More ideas about plugin system

After some time spent on writing SQLAlchemy plugin and a discussion with @JukkaL here are some comments and ideas about our plugin :

• Currently, member signature hook is not called on __init__ and __new__. Current workaround is to use function hook, but it is still painful. The solution would require moving type_object_type() (and maybe a bunch of other helpers) from checkmember.py to a separate module (which is probably a good idea on its own). We can also move some helpers from checker.py related to type objects to the same module.

• During semantic analysis types of expressions are not known. This poses a problem in attrs and even more in SQLAlchemy: When generating an __init__ method for a model, plugin needs to figure out what is a type of attributes from definitions like name = Column(String(40)). Currently I do this using some heuristics that works in 95% of cases (and so does attrs plugin), but a better solution is probably to just store names of "magic" classes (SQLAlchemy models, dataclasses etc.) in the plugin state, add a dummy __init__ to symbol table, and defer figuring out actual type to the method signature hook that is called in type checking (depends on fixing previous bullet).

• It is better to move calling semantic analysis hooks to the very end, or (probably even better) just allow them to defer current target (but this is currently not possible).

• We should not allow most type operations in semantic analysis hooks (we can't enforce it, but should at least document it clearly). Some operations are still OK (like map_instance_to_supertype() or erase_type()).

• The type checking hooks should also be able to defer current target using a special return value or an exception (e.g. __init__ signature hook in the SQLAlchemy example above if one would use a user-defined type in an import cycle).

Probably, most (or all) of decisions should be made after the upcoming semanal.py refactoring.

(Also function hook is inconvenient, but @gvanrossum already mentioned this in https://github.com/python/mypy/issues/5409 that has a bunch of good ideas about improving plugin system.)

Beginne damit, die pluginbezogene Logik in checkmember.py und checker.py zu lesen, und prüfe anschließend die geplante Refaktorierung von semanal.py sowie das verknüpfte Issue #5409. Das Issue führt mehrere Designfragen zu Hooks für semantische Analyse und Typprüfung auf, definiert jedoch weder eine einzelne Änderung noch ein konkretes Abschlusskriterium.