Sets in Python

[Sion Arrowsmith]

sapsi  <saptarshi.guha at gmail.com> wrote:
> Why can't lists be hashed?

Several people have answered "because they're mutable" without
explaining why mutability precludes hashing. So:

Consider a dict (dicts have been in Python a *lot* longer than
sets, and have the same restriction) which allowed lists as
keys:

d = {}
k = [1, 2]
d[k] = None

Now, if I were to do:

k.append(3)

what would you expect:

d.keys()

to return? Did d magically rehash k when it was modified? Did d[k]
take a copy of k, and if so, how deep was the copy (consider
d[[1, k]] = None followed by a modification to k)? Leaving the hash
unchanged and relying on collision detection to resolve won't work,
since you may go directly for d[[1, 2, 3]] and not spot that
there's already an entry for it since it's been hashed under [1, 2].

"Practicality beats purity" and the design decision was to simply
sidestep these issues by disallowing mutable dict keys. And as the
set implementation is based on the dict implementation, it applies
to sets to.

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