Why is using mutable objects as keys in a Python dictionary not allowed?

In Python, dictionary keys must be hashable and immutable objects. Mutable objects, such as lists or dictionaries, cannot be used as keys because:

• Hash must remain constant: When a key is added to a dictionary, its hash is computed. If the object changes, the hash will change, and the dictionary will not be able to find that key.
• Violation of dictionary integrity: Changing a key after adding it will lead to errors when searching or deleting elements.

For example, attempting to use a list as a key will cause an error:

my_dict = {}
key = [1, 2, 3]
my_dict[key] = 'value'  # TypeError: unhashable type: 'list'

Therefore, immutable types such as strings, numbers, and tuples (if they contain only immutable elements) are used as keys.