Steven D'Aprano wrote:
isinstance([], List[int])
isinstance([], List[str])
How can a single value be an instance of two mutually incompatible
types?
I don't think there's any contradiction there, because the
compatibility rules are different for static and runtime types.
Statically, when you assign something of type A to a variable
of type B, you're assering that *all* values of type A are
compatible with B. But at runtime, you're only asserting that
one *particular* value is compatible with B.
> It just seems
perverse to me to say that it is "meaningless" (in Mark's words) to ask
whether
isinstance(['a', 'b'], List[int])
isinstance(123, List[str])
for example). If static type checking has any meaning at all, then the
answers to those two surely have to be False.
I doubt whether Mark meant "separate" to imply "unrelated".
Static and runtime types are clearly related, although the
relationship is not one-to-one and involves complicated
overlaps.
To my mind, the question isn't whether tests like that are
meaningful -- clearly they are. The question is whether we
should attempt to support answering them at run time, given
that doing so in the general case requires unbounded amounts
of computation.
--
Greg
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