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.

Python 2.7 urllib の例と、issue でのプロトコル、オプションのメソッド、シグネチャに関する議論から始めます。条件付きで利用可能なメンバーを表すにはどの宣言メカニズムを使うべきかを定義し、続いて、それらのシグネチャが引き続き認識され、条件付きの属性アクセスが mypy に受け入れられることを確認します。