How do Python dictionaries handle keys and values, and what makes a key valid?

How do Python dictionaries handle keys and values, and what makes a key valid?

• A. Values are stored in key-value pairs.
• B. Keys must be hashable and unique; values are stored in key-value pairs.
• C. Keys can be any object; duplicates allowed.
• D. Dictionaries require keys to be strings only.

Answer: (B)

Dictionaries store data as pairs that map each key to a value. For a key to be valid, it must be hashable and unique within the dictionary. Hashable means Python can compute a stable hash value for the key and compare it reliably to find the right entry; this usually requires the key to be immutable. Because of that, common valid keys are strings, numbers, or tuples of immutable elements. Mutable objects like lists or dicts can’t be keys because their contents can change, which would break the hashing and lookups. Values can be any Python object, and different keys can map to the same value. If you assign a value to an existing key, the old value is replaced with the new one, since keys must be unique inside a dictionary. This combination—hashable, unique keys with arbitrary values—lets dictionaries quickly locate and retrieve the corresponding value for any given key.

Dictionaries store data as pairs that map each key to a value. For a key to be valid, it must be hashable and unique within the dictionary. Hashable means Python can compute a stable hash value for the key and compare it reliably to find the right entry; this usually requires the key to be immutable. Because of that, common valid keys are strings, numbers, or tuples of immutable elements. Mutable objects like lists or dicts can’t be keys because their contents can change, which would break the hashing and lookups.

Values can be any Python object, and different keys can map to the same value. If you assign a value to an existing key, the old value is replaced with the new one, since keys must be unique inside a dictionary. This combination—hashable, unique keys with arbitrary values—lets dictionaries quickly locate and retrieve the corresponding value for any given key.