structural.py

from typing import Protocol

class MyProto(Protocol):
   value: str

@dataclass
class A:
   value: str

@dataclass
class B:
   value: str
   image: str

def my_func(obj: MyProto) -> str:  # note the changed the type annotation
    return obj.value
my_func(B("hi", "a gzipped photo of a cat"))  # no error