There has been some discussion here and there concerning the differences between runtime types and static types (mypy etc.). What I write below is not really an idea or proposal---just a perspective, or a topic that people may want to discuss. Since the discussion on this is currently very fuzzy and scattered and not really happening either AFAICT (I've probably missed many discussions, though). Anyway, I thought I'd give it a shot: Clearly, there needs to be some sort of distinction between runtime classes/types and static types, because static types can be more precise than Python's dynamic runtime semantics. For example, Iterable[int] is an iterable that contains integers. For a static type checker, it is clear what this means. But at runtime, it may be impossible to figure out whether an iterable is really of this type without consuming the whole iterable and checking whether each yielded element is an integer. Even that is not possible if the iterable is infinite. Even Sequence[int] is problematic, because checking the types of all elements of the sequence could take a long time. Since things like isinstance(it, Iterable[int]) cannot guarantee a proper answer, one easily arrives at the conclusion that static types and runtime classes are just two separate things and that one cannot require that all types support something like isinstance at runtime. On the other hand, there are many runtime things that can or could be done using (type) annotations, for example: Multidispatch (example with hypothetical syntax below): @overload def concatenate(parts: Iterable[str]) -> str: return "".join(parts) @overload def concatenate(parts: Iterable[bytes]) -> bytes: return b"".join(parts) @overload def concatenate(parts: Iterable[Iterable]) -> Iterable: return itertools.chain(*parts) or runtime type checking: @check_types def load_from_file(filename: Union[os.PathLike, str, bytes]): with open(filename) as f: return do_stuff_with(f.read()) which would automatically give a nice error message if, say, a file object is given as argument instead of a path to a file. However useful (and efficient) these things might be, the runtime type checks are problematic, as discussed above. Furthermore, other differences between runtime and static typing may emerge (or have emerged), which will complicate the matter further. For instance, the runtime __annotations__ of classes, modules and functions may in some cases contain something completely different from what a type checker thinks the type should be. These and other incompatibilities between runtime and static typing will create two (or more) different kinds of type-annotated Python: runtime-oriented Python and Python with static type checking. These may be incompatible in both directions: a static type checker may complain about code that is perfectly valid for the runtime folks, and code written for static type checking may not be able to use new Python techniques that make use of type hints at runtime. There may not even be a fully functional subset of the two "languages". Different libraries will adhere to different standards and will not be compatible with each other. The split will be much worse and more difficult to understand than Python 2 vs 3, peoples around the world will suffer like never before, and programming in Python will become a very complicated mess. One way of solving the problem would be that type annotations are only a static concept, like with stubs or comment-based type annotations. This would also be nice from a memory and performance perspective, as evaluating and storing the annotations would not occupy memory (although both issues and some more might be nicely solved by making the annotations lazily ealuated). However, leaving out runtime effects of type annotations is not the approach taken, and runtime introspection of annotations seems to have some promising applications as well. And for many cases, the traditional Python class actually acts very nicely as both the runtime and static type. So if type annotations will be both for runtime and for static checking, how to make everything work for both static and runtime typing? Since a writer of a library does not know what the type hints will be used for by the library users, it is very important that there is only one way of making type annotations which will work regardless of what the annotations are used for in the end. This will also make it much easier to learn Python typing. Regarding runtime types and isinstance, let's look at the Iterable[int] example. For this case, there are a few options: 1) Don't implement isinstance This is problematic for runtime uses of annotations. 2) isinstance([1, '2', 'three'], Iterable[int]) returns True This is in fact now the case. This is ok for many runtime situations, but lacks precision compared to the static version. One may want to distinguish between Iterable[int] and Iterable[str] at runtime (e.g. the multidispatch example above). 3) Check as much as you can at runtime There could be something like Reiterable, which means the object is not consumed by iterating over it, so one could actually check if all elements are instances of int. This would be useful in some situations, but not available for every object. Furthermore, the check could take an arbitrary amount of time so it is not really suitable for things like multidispatch or some matching constructs etc., where the performance overhead of the type check is really important. 4) Do a deeper check than in (2) but trust the annotations For example, an instance of a class that has a method like def __iter__(self) -> Iterator[int]: some code could be identified as Iterable[int] at runtime, even if it is not guaranteed that all elements are really integers. On the other hand, an object returned by def get_ints() -> Iterable[int]: some code does not know its own annotations, so the check is difficult to do at runtime. And of course, there may not be annotations available. 5) Something else? And what about PEP544 (protocols), which is being drafted? The PEP seems to aim for having type objects that represent duck-typing protocols/interfaces. Checking whether a protocol is implemented by an object or type is clearly a useful thing to do at runtime, but it is not really clear if isinstance would be a guaranteed feature for PEP544 Protocols. So one question is, is it possible to draw the lines between what works with isinstance and what doesn't, and between what details are checked by isinstance and what aren't? -- Or should insinstance be reserved for a more limited purpose, and add another check function, say `implements(...)`, which would perhaps guarantee some answer for all combinations of object and type? I'll stop here---this email is probably already much longer than a single email should be ;) -- Koos -- + Koos Zevenhoven + http://twitter.com/k7hoven +