> What I can’t understand is the argument that occasional use of `isinstance` is bad, but also that pervasive nominal type-checking via annotations is good. it

> What I can’t understand is the argument that occasional use of `isinstance` is bad, but also that pervasive nominal type-checking via annotations is good. it doesn't have to be nominal, MyPy supports structural sub-typing through Protocols [1] [1] https://mypy.readthedocs.io/en/stable/protocols.html https://mypy.readthedocs.io/en/stable/protocols.html