Enumerate

In a collection, an element has both an index (where it is located) and a value (what it is). In a loop we often want both of these things.

With enumerate, we receive generated tuples containing an index and a value. Enumerate() is a built-in. It is often used in for-loops for iteration.

Initial example

This example uses a string list. Enumerate() acts on any iterable, even a tuple, but a list is a common target for it.

animals = ["cat", "bird", "dog"]

# Use enumerate to get indexes and elements from an iterable.
# ... This unpacks a tuple.
for i, element in enumerate(animals):
    print(i, element)

# Does not unpack the tuple.
for x in enumerate(animals):
    print(x, "UNPACKED =", x[0], x[1])
0 cat
1 bird
2 dog
(0, 'cat') UNPACKED = 0 cat
(1, 'bird') UNPACKED = 1 bird
(2, 'dog') UNPACKED = 2 dog

String

In Python we use strings throughout many programs. With enumerate() we have a good way to loop over the characters in a string, while keeping track of indexes.

word = "one"

# Enumerate over a string.
# ... This is a good way to get indexes and chars from a string.
for i, value in enumerate(word):
    print(i, value)
0 o
1 n
2 e

Not iterable error

The enumerate() built-in must receive an iterable thing. An argument like an integer will cause an error as an integer cannot be looped over.

number = 10

# We cannot enumerate an object that is not iterable.
for i, value in enumerate(number):
    pass
Traceback (most recent call last):
  File "C:\programs\file.py", line 7, in <module>
    for i, value in enumerate(number):
TypeError: 'int' object is not iterable

Powerful tool

Enumerate() is a powerful feature in the Python language. With it we access both indexes and elements themselves.

No information is missing, and we can use a simple for-loop without any complicated index management. With simpler programs, we tend to have fewer program bugs.