The current descrintro.html says:

"__new__ must return an object... If you return an
existing object, the constructor call will still call
its __init__ method. If you return an object of a
different class, its __init__ method will be called..."

This suggests that __new__ will call __init__ of the
object even if the object is a different type.  The
code in typeobject.c disagrees with this:

static PyObject *
type_call(PyTypeObject *type, PyObject *args, PyObject
*kwds)
{
	...
	/* If the returned object is not an instance of type,
	   it won't be initialized. */
	if (!PyType_IsSubtype(obj->ob_type, type))
		return obj;
	...
}

Suggested fix:

"__new__ must return an object... If you return an
existing object, the constructor call will still call
its __init__ method unless the object is an instance of
a different class..."

Logged In: YES 
user_id=99874

I agree strongly that the docs need to be clarified.

I spent a while playing around with it and confirmed that
your reading of it appears to be correct, and your proposed
wording is simple and straightforward.

I second the request to make this change.

Documentation had several updates in these 2+ years.

Now it says:

"""
If __new__() returns an instance of cls, then the new instance’s
__init__() method will be invoked like __init__(self[, ...]), where self
is the new instance and the remaining arguments are the same as were
passed to __new__().

If __new__() does not return an instance of cls, then the new instance’s
__init__() method will not be invoked.
"""

, which is clearer.

Thanks!