Inherit multiple cdef (mixin) classes

[jbrock...@gmail.com]

I have a python class that needs to subclass each of two cdef classes, which in general is not allowed.  The hope: if one of them is a mixin that doesn't implement __init__/__cinit__ or any cdef attributes, can this be worked around?

[da-woods]

My understanding is that it does work if the memory layouts are compatible (although I'm personally a little vague on what "compatible" means)

Certainly a genuinely empty cdef class + a "regular" cdef class does work:

cdef class C:
    cdef int a
    cdef float b

cdef class D:
    pass

then in Python I can successfully do:

class E(C, D):
   ...

However, if I add a cdef method to D then that creates a vtable pointer and you can no longer do this inheritance. Adding a def function doesn't break it though.

__init__ isn't a problem provided that you use super().

__cinit__ doesn't prevent you from subclassing it, although doesn't appear to get called for one of the classes (the rules about which one isn't obvious to me). Thus it's pretty pretty useless...

So it may work (under limited circumstances), but you may find it limited to the point of uselessness

David

On 04/04/2021 18:46, jbrock...@gmail.com wrote:

I have a python class that needs to subclass each of two cdef classes, which in general is not allowed.  The hope: if one of them is a mixin that doesn't implement __init__/__cinit__ or any cdef attributes, can this be worked around?

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[Brock Mendel]

That's very helpful, thank you.

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