Read this for more on indexing.
3. Traversing a list
The most common way to traverse the elements of a list is with a for loop. The syntax is the same as for strings:
for cheese in cheeses:
This works well if you only need to read the elements of the list. But if you want to write or update the elements, you need the indices. A common way to do that is to combine the built-in functions range and len:
for i in range(len(numbers)):
numbers[i] = numbers[i] * 2
This loop traverses the list and updates each element. len returns the number of elements in the list. range returns a list of indices from 0 to n−1, where n is the length of the list. Each time through the loop i gets the index of the next element. The assignment statement in the body uses i to read the old value of the element and to assign the new value.
A for loop over an empty list never runs the body:
for x in :
print('This never happens.')
Although a list can contain another list, the nested list still counts as a single element. The length of this list is four:
['spam', 1, ['Brie', 'Roquefort', 'Pol le Veq'], [1, 2, 3]]