What can be a key in a Python dictionary?

In Python, a dictionary key can be any immutable and hashable object. This means that the key must have:

• A __hash__() method that returns a hash code.
• An __eq__() method for comparison with other keys.

Typical examples of keys:

• Numbers (int, float, bool)
• Strings (str)
• Tuples (tuple), if all elements inside are also hashable
• frozenset

Example:

my_dict = {
    42: 'number',
    'key': 'string',
    (1, 2): 'tuple'
}

Mutable types such as lists, sets, or dictionaries cannot be keys because their contents can change, which would break the integrity of the hash table.