 Python makes a distinction between lists and tuples . Lists are written as [ 1 , 2 , 3 ] , are mutable , and cannot be used as the keys of dictionaries ( dictionary keys must be immutable in Python ) . <unk> are written as ( 1 , 2 , 3 ) , are immutable and thus can be used as the keys of dictionaries , provided all elements of the tuple are immutable . The parentheses around the tuple are optional in some contexts . <unk> can appear on the left side of an equal sign ; hence a statement like x , y = 
