A slicing (or slice) selects a range of items in a sequence (string, tuple or list) object:
slicing: primary "[" [condition] ":" [condition] "]"
The primary must evaluate to a sequence object. The lower and upper
bound expressions, if present, must evaluate to plain integers;
defaults are zero and the sequence's length, respectively. If either
bound is negative, the sequence's length is added to it. The slicing
now selects all items with index k such that
i <= k < j where i
and j are the specified lower and upper bounds. This may be an
empty sequence. It is not an error if i or j lie outside the
range of valid indexes (such items don't exist so they aren't
selected).