Optional class and protocol fields and methods #601
Description
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.nextCurrently 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.