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Enumerate

Enumerate is a built-in function of Python. Its usefulness can not be summarized in a single line. Yet most of the newcomers and even some advanced programmers are unaware of it. It allows us to loop over something and have an automatic counter. Here is an example:

my_list = ['apple', 'banana', 'grapes', 'pear']
for counter, value in enumerate(my_list):
    print counter, value

# Output:
# 0 apple
# 1 banana
# 2 grapes
# 3 pear

And there is more! enumerate also accepts an optional argument that allows us to specify the starting index of the counter.

my_list = ['apple', 'banana', 'grapes', 'pear']
for c, value in enumerate(my_list, 1):
    print(c, value)

# Output:
# 1 apple
# 2 banana
# 3 grapes
# 4 pear

An example of where the optional argument of enumerate comes in handy is creating tuples containing the index and list item using a list. Here is an example:

my_list = ['apple', 'banana', 'grapes', 'pear']
counter_list = list(enumerate(my_list, 1))
print(counter_list)
# Output: [(1, 'apple'), (2, 'banana'), (3, 'grapes'), (4, 'pear')]