[Python-Dev] Re: Pre-PEP: Mutable keys in dictionaries

[Guido van Rossum]

> Can __temporary_immutable__ work on a multi-threaded system?  What is to 
> prevent the (known to be mutable) key from changing underneath you during 
> the lookup?  Perhaps I am misunderstanding, but it seems to me that you 
> must either copy the mutable key or be able to block all writers to the key 
> during the lookup.

The sets module has this problem now.  Our approach to this is to
document that sets are not thread-safe.

--Guido van Rossum (home page: http://www.python.org/~guido/)