How to type hint with walrus operator?

Question

I am trying to type hint a walrus operator expression, i.e.

while (var: int := some_func()): ...

How can I do this?

Acerbate

Answer 1

It's not possible. From PEP 572

Inline type annotations are not supported:

You need to declare the variable before the while loop, and you can specify the type there.

var: int
while var := some_func():
    ...

Fukuoka

Is the = 0 necessary? It seems like var: int would be enough. – Brennen

If you don't initialize it with an integer, you need var: Optional[int] so it doesn't violate the type. – Fukuoka

Interesting. var: int doesn't actually create var, so it never has the value None. Why would the type be violated? – Brennen

I was just going by what the PEP said. It suggested var: Optional[int] = None – Fukuoka

I don't think the Optional hint is required; it's just the one used as an example. var: int alone should be fine, as long as some_func is guaranteed to return an int. – Protonema

Answer 2

I don't believe you can.

A variable can be annotated because the grammar rule for assignment is

assignment:
    | NAME ':' expression ['=' annotated_rhs ] 

    ...

Note that the type hint is explicit between the : following the name and the =.

An assignment expression, on the other hand, only provides for a name, no type hint, preceding the :=:

named_expression:
    | NAME ':=' ~ expression 
    | expression !':='

Protonema

this seems like an oversight? I would expect that this operator would be implemented with proper typing – Acerbate

Python's grammar is intentionally restricted so that it can be parsed by an LL(1) parser. I'm fuzzy on the details (it's been a long time since I've studied parsers), but I think something like if x: int := f(y): can't be expressed with an LL(1) grammar; the lookahead needed to tell if the : following the x introduces a type hint or finishes the condition simply isn't available to an LL(1) parser. Python now uses a PEG parser, which I think is much more flexible, but := predates the introduction of the new parser. – Protonema

The walrus operator seems like an incomplete/flawed concept if it can't even be used with instance variables and type hints. – Silvanasilvano