问题
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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?
回答1:
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
tuplemethod which will copy only when necessary.
回答2:
Because otherwise there wouldn't be an immutable sequence type! If you want a mutable tuple you just use a list.
Using immutable types when appropriate has various performance perks, and you couldn't easily use a dict with tuple keys if they were made mutable.
来源：https://stackoverflow.com/questions/9714815/why-tuple-is-not-mutable-in-python