Python Enumerate

Iterating with for in

In a variety of languages, especially c-based languages, iterating over lists, or arrays involves incrementing a variable until it reaches a certain size. Python for-loops actually lack this syntax, so for in tend to get used a lot.

from string import ascii_lowercase as ascii

for c in ascii:
    print(c)

Output:

a
b
c
d
...

Iterating with indices

Thus what you end up doing to access a list via indices, is:

from string import ascii_lowercase as ascii

for i in range(len(ascii)):
    print(i, ascii[i])

Output:

0 a
1 b
2 c
3 d
...

Using enumerate

The pythontic way, when you want to access both the index and value directly, without the extra lookup, is to use enumerate.

for i, c in enumerate(ascii):
    print(i, c)

Output:

0 a
1 b
2 c
3 d
...

Indexing from an arbirary number

If a second argument is passed into enumerate, it will begin indexing from that number. For example, to start indexing from 1:

for i, c in enumerate(ascii, 1):
    print(i, c)

Output:

1 a
2 b
3 c
4 d
...