What types of elements can be in a set in Python?

In a set in Python, only hashable (immutable) objects can be included, since sets are implemented based on a hash table. This means that the elements of a set must be immutable and have a __hash__ method.

Examples of permissible elements:

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

Disallowed elements:

• Lists (list)
• Dictionaries (dict)
• Sets (set)

Example:

s = {1, 'hello', (2, 3)}  # correct

# s = {1, [2, 3]}  # will raise a TypeError because list is not hashable