Tuples are immutable which makes them hashable and thus usable as keys for dictionaries, where (mutable) lists are not:
>>> d = {}
>>> l = [1, 2, 3]
>>> t = (1, 2, 3)
>>> d[l] = []
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: unhashable type: 'list'
>>> d[t] = []
>>>
#tuples #dicts