Python/C Inconsistency: Detecting classes that just implement `__eq__`

[jamadden]

Consider:

from BTrees import family64

class WithEq(object):
    def __eq__(self, other):
        return NotImplemented

s = family64.OO.TreeSet()
s.add(WithEq())

The WithEq class has default comparison: it doesn't define any sorting methods.

The C implementation doesn't detect this (because just defining __eq__ is enough to fill in the tp_richcompare slot):

$ python /tmp/foo.py
$

The Python implementation does detect this:

$ PURE_PYTHON=1 python /tmp/foo.py
Traceback (most recent call last):
  File "/tmp/foo.py", line 7, in <module>
    s.add(WithEq())
  File "//BTrees/src/BTrees/_base.py", line 1406, in add
    return self._set(self._to_key(key))[0]
  File "//BTrees/src/BTrees/_datatypes.py", line 255, in __call__
    raise TypeError("Object of class %s has default comparison" % (type(item).__name__,))
TypeError: Object of class WithEq has default comparison

Can we improve the C implementation?

BTrees/src/BTrees/objectkeymacros.h

Lines 23 to 33 in 86fd464

	#ifdef PY3K
	if (Py_TYPE(arg)->tp_richcompare == Py_TYPE(object_)->tp_richcompare)
	#else
	if ((Py_TYPE(arg)->tp_richcompare == NULL
	&& Py_TYPE(arg)->tp_compare == Py_TYPE(object_)->tp_compare)
	/* Also exclude new-style classes. On Python 2, they can be compared,
	but order by address, making them not suitable for BTrees. */
	|| PyType_CheckExact(arg)
	/* But let classes with a meta class that implements comparison through. */
	|| (PyType_Check(arg) && Py_TYPE(arg)->tp_richcompare == PyType_Type.tp_richcompare)
	)

A real-world class that demonstrates this is persistent.wref.WeakRef.