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

Question

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)

You need to get the class object in scope somehow, either by importing it, or here, you can simply do `PyCapsule = type(capsule)` then use `isinstance(whatever, PyCapsule)` — juanpa.arrivillaga, Feb 19 '20 at 22:34
@juanpa.arrivillaga I'm looking for the way to import it; what I would like to do is have a function check that the user passed in a PyCapsule, without having to instantiate a PyCapsule and extracting the type out. I've updated my OP to include that. — Matthew Moisen, Feb 19 '20 at 23:06
It may not be exposed as an import anywhere, unfortunately. But what harm is there in instantiating a PyCapsule once in the global scope simply to get it's type? — juanpa.arrivillaga, Feb 19 '20 at 23:07
@juanpa.arrivillaga If it's not exposed as an import anywhere, I think I might have to do that. — Matthew Moisen, Feb 19 '20 at 23:08
Well, that sounds like a solution. It isn't particularly bad, although it is a bit inelegant I suppose. — juanpa.arrivillaga, Feb 19 '20 at 23:08

score 6 · Answer 1 · answered Jun 08 '20 at 08:51

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'

score 2 · Accepted Answer · answered Feb 20 '20 at 12:01

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(capsulepointerpointer)).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.

Olivier Melançon · Answer 3 · 2020-02-20T01:14:00.787

1

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):
    ...

edited Feb 20 '20 at 01:14

answered Feb 19 '20 at 22:35

Olivier Melançon

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

3 Answers3