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Exercise 5

Which of the following values can't be used as a key in a dict object, and why?

'cat'
(3, 1, 4, 1, 5, 9, 2)
['a', 'b']
{'a': 1, 'b': 2}
range(5)
{1, 4, 9, 16, 25}
3
0.0
frozenset({1, 4, 9, 16, 25})

Solution

The following items can't be used as keys:

['a', 'b']
{'a': 1, 'b': 2}
{1, 4, 9, 16, 25}

The first value is a list, the second another dict, and the last a set. Since all 3 types are mutable, they can't be used as dict keys. All remaining items are immutable built-in objects; they are acceptable dict keys.

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