That code is quite old. This comment tries to explain it:

```

/* Check that the use doesn't do something silly and unsafe like

object.__new__(dict). To do this, we check that the

most derived base that's not a heap type is this type. */

```

I think you may have to special-case this and arrange for B.__new__() to be called, like it or not.

(If you want us to change the code, please file a bpo bug report. I know that's no fun, but it's the way to get the right people involved.)

On Mon, Feb 1, 2021 at 3:27 AM Phil Thompson via Python-Dev <python-dev@python.org> wrote:

Hi,

I'm trying to understand the purpose of the check in tp_new_wrapper() of
typeobject.c that results in the "is not safe" exception.

I have the following class hierarchy...

B -> A -> object

...where B and A are implemented in C. Class A has an implementation of
tp_new which does a few context-specific checks before calling
PyBaseObject_Type.tp_new() directly to actually create the object. This
works fine.

However I want to allow class B to be used with a Python mixin. A's
tp_new() then has to do something similar to super().__new__(). I have
tried to implement this by locating the type object after A in B's MRO,
getting it's '__new__' attribute and calling it (using PyObject_Call())
with B passed as the only argument. However I then get the "is not safe"
exception, specifically...

TypeError: object.__new__(B) is not safe, use B.__new__()

I take the same approach for __init__() and that works fine.

If I comment out the check in tp_new_wrapper() then everything works
fine.

So, am I doing something unsafe? If so, what?

Or, is the check at fault in not allowing the case of a C extension type
with its own tp_new?

Thanks,
Phil
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