Python abc模块:同时扩展抽象基类和异常派生类会导致令人惊讶的行为

Question

扩展抽象基类和从"object"派生的类的工作方式与您期望的一样:如果您没有实现所有的抽象方法和属性，您将得到一个错误。

奇怪的是，用扩展了"Exception"的类替换对象派生类允许你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."

收益率…

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"，所以为什么我离开实例化"myCustomException"?

EDIT:为了记录，这是我最终采用的hackish解决方案。不完全等同，但对我的目的有效。

# "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."

Answer 1

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

当您尝试实例化myConcreteClass_1时，它最终会从Exception类调用__new__。当需要实例化myConcreteClass_2时，它从object中调用__new__:

>>> 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

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

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;
}

比较__new__对object的实现:

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);
}

你可以看到object.__new__有代码抛出错误，当有抽象方法没有被覆盖，但BaseException.__new__没有。

Answer 2

Dano的回答是准确的，但缺少一个变通方法。您可以在自己的__new__方法中重现目标代码:

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()

产生两个预期的异常