[Python-ideas] Runtime types vs static types
greg.ewing at canterbury.ac.nz
Sun Jul 2 18:59:15 EDT 2017
Steven D'Aprano wrote:
> isinstance(, List[int])
> isinstance(, List[str])
> How can a single value be an instance of two mutually incompatible
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
> 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
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
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