问题
Extending both an abstract base class and a class derived from "object" works as you would expect: if you you haven't implemented all abstract methods and properties, you get an error.
Strangely, replacing the object-derived class with an class that extends "Exception" allows you to create instances of classes which do not implement all the required abstract methods and properties.
For example:
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."
...yields...
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
I know that "Exception" and therefore "myCustomException" have no attribute "foo", so why am I getting away with instantiating "myCustomException"?
EDIT: For the record, this is the hackish workaround I ended up going with. Not truly equivalent, but works for my purposes.
# "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."
回答1:
It looks like this is because the __new__ method for BaseException doesn't care about abstract methods/properties.
When you try to instantiate myConcreteClass_1, it ends up calling __new__ from the Exception class. When want to instantiate myConcreteClass_2, it calls the __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 class doesn't provide a __new__ method, but it's parent, BaseException, does:
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;
}
Compare this to the __new__ implementation for object:
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);
}
As you can see object.__new__ has code to throw an error when there are abstract methods that aren't overridden, but BaseException.__new__ does not.
回答2:
Dano's answer is accurate but is missing a workaround. You can reproduce the object code in your own __new__ method:
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()
Which yields the two expected exceptions.
来源：https://stackoverflow.com/questions/24792881/python-abc-module-extending-both-an-abstract-base-class-and-an-exception-derive