["python"]
Colon (:) in Python list index

I'm new to Python. I see : used in list indices especially when it's associated with function calls. Python 2.7 documentation suggests that lists.append translates to a[len(a):] = [x]. Why does one need to suffix len(a) with a colon? I understand that : is used to identify keys in dictionary.
: 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.