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Why are python strings and tuples are made immutable?

What lower-level design makes tuple not mutable in Python? Why this feature is useful?

A few reasons:

• Mutable objects like lists cannot be used as dictionary keys or set members in Python, since they are not hashable. If lists were given __hash__ methods based on their contents, the values returned could change as the contents change, which violates the contract for hash values.
• If Python only had mutable sequences, constructors which accepted sequences would often need to copy them to ensure that the sequences couldn't be modified by other code. Constructors can avoid defensive copying by only accepting tuples. Better yet, they can pass sequence arguments through the tuple method which will copy only when necessary.