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Restrictions on Dictionary Keys and Values

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Almost any type of value can be used as a dictionary key in Python. You can even use built-in objects like types and functions. However, there are a couple restrictions that dictionary keys must abide by.

First, a given key can appear in a dictionary only once. Duplicate keys are not allowed. A dictionary maps each key to a corresponding value, so it doesn’t make sense to map a particular key more than once. If you specify a key a second time during the initial creation of a dictionary, then the second occurrence will override the first.

Second, a dictionary key must be of a type that is immutable. For example, you can use an integer, float, string, or Boolean as a dictionary key. However, neither a list nor another dictionary can serve as a dictionary key, because lists and dictionaries are mutable. Values, on the other hand, can be any type and can be used more than once.

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