On 05/09/16 23:16, Greg Ewing wrote:
Mark Shannon wrote:
Unless of course, others may have a different idea of what the "type
of a variable" means.
To me, it means it means that for all assignments `var = expr`
the type of `expr` must be a subtype of the variable,
and for all uses of var, the type of the use is the same as the type
of the variable.
I think it means that, at any given point in time, the
value of the variable is of the type of the variable or
some subtype thereof. That interpretation leaves the
type checker free to make more precise inferences if
it can. For example, in...
How does that differ from annotating the type of the expression?
def foo()->int:
x:Optional[int] = bar()
if x is None:
return -1
return x
...the type checker could notice that, on the branch
containing 'return x', the value of x must be of type
int, so the code is okay.
The issue is not whether the checker can tell that the type of the
*expression* is int, but whether it is forced to use the type of the
*variable*. The current wording of PEP 526 strongly implies the latter.
Cheers,
Mark.
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