[issue24912] The type of cached objects is mutable

[Mark Shannon]

Mark Shannon added the comment:

Please revert c0d25de5919e.
Breaking the interpreter in order to facilitate some obscure use case is unacceptable.

If you want to do fancy stuff with modules, fine. Take a look at the source of the six module 
https://bitbucket.org/gutworth/six/src/cd1e81d33eaf3d6944f859c2aa7d5d3f515013c8/six.py?at=default for some tips.

I think immutability is a fundamental property of an object. The "consenting" adults ideas is fine for accessibility. However, making previously immutable object mutable forces developers to use lots of defensive copying and causes obscure bugs (like this one).

I do not regard the immutable of numbers as an historical accident, but as vitally important for any sort of numerical reasoning.
Just take a look at a language with mutable strings to see the problems that causes.

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<http://bugs.python.org/issue24912>
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