Python C API - How to inherit from your own python class? - python-c-api

Question

The newtypes tutorial shows you how to inherit from a base python class. Can you inherit from your own python class? Something like this?
PyObject *mod = PyImport_AddModule("foomod");
PyObject *o = PyObject_GetAttrString(mod, "BaseClass");
PyTypeObject *t = o->ob_type;
FooType.tp_base = t;
if (PyType_Ready(&FooType ) < 0) return NULL;
though you need to define your struct with the base class as the first member per the documentation so it sounds like this is not possible? ie how would I setup the Foo struct?
typedef struct {
PyListObject list;
int state;
} SubListObject;
What I'm really trying to do is subclass _UnixSelectorEventLoop and it seems like my only solution is to define a python class that derives from my C class and from _UnixSelectorEventLoop with my C class listed first so that it can override methods in the other base class.

Answer 1

I think you're basically right on your assessment:
it seems like my only solution is to define a python class that derives from my C class and from _UnixSelectorEventLoop with my C class listed first so that it can override methods in the other base class.
You can't define a class that inherits from a Python class because it'd need to start with a C struct of basically arbitrary size.
There's a couple of other options that you might like to consider:
You could create a class the manual way by calling PyType_Type. See this useful answer on a question about multiple inheritance which is another sort of inheritance that the C API struggles with. This probably limits you too much, since you can't have C attributes, but you can have C functions.
You could do "inheritance by composition" - i.e. have you _UnixSelectorEventLoop as part of the object, then forward __getattr__ and __setattr__ to it in the event of unknown attributes. It's probably easier to see what I mean with Python code (which is simply but tediously transformed into C API code)
class YourClass:
def __init__(self,...):
self.state = 0
self._usel = _UnixSelectorEventLoop()
def __getattr__(self, name):
return getattr(self._usel, 'name')
def __setattr__(self, name, value):
if name in self.__dict__:
object.__setattr__(self, name, value)
else:
setattr(self._usel, name, value)
# maybe __hasattr__ and __delattr__ too?
I'm hoping to avoid having to write this C API code myself, but the slots are tp_getattro and tp_setattro. Note that __getattr__ will need to be more comprehensive in the C version, since it acts closer to the __getattribute__ in Python. The flipside is that isinstance and issubclass will fail, which may or may not be an issue for you.