CODEWITHSUNDEEP
pythonpython-3.xpython-c-api
Matthew Moisen
Matthew Moisen

Reputation: 18299

How to check if an object in Python is a PyCapsule?

I have a C Extension that receives and accepts PyCapsule objects.

In my python wrapper, how can I check if a python object is of the PyCapsule type object?

>>> # My C extension
>>> foo = Foo()
>>> capsule = foo.to_capsule()  # returns a PyCapsule object from the C extension
>>> capsule
<capsule object "foo" at 0xf707df08>
>>> type(capsule)
<class 'PyCapsule'>
isinstance(capsule, PyCapsule)
NameError: name 'PyCapsule' is not defined

What I'm trying to do is write a function like:

def push_capsule(capsule):
    # check that the `capsule` is of type PyCapsule
    # c_extension.push_capsule(capsule)

Upvotes: 3

Views: 3416

Answers (3)

DavidW
DavidW

Reputation: 30931

It's a bit of a mess but you can get it from ctypes:

def get_capsule_type():
    class PyTypeObject(ctypes.Structure):
        pass  # don't need to define the full structure
    capsuletype = PyTypeObject.in_dll(ctypes.pythonapi, "PyCapsule_Type")
    capsuletypepointer = ctypes.pointer(capsuletype)
    return ctypes.py_object.from_address(ctypes.addressof(capsuletypepointer)).value

Creating a py_object from an address looks to require an address which contains a PyObject*, rather than the PyObject* itself, hence the extra layer of indirection.

Upvotes: 2

Wenzel Jakob
Wenzel Jakob

Reputation: 695

The PyCapsule type is not directly accessible from Python as you have noted. To detect it without the heavy ctypes dependency (in the accepted answer by DavidW), I would do the following instead:

def is_capsule(o):
    t = type(o)
    return t.__module__ == 'builtins' and t.__name__ == 'PyCapsule'

Upvotes: 7

Olivier Melan&#231;on
Olivier Melan&#231;on

Reputation: 22324

In general, first check if your API offers some way to access the class you want to reference.

If it does not, recover the class from a dummy instance.

PyCapsule = type(Foo().to_capsule())

...

if isinstance(bar, PyCapsule):
    ...

Upvotes: 1

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