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.)

Start by reading the plugin-related logic in checkmember.py and checker.py, then review the planned semanal.py refactoring and the linked issue #5409. The issue lists several design questions about semantic-analysis and type-checking hooks, but it does not define a single change or a concrete done criterion.