Fix rmm ._lib imports

[Matt711]

Description

This PR fixes a bug in #1676. It makes sure that rmm imports work correctly using both from rmm._lib... and import rmm._lib... syntax.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[Matt711]

These classes are cdef'd, why are they importable from python? Should they be cpdef'd?

[wence-]

A cdef class is a normal python class that can also hold C properties. (https://cython.readthedocs.io/en/latest/src/tutorial/cdef_classes.html)

cpdef is for (predominantly) functions which should have a "fast-path" calling route from typed code in cython, and also a "slow-path" python-like calling route.

[Matt711]

So its only cpdef functions that can be called from C and Python?

Edit: I think that's true https://cython.readthedocs.io/en/latest/src/userguide/pyrex_differences.html#cpdef-functions

[vyasr]

Not exactly.

• cdef functions can only be called from Cython and in typed contexts (i.e. you cannot pass arbitrary Python objects to them, you need to pass objects that have already been cdefed appropriately themselves).
• def functions can be called from Python or Cython. They are pure Python functions, so all arguments are treated as normal Python objects.
• cpdef functions can be called from Python or Cython. When called from Python, they behave like def functions. When called from Cython with untyped arguments, they also behave like def functions. When called from Cython with typed arguments, they behave like cdef functions.

There's more information here.

[Matt711]

Thanks for the clarification (def and cpdef make sense to me). Can you give an example of a "typed context"?

[Matt711]

Can a typed context be in a *.py file?

[Matt711]

Is it the body of cpdef and cdef functions/methods?

[vyasr]

cdef class A:
    cpdef class f(self):
        pass

A().f()  # Called like a Python def function
cdef A a = A()
a.f() # Called like a cdef function.

So no, a typed context cannot be in a *.py file, or at least not in the way that we use Cython. With newer versions of Cython you can use py files and compile them too, but the point is that you have to use Cython-specific syntax.

[vyasr]

LGTM. I notice that helper.py is missing, but since that only contains a cdef function it's correct not to have it here since it doesn't expose anything that would be visible to Python.

[vyasr]

I'm going to go ahead and merge because this is currently breaking some users. If we have more changes that we want, we can address them in follow-ups, but I'd like to be able to experiment ASAP to make sure that the underlying issues are resolved in all cases.

[vyasr]

/merge