def get_from_dict(d: dict, key: str, value_type = int) -> type(value_type):
    value = d.get(key)
    if value is None:
        raise RuntimeError(f"Tried to fetch {key} from dictionary {d} but dictionary did not have the key.")

    return value_type(value)

import inspect
from functools import wraps
from typing import Any

def set_rettype_as_var(variable_name):
    def decorator(f):
        @wraps(f)
        def wrapper(*args, **kwargs_inner):
            return f(*args, **kwargs_inner)
        
        desired_type = Any
        signature = inspect.getfullargspec(f)
        if signature.kwonlydefaults and variable_name in signature.kwonlydefaults:
            desired_type = type(signature.kwonlydefaults.get(variable_name))
            # probably, you can take into account signature.annotations ... 
            
        if signature.args and variable_name in signature.args:
            if signature.defaults is not None:
                desired_type = type(signature.defaults[signature.args.index(variable_name)])
            else:
                desired_type = signature.annotations.get(variable_name, Any)
            
        # maybe you will need to define some other conditions .... 
        
        wrapper.__annotations__['return'] = desired_type
        return wrapper
    return decorator

@set_rettype_as_var('x')
def a(x:float):
    pass

a.__annotations__

Error:
{'x': float, 'return': float}