Guido van Rossum
The link between hashing and immutability is because objects whose hash would change are common, e.g. lists, and using them as dict keys would be very hard to debug for users most likely to make this mistake. […]
[…] But the real question here isn't "why aren't all things hashable" but "why can't you put mutable values into a set". […]
Hashing comes into play because all of Python's common data structures use hashing to optimize lookup -- but if we used a different data structure, e.g. something based on sorting the keys, we'd still have the mutability problem. And we'd have worse problems, because values would have to be sortable, which is a stricter condition than being immutable.
In any case, you can't solve this problem by making all values hashable.
That was a great explanation; you answered several points on which I was vague, and you addressed some things I didn't even know were problems. I'd love to see that edited to a blog post we can reference in a single article, if you have the time. -- \ “I went to the museum where they had all the heads and arms | `\ from the statues that are in all the other museums.” —Steven | _o__) Wright | Ben Finney