[Python] How to Check the Actual Type of a Generic Parameter in a Python Generic Function to...

I am working with a generic function in Python and trying to understand how to properly check the actual type of a generic parameter to narrow the type of all parameters bound to the same generic type variable T. Here is an example of my code:


T = TypeVar('T', int, str)

def foo(a: T, b: T) -> None:
if isinstance(a, int):
b.as_integer_ratio() # Type checker error


When I run this code a type checker (e.g., Pylance), I get the following error:

Cannot access attribute "as_integer_ratio" for class "str*"
Attribute "as_integer_ratio" is unknown
Pylance(reportAttributeAccessIssue)


I expected that since a and b are both annotated with the same type variable T, checking that a is an int would also narrow b to int. Why does the type checker still consider b to potentially be a str?

My questions are:

• Why does the type checker raise this error? I thought the type variable T ensures that a and b are always of the same type.
• How can I correctly express that a and b must be of the same type, and narrow their types based on a runtime check like isinstance?
• Under what circumstances would type narrowing for all parameters of type T fail or lead to errors?

Any insights or examples on how to handle type narrowing with generics in Python would be greatly appreciated!

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