Optional class and protocol fields and methods

Sometimes, implementations check for the existence of a method on an object before using it. Take this example from Python 2.7's urllib:

class addbase:
    def __init__(self, fp):
        self.fp = fp
        self.read = self.fp.read
        self.readline = self.fp.readline
        if hasattr(self.fp, "readlines"): self.readlines = self.fp.readlines
        if hasattr(self.fp, "fileno"):
            self.fileno = self.fp.fileno
        else:
            self.fileno = lambda: None
        if hasattr(self.fp, "__iter__"):
            self.__iter__ = self.fp.__iter__
            if hasattr(self.fp, "next"):
                self.next = self.fp.next

Currently this is best modeled by leaving these methods out from a protocol. But this means that their signature is unknown, and mypy will complain about the non-existing attribute. It would be useful to be able to model this somehow, for example by using a decorator for such methods.

Start with the Python 2.7 urllib example and the issue's discussion of protocols, optional methods, and signatures. Define what declaration mechanism should represent conditionally available members, then verify that their signatures remain known and that conditional attribute access is accepted by mypy.