Steven D'Aprano <steve+comp.lang.pyt...@pearwood.info> writes:
>> L[0].append(5) # mutate L, in some reasonable sense of "mutate"
>
> You haven't mutated the tuple called "L". You've mutated its internal
> components, which are lists. If you wanted them to also be immutable, you
> should have used tuples :)
Obviously you are correct in one reasonable sense of "mutate". In
another sense, if L is immutable and you evaluate a purely computational
function on it two different times, you should get the same result both
times; for example, if you pickle L twice you should get two identical
pickles. That's what I think of "immutable" as meaning, and obviously
isn't the case with this L.
> The real point I was trying to make is that there are two definitions of
> "constant" that we care about: immutability and resistance to name re-
> binding. (Unbindability?) Python already gives us all the tools we need
> for immutability (modulo the usual disclaimers about "we're all adult
> here", "private attributes are private by convention", etc.). What Python
> doesn't have is any way of prohibiting name rebinding short of creating a
> new reserved word like None.
I think I'd also want to be able to make instance attributes
un-rebindable. E.g.
class Foo(object):
@const
def wibble(self, a): ...
it shouldn't be possible to redefine x.wibble if x is a Foo. That
should allow for compile-time binding in a lot of practical code.
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