文字列、タプルまたはリストの、要素の範囲を選択できます。
slicing ::= primary "[" slice_list "]"
slice_list ::= slice_item ("," slice_item)* [","]
slice_item ::= expression | proper_slice
proper_slice ::= [lower_bound] ":" [upper_bound] [ ":" [stride] ]
lower_bound ::= expression
upper_bound ::= expression
stride ::= expression
6.3.3. スライス表記 (slicing) - 6. 式 (expression) — Python 3.x ドキュメント