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Yeah I agree with you completely, I think we just use different terminology to describe the same thing. It sounds like you use "type" to mean "concrete type", i

Yeah I agree with you completely, I think we just use different terminology to describe the same thing. It sounds like you use "type" to mean "concrete type", i.e. the thing it actually is, and not the thing it acts like, and then "protocol/interface" to describe the behaviour, the thing we can use it as. As somebody who mostly uses statically-typed languages, I use "type" to mean "the thing my static type checker knows this as", which could be a concrete type or simply an interface, and then obviously I use the term "concrete type" when I need to be more explicit. I like the idea of a hasprotocol or supportsprotocol predicate, certainly over usage of isinstance, since one of the beauties of duck typing is that it works without requiring the type hierarchy. The tricky part will be in allowing you to define your own protocols, like "has these attributes/methods". An example would be the difference (or lack thereof) between classes and namedtuples.

benhoyt 10y ago
Interestingly, I think that's what isinstance() actually does when it's called on abstract base classes like collections.abc.Iterable -- it looks to see whether the type supports __iter__ and whatnot. See the abc module and it's implementation here: * https://docs.python.org/3.4/library/collections.abc.html https://docs.python.org/3.4/library/collections.abc.html * https://hg.python.org/cpython/file/3.4/Lib/_collections_abc.py https://hg.python.org/cpython/file/3.4/Lib/_collections_abc....
- clusmore 10y ago
  Pasting the relevant snippet from your link: def __subclasshook__(cls, C): if cls is Iterable: if any("__iter__" in B.__dict__ for B in C.__mro__): return True return NotImplemented Wow, hadn't seen this before. That's exactly what I mean. If I can define my own, e.g. Addable, which does a similar check for the presence of __add__ (and __radd__) and MyPy uses the subclasshook, then that'd be perfect.
  - dbcurtis 10y ago
    Get _Fluent_Python_ by Ramalho and read chapter 11.

benhoyt 10y ago

Interestingly, I think that's what isinstance() actually does when it's called on abstract base classes like collections.abc.Iterable -- it looks to see whether the type supports __iter__ and whatnot. See the abc module and it's implementation here: * https://docs.python.org/3.4/library/collections.abc.html https://docs.python.org/3.4/library/collections.abc.html * https://hg.python.org/cpython/file/3.4/Lib/_collections_abc.py https://hg.python.org/cpython/file/3.4/Lib/_collections_abc....

clusmore 10y ago
Pasting the relevant snippet from your link: def __subclasshook__(cls, C): if cls is Iterable: if any("__iter__" in B.__dict__ for B in C.__mro__): return True return NotImplemented Wow, hadn't seen this before. That's exactly what I mean. If I can define my own, e.g. Addable, which does a similar check for the presence of __add__ (and __radd__) and MyPy uses the subclasshook, then that'd be perfect.
- dbcurtis 10y ago
  Get _Fluent_Python_ by Ramalho and read chapter 11.