Question

What’s the correct way to prevent invoking (creating an instance of) a C type from Python?

I’ve considered providing a tp_init that raises an exception, but as I understand it that would still allow __new__ to be called directly on the type.

A C function returns instances of this type — that’s the only way instances of this type are intended to be created.

Edit: My intention is that users of my type will get an exception if they accidentally use it wrongly. The C code is such that calling a function on an object incorrectly created from Python would crash. I realise this is unusual: all of my C extension types so far have worked nicely when instantiated from Python. My question is whether there is a usual way to provide this restriction.

Answer 1

Simple: leave the tp_new slot of the type empty.

>>> Foo()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: cannot create 'foo.Foo' instances
>>> Foo.__new__(Foo)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: object.__new__(foo.Foo) is not safe, use foo.Foo.__new__()

If you inherit from a type other than the base object type, you will have to set tp_new to NULL after calling PyType_Ready().