Answer:
In Python, dictionary keys must be of an immutable data type. This means that you can use certain types of objects as keys, while others are not allowed. The reason for this restriction is related to how dictionaries are implemented and how they efficiently retrieve and store values based on keys.
Here are the key characteristics of objects that can be used as keys in dictionaries:
1. Immutable: Keys must be of a data type that cannot be modified after creation. Immutable objects are constant and cannot change their state or value. This is important because if a key were to change its value, it could lead to unpredictable behavior when trying to look up values in the dictionary.
2. Hashable: Dictionaries use a hashing mechanism to efficiently store and retrieve values based on keys. Hashing requires that the key's value doesn't change and that the same key always produces the same hash value. Immutable objects like numbers, strings, tuples, and frozensets have consistent hash values, making them suitable for use as dictionary keys.
Here are some examples of objects that can be used as keys in dictionaries:
- Integers: e.g., `1`, `42`, `-5`
- Strings: e.g., `"apple"`, `"banana"`, `"hello"`
- Tuples (containing only hashable elements): e.g., `(1, 2, 3)`, `("John", "Doe")`
- Frozensets (since they are immutable): e.g., `frozenset([1, 2, 3])`
Here are examples of objects that cannot be used as keys in dictionaries:
- Lists (since they are mutable): e.g., `[1, 2, 3]`
- Dictionaries (since they are mutable): e.g., `{"name": "John", "age": 30}`
- Sets (since they are mutable): e.g., `set([1, 2, 3])`
When you try to use a mutable object as a dictionary key, Python will raise a `TypeError` because it cannot guarantee the immutability and consistent hashing required for dictionary keys.
In summary, dictionary keys in Python must be of an immutable and hashable data type to ensure the integrity and efficiency of dictionary operations. Immutable objects like numbers, strings, tuples, and frozensets are commonly used as keys.
Step-by-step explanation: