Narrowing Variable Types

[sghegoiu-rokos]

Hi,

Was wondering what the suggested method is for narrowing a variable type when getting it with Variable.get('test')

Is it possible to tell the type checker that the value of test is a string? Would be very helpful

[bdalpe]

Hi @sghegoiu-rokos,

Yes, you can include a type hint as part of your code. This allows your IDE or type checker to statically validate the type.

from prefect import flow
from prefect.variables import Variable

@flow
def my_flow():
    var: str = Variable.get("my_variable")  # note the addition of `: str` type hint after the variable name here
    # ... rest of the flow logic ...

NB: This type hint is not enforced at runtime. If you need runtime type enforcement, you'll need to use isinstance conditional statements.

[sghegoiu-rokos]

thanks for the answer @bdalpe, unfortunately that doesnt seem to work for me

[bdalpe]

Based on your screenshot, I'm assuming you're using VSCode as your IDE?

Which version of Prefect do you have installed as part of your project dependencies?

[sghegoiu-rokos]

that's correct - ive tried with both pylance and pyright

[bdalpe]

A couple of options here:

1. Use cast (type-only, no runtime check)

from typing import cast

var: str = cast(str, Variable.get("my_variable"))

2. Force it to be a string at runtime

var = str(Variable.get("my_variable"))

3. Tell Prefect you expect a string (and explicitly validate at runtime that it is one)

v = Variable.get("my_variable")
if not isinstance(v, str):
    raise TypeError(f"Expected str for my_variable, got {type(v).__name__}")
var: str = v

[sghegoiu-rokos]

Yes, I can get around it with cast. A somewhat nicer interface would be to allow people to declare (or perhaps infer) when a Variable is declared its type - that way Prefect could offer type safety & narrowing directly.

[zzstoatzz]

inferring might be tricky in general, but we could allow something like this

var = Variable[str].get("my_variable")

which sounds like a reasonable feature request

[sghegoiu-rokos]

that is exactly what im looking for

[bdalpe]

Created #19752 as a feature enhancement request

[zzstoatzz]

this is merged!