner.

Now, let’s try modifying a list using enumerate:

the_list = [10, 20, 30, 40]

print(f"Before: {the_list}")

for i_element, element in enumerate(the_list):
    the_list[i_element] = element * 100

print(f"After: {the_list}")

Before: [10, 20, 30, 40]
After: [1000, 2000, 3000, 4000]

It worked, and it is indeed a clean code.