以任何方式使用都会引发异常的对象

问题描述

我需要创建一个对象，该对象以任何方式使用时都会引发自定义异常UnusableObjectError(尽管创建该对象不应创建异常).

I need to create an object that would raise a custom exception, UnusableObjectError, when it is used in any way (creating it should not create an exception though).

a = UnusableClass()     # No error
b = UnusableClass()     # No error
a == 4                  # Raises UnusableObjectError
'x' in a                # Raises UnusableObjectError
for i in a:             # Raises UnusableObjectError
    print(i)

# ..and so on

---

我想出了下面的代码，该代码似乎表现出预期的效果.

---

I came up with the code below which seems to behave as expected.

class UnusableObjectError(Exception):
    pass

CLASSES_WITH_MAGIC_METHODS = (str(), object, float(), dict())

# Combines all magic methods I can think of.
MAGIC_METHODS_TO_CHANGE = set()
for i in CLASSES_WITH_MAGIC_METHODS:
    MAGIC_METHODS_TO_CHANGE |= set(dir(i))
MAGIC_METHODS_TO_CHANGE.add('__call__')
# __init__ and __new__ must not raise an UnusableObjectError
# otherwise it would raise error even on creation of objects.
MAGIC_METHODS_TO_CHANGE -= {'__class__', '__init__', '__new__'}


def error_func(*args, **kwargs):
    """(nearly) all magic methods will be set to this function."""
    raise UnusableObjectError


class UnusableClass(object):
    pass

for i in MAGIC_METHODS_TO_CHANGE:
    setattr(UnusableClass, i, error_func)

问题:
是否已经存在行为符合所描述的类?

Questions:
Is there an already existing class that behaves as described?

如果没有，我的UnusableClass()是否存在任何缺陷(例如，使用类实例的情况不会引发错误)，如果是，我该如何解决这些缺陷?

If not, is there any flaw in my UnusableClass() (e.g., situations when using the instances of the class wouldn't raise an error) and if so, how can I fix those flaws?

推荐答案

转出元类和dunder(双下划线)方法不能很好地结合在一起(不幸的是，因为那样会更简化地实现此方法) ).

Turns out metaclasses and dunder (double underscore) methods don't go well together (which is unfortunate, since that would have been a more streamlined way to implement this).

我找不到任何可导入的魔术方法名称列表，因此我创建了一个名称并将其放在PyPi上( https://pypi.python.org/pypi/magicmethods/0.1.1 ).有了它，UnusableClass的实现可以写成一个简单的类装饰器:

I couldn't find any importable listing of magic method names, so I created one and put it on PyPi (https://pypi.python.org/pypi/magicmethods/0.1.1). With it, the implementation of UnusableClass can be written as a simple class decorator:

import magicmethods

class UnusableObjectError(Exception):
    pass

def unusable(cls):
    def _unusable(*args, **kwargs):
        raise UnusableObjectError()

    for name in set(magicmethods.all) - set(magicmethods.lifecycle):
        setattr(cls, name, _unusable)
    return cls

@unusable
class UnusableClass(object):
    pass

magicmethods.lifecycle包含__new__，__init__和__del__.您可能需要对此进行调整..

magicmethods.lifecycle contains __new__, __init__, and __del__. You might want to adjust this..

此实现还可以处理:

a = UnusableClass()
with a:
    print 'oops'

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