New submission from Pekka Klärck <pekka.kla...@gmail.com>: = Introduction =
In Python 3.5 and 3.6 types defined in the typing module are instances of `type` and also subclasses of the "real" type they represent. For example, both `isinstance(typing.List, type)` and `issubclass(typing.List, list)` return true. In Python 3.7 the former returns false and the latter causes a TypeError. I could find anything related to these changes in the Python 3.7 release notes or from the documentation of the typing module. I explain my use case and the problems these changes have caused below. = Use case = I'm implementing automatic argument conversion to Robot Framework, a generic open source test automation framework, based on function annotations. The idea is that if a user has defined a keyword like def example(arg: int): # ... we can convert argument passed in plain text test data like Example 42 into the correct type automatically. For more details see this issue in our tracker: https://github.com/robotframework/robotframework/issues/2890 = Problem 1 = I have implemented converters for different types and use annotations to find out the expected type for each argument. To exclude non-type annotations, my code uses `isinstance(annotation, type)` but in Python 3.7 this excludes also types defined in the typing module. I could apparently use `isinstance(annoation, (type, typing._GenericAlias))`, but touching private parts like is fragile and feels wrong in general. = Problem 2 = Each converter I've implemented is mapped to a certain type (e.g. `list`) and, when applicable, also to an abc (e.g. `collections.abc.MutableSequence`). When finding a correct converter for a certain type, the code uses an equivalent of `issubclass(type_, (converter.type, converter.abc))`. In Python 3.5 and 3.6 this works also if the used type is defined in the typing module but with Python 3.7 it causes a TypeError. I guess I could handle the types in the typing module by explicitly mapping converters also to these types (e.g. `typing.List`) and then using something like `type_ is converter.typing`. The problem is that although it would work with types like `List`, it wouldn't work if types are used like `List[int]`. ---------- messages: 324518 nosy: pekka.klarck priority: normal severity: normal status: open title: Types in `typing` not anymore instances of `type` or subclasses of "real" types _______________________________________ Python tracker <rep...@bugs.python.org> <https://bugs.python.org/issue34568> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com