[Python-ideas] What should a good typechecker do?

[Stephen J. Turnbull]

Guido van Rossum writes:

 > But that's not what type comments mean! They don't annotate the
 > expression. They annotate the variable.

In PEP 484.  But syntactically, AFAICS in an initialization that's a
distinction without a difference.  It would be perfectly possible to
write a checker that allows

    if cond:
        x: str = "a string, what else?"
    else:
        x: int = 1

and infers the union, and even infers the types from the expressions
in

    if cond:
        x = "a string, what else?"
    else:
        x = 1

Once you have that, then the only things that are really being typed
are the initializer expressions.

I don't understand why some people seem to think that is a *good*
typechecker (if that's it's normal mode of operation).  But as a tool
to identify the types that untyped (or ambiguously-typed) code
actually uses, review them, and help produce a precisely typed
version, it seems perfectly plausible to me.