Python’s built-in enumerate function takes in any iterable sequence, such as a list, and yields the elements of the sequence along with an index number. This is useful for adding a count to every element of a list. The basic syntax of this function is:

enumerate(sequence, start=0)

where sequence is the list or iterable data container and start=0 is what you want the counter to start at.

Without enumerate, we can print the the elements of a list and number them via a for loop by setting up and incrementing a counter. For example:

fruit_list = ['apple', 'banana', 'cherry']

counter = 1
for item in fruit_list:
    print '{0}. {1}'.format(counter, item)
    counter += 1

Which yields:

1. apple
2. banana
3. cherry

Using enumerate, we can do the same thing, but without setting up and incrementing a counter:

fruit_list = ['apple', 'banana', 'cherry']

for counter, item in enumerate(fruit_list, start=1):
    print '{0}. {1}'.format(counter, item)

Which yields:

1. apple
2. banana
3. cherry

Here are some other use cases:

Using enumerate to create a list of tuples:

print list(enumerate(fruit_list))

Which yields:

[(0, 'apple'), (1, 'banana'), (2, 'cherry')]

Using enumerate to create a dictionary of key-value pairs:

print dict(enumerate(fruit_list))

Which yields:

{0: 'apple', 1: 'banana', 2: 'cherry'}

For more information see:
https://docs.python.org/2/library/functions.html#enumerate