Why can't any data type be used as a dictionary key in Python?

In Python, a dictionary key can only be a hashable (immutable) object. This is because the dictionary is implemented as a hash table, and its hash is used for quick key lookup.

If the object is mutable (for example, a list or a dictionary), its hash may change over time, which would break the integrity of the data structure and lead to errors.

Therefore, keys must be:

• Immutable (for example, strings, numbers, tuples of immutable elements).
• Implementing the __hash__ and __eq__ methods to compare and index correctly.

Example:

# Correct
my_dict = {('a', 1): 'value'}

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