In a dictionary, keys must be:

In a dictionary, keys must be immutable and unique. This means that the keys cannot be changed after they are created, which is a requirement to maintain the integrity and consistency of the dictionary data structure. Immutable data types in Python include numbers, strings, and tuples. When a key is immutable, it can be hashed, which allows Python to efficiently locate the key and its associated value.

Additionally, keys must be unique within a dictionary; if the same key is used to insert another value, it will overwrite the previous value associated with that key. This ensures that each key presents a distinct reference point for retrieving the corresponding value in a dictionary.

In contrast, mutable types, such as lists and dictionaries themselves, cannot be used as keys because their contents can change, leading to ambiguity and inconsistencies when trying to access values. Thus, the requirement for uniqueness and immutability is crucial for the functionality of dictionaries in Python.