Typehinting an unpack assignment [duplicate]

Question

Does anyone know how to properly typehint an unpack assignment - if it's even possible?

To clarify what I mean:

data = some_function_without_typehint()

x: str = data[0]
y: int = data[1]
# x any y have typehint support

x,y = data # no typehint support

# is anything like this possible in v3.8+ ?
x: str, y: int = data
x,y : str, int = data

Answer 1

Variable annotations aren't allowed with unpacking. You have to annotate the names separately from the actual assignment.

x: str
y: str
x, y = data

The rules can be seen in the definition of an annotated assignment

annotated_assignment_stmt ::=  augtarget ":" expression
                               ["=" (starred_expression | yield_expression)]

and the definition of an augtarget:

augtarget                 ::=  identifier | attributeref | subscription | slicing

The lack of multiple targets is explicitly called out:

The difference from normal Assignment statements is that only single target is allowed.

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(It's not entirely clear to me why you can't annotate the targets individually. I usually just assume it has something to do with ambiguity and LL(1) grammars. Too much lookahead needed, perhaps, to tell if x: str, y is x annotated with str, y or x annotated with str and y being another target.)

(After typing that, it seems likely. While the new PEG grammar is not so restricted, there may not be a compelling reason to change existing grammar rules to take advantage of it.)