Colon (:) in Python list index

: is the delimiter of the slice syntax to 'slice out' sub-parts in sequences , [start:end]

[1:5] is equivalent to "from 1 to 5" (5 not included)
[1:] is equivalent to "1 to end"
[len(a):] is equivalent to "from length of a to end"

Watch https://youtu.be/tKTZoB2Vjuk?t=41m40s at around 40:00 he starts explaining that.

Works with tuples and strings, too.

slicing operator. http://docs.python.org/tutorial/introduction.html#strings and scroll down a bit

a[len(a):] - This gets you the length of a to the end. It selects a range. If you reverse a[:len(a)] it will get you the beginning to whatever is len(a).