Python abc 模块：扩展抽象基类和异常派生类会导致令人惊讶的行为

扩展抽象基类和从“对象”派生的类可以按照您的预期工作：如果您尚未实现所有抽象方法和属性，则会出现错误。

奇怪的是，用扩展“Exception”的类替换对象派生类允许您创建不实现所有必需的抽象方法和属性的类的实例。

例如：

import abc

# The superclasses
class myABC( object ):
    __metaclass__ = abc.ABCMeta

    @abc.abstractproperty
    def foo(self):
        pass

class myCustomException( Exception ):
    pass

class myObjectDerivedClass( object ):
    pass

# Mix them in different ways
class myConcreteClass_1(myCustomException, myABC):
    pass

class myConcreteClass_2(myObjectDerivedClass, myABC):
    pass

# Get surprising results
if __name__=='__main__':
    a = myConcreteClass_1()
    print "First instantiation done. We shouldn't get this far, but we do."
    b = myConcreteClass_2()
    print "Second instantiation done. We never reach here, which is good."

...产量...

First instantiation done. We shouldn't get this far, but we do.
Traceback (most recent call last):
  File "C:/Users/grahamf/PycharmProjects/mss/Modules/mssdevice/sutter/sutter/test.py", line 28, in <module>
    b = myConcreteClass_2()
TypeError: Can't instantiate abstract class myConcreteClass_2 with abstract methods foo

我知道“Exception”因此“myCustomException”没有属性“foo”，那么为什么我要实例化“myCustomException”呢？

EDIT:根据记录，这是我最终采用的黑客解决方法。并不真正等同，但适合我的目的。

# "abstract" base class
class MyBaseClass( Exception ):
    def __init__(self):
        if not hasattr(self, 'foo'):
            raise NotImplementedError("Please implement abstract property foo")


class MyConcreteClass( MyBaseClass ):
    pass

if __name__=='__main__':
    a = MyConcreteClass()
    print "We never reach here, which is good."

看起来这是因为__new__方法用于BaseException不关心抽象方法/属性。

当你尝试实例化时myConcreteClass_1，它最终调用__new__来自Exception班级。当想要实例化时myConcreteClass_2，它调用__new__ from object:

>>> what.myConcreteClass_1.__new__()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: exceptions.Exception.__new__(): not enough arguments
>>> what.myConcreteClass_2.__new__()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: object.__new__(): not enough arguments

The Exception类不提供__new__方法，但它是父方法，BaseException, does http://hg.python.org/cpython/file/db302b88fdb6/Objects/exceptions.c#l31:

static PyObject *
BaseException_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
{
    PyBaseExceptionObject *self;

    self = (PyBaseExceptionObject *)type->tp_alloc(type, 0);
    if (!self)
        return NULL;
    /* the dict is created on the fly in PyObject_GenericSetAttr */
    self->dict = NULL;
    self->traceback = self->cause = self->context = NULL;
    self->suppress_context = 0;

    if (args) {
        self->args = args;
        Py_INCREF(args);
        return (PyObject *)self;
    }

    self->args = PyTuple_New(0);
    if (!self->args) {
        Py_DECREF(self);
        return NULL;
    }

    return (PyObject *)self;
}

将此与__new__实施object http://hg.python.org/cpython/file/db302b88fdb6/Objects/typeobject.c#l3233:

static PyObject *
object_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
{
    if (excess_args(args, kwds) &&
        (type->tp_init == object_init || type->tp_new != object_new)) {
        PyErr_SetString(PyExc_TypeError, "object() takes no parameters");
        return NULL;
    }

    if (type->tp_flags & Py_TPFLAGS_IS_ABSTRACT) {
        PyObject *abstract_methods = NULL;
        PyObject *builtins;
        PyObject *sorted;
        PyObject *sorted_methods = NULL;
        PyObject *joined = NULL;
        PyObject *comma;
        _Py_static_string(comma_id, ", ");
        _Py_IDENTIFIER(sorted);

        /* Compute ", ".join(sorted(type.__abstractmethods__))
           into joined. */
        abstract_methods = type_abstractmethods(type, NULL);
        if (abstract_methods == NULL)
            goto error;
        builtins = PyEval_GetBuiltins();
        if (builtins == NULL)
            goto error;
        sorted = _PyDict_GetItemId(builtins, &PyId_sorted);
        if (sorted == NULL)
            goto error;
        sorted_methods = PyObject_CallFunctionObjArgs(sorted,
                                                      abstract_methods,
                                                      NULL);
        if (sorted_methods == NULL)
            goto error;
        comma = _PyUnicode_FromId(&comma_id);
        if (comma == NULL)
            goto error;
        joined = PyUnicode_Join(comma, sorted_methods);
        if (joined == NULL)
            goto error;

        PyErr_Format(PyExc_TypeError,
                     "Can't instantiate abstract class %s "
                     "with abstract methods %U",
                     type->tp_name,
                     joined);
    error:
        Py_XDECREF(joined);
        Py_XDECREF(sorted_methods);
        Py_XDECREF(abstract_methods);
        return NULL;
    }
    return type->tp_alloc(type, 0);
}

如你看到的object.__new__当存在未被重写的抽象方法时，有代码抛出错误，但是BaseException.__new__才不是。