7

扩展抽象基类和派生自“对象”的类可以按您的预期工作：如果您还没有实现所有抽象方法和属性，则会出现错误。

奇怪的是，用扩展“异常”的类替换对象派生类允许您创建不实现所有必需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“异常”因此“myCustomException”没有属性“foo”，那么为什么我要实例化“myCustomException”呢？

编辑：为了记录，这是我最终采用的骇人听闻的解决方法。不是真正等效的，但适用于我的目的。

# "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."
4

2 回答 2

6

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

当您尝试实例化myConcreteClass_1时，它最终会__new__从Exception类中调用。当想要实例化myConcreteClass_2时，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

该类Exception不提供__new__方法，但它的父类BaseException,提供：

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__实现进行object比较：

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);
}

如您所见object.__new__，当存在未被覆盖但没有被覆盖的抽象方法时，有代码会引发错误BaseException.__new__。

于 2014-07-17T05:09:19.237 回答
0

Dano 的回答是准确的，但缺少解决方法。您可以用自己的__new__方法重现目标代码：

import abc, traceback

# The superclasses
class MyABC(abc.ABC):

    @property
    @abc.abstractmethod
    def foo(self):
        pass

class MyCustomException( Exception ):
    pass

class MyObjectDerivedClass( object ):
    pass

# Mix them in different ways
class MyConcreteClass_1(MyCustomException, MyABC):

    def __new__(cls, *args, **kwargs):
        ''' Same abstract checks than in object.__new__ '''
        res = super().__new__(cls, *args, **kwargs)
        if cls.__abstractmethods__:
            raise TypeError(f"Can't instantiate abstract class {cls.__name__} with abstract methods {','.join(sorted(cls.__abstractmethods__))}")
        return res

class MyConcreteClass_2(MyObjectDerivedClass, MyABC):
    pass

# No longer get surprising results
if __name__=='__main__':
    try:
        a = MyConcreteClass_1()
    except TypeError:
        traceback.print_exc()
    try:
        b = MyConcreteClass_2()
    except TypeError:
        traceback.print_exc()

这产生了两个预期的异常。

于 2019-11-15T13:54:25.707 回答