通过类层次结构合并字典类属性

Question

我试图使以下代码工作。我想创建一个类属性，它是一个默认值的字典，通过类层次结构自动从子节点更新到父节点。理想情况下，我希望在父对象中使用魔法，以便子对象可以根据需要简单地覆盖值。我也愿意听取关于如何重新设计这个的建议，如果有一个关于这种事情的习语。

class A(object):
  DEFAULTS = {'a': 'default a', 'd': 'test d'}
  def __init__(self, *args, **kwargs):
    pass
    # but can i do something with super? this fails but is the
    # approximate idea of what I want...
    # self.DEFAULTS.update(super(self.__class__, self).DEFAULTS)
class B(A):
  DEFAULTS = {'b': 'default b'}
class C(A):
  DEFAULTS = {'a': 'a overridden in C'}
class D(C):
  DEFAULTS = {'d': 'd overridden in D'}
def test():
  a = A()
  b = B()
  c = C()
  d = D()
  print a.DEFAULTS
  print b.DEFAULTS
  print c.DEFAULTS
  print d.DEFAULTS
  assert (a.DEFAULTS == {'a': 'default a', 'd': 'test d'})
  assert (b.DEFAULTS == {'a': 'default a', 'b': 'default b', 'd': 'test d'})
  assert (c.DEFAULTS == {'a': 'overridden in c', 'd': 'test d'})
  assert (d.DEFAULTS == {'a': 'overridden in c', 'd': 'd overridden in D'})
test()

当然，现在这会导致以下输出:

{'a': 'default a', 'd': 'test d'}
{'b': 'default b'}
{'a': 'a overridden in C'}
{'d': 'd overridden in D'}
Traceback (most recent call last):
  File "experimental/users/edw/python/class_magic.py", line 36, in <module>
    test()
  File "experimental/users/edw/python/class_magic.py", line 32, in test
    assert (b.DEFAULTS == {'a': 'default a', 'b': 'default b', 'd': 'test d'})
AssertionError

Answer 1

感谢sobolevn提供的元类示例。我在此基础上将默认值移动到子类中，而不是像原始问题中的示例那样移动到单个外部数据结构中。如果正在复制的字典变大，性能可能会出现问题，但由于这是配置代码，因此在实践中可能会出现意想不到的情况。

class DefaultsMetaBase(type):
  """Automatically merges DEFAULTS from all parent classes."""
  def __call__(self, *args, **kwargs):
    obj = type.__call__(self)
    for klass in obj.__class__.__mro__:
      if klass == obj.__class__ or klass == Base or not issubclass(klass, Base):
        continue
      if hasattr(klass, 'DEFAULTS'):
        d = klass.DEFAULTS.copy()
        d.update(obj.DEFAULTS)
        obj.DEFAULTS = d
    return obj
class Base(object):
  __metaclass__ = DefaultsMetaBase
class A(Base):
  DEFAULTS = {'a': 'default a', 'd': 'test d'}
class B(A):
  DEFAULTS = {'b': 'default b'}
class C(A):
  DEFAULTS = {'a': 'a overridden in C'}
  some_setting = ['a', 'b', 'c', 'd']
class D(C):
  DEFAULTS = {'d': 'd overridden in D'}
  some_setting = ['q', 'r', 's']
  another_setting = 'moar roar'
class F(D):
  another_setting = 'less roar'
def test():
  a = A()
  b = B()
  c = C()
  d = D()
  f = F()
  assert (a.DEFAULTS == {'a': 'default a', 'd': 'test d'})
  assert (b.DEFAULTS == {'a': 'default a', 'b': 'default b', 'd': 'test d'})
  assert (c.DEFAULTS == {'a': 'a overridden in C', 'd': 'test d'})
  assert (d.DEFAULTS == {'a': 'a overridden in C', 'd': 'd overridden in D'})
  assert (f.DEFAULTS == {'a': 'a overridden in C', 'd': 'd overridden in D'})
  print 'pass!'
test()

Answer 2

这符合您的需求吗?

方法示例:

class A(object):
    _DEFAULTS = {'a': 'a value'}
    @classmethod
    def get_defaults(cls):
        return cls._DEFAULTS
class B(A):
    _B_DEFAULTS = {'b': 'b value'}
    @classmethod
    def get_defaults(cls):
        defaults = super(B, cls).get_defaults()
        defaults.update(cls._B_DEFAULTS)
        return defaults

元类的例子:

# Some data-structure (it should be refactored): 
_DEFAULTS = {'A': {'a': 'a_value'}, 'B': {'b': 'b_value'}}
class DefaultsInitializer(type):
     def __call__(self, *args, **kwargs):
          obj = type.__call__(self)
          obj.defaults = _DEFAULTS[obj.__class__.__name__]
          for klass in obj.__class__.__bases__:
               if klass.__name__ in _DEFAULTS:
                    obj.defaults.update(_DEFAULTS[klass.__name__])
          return obj

和一些类:

>>> class A(object):
...      __metaclass__ = DefaultsInitializer
... 
>>> a = A()
>>> a.defaults
{'a': 'a_value'}
>>> class B(A):
...      pass
... 
>>> b = B()
>>> b.defaults
{'a': 'a_value', 'b': 'b_value'}

Answer 3

听起来你想要的东西就像类属性通常发生的那样——子属性继承父属性，如果需要，可以用自己的值隐藏父属性。

。

class A(object):
    a = 'a'
class B(A):
    b = 'b'
class C(A):
    a = 'c'
assert A.a == 'a'and A().a == 'a'
assert B.a == 'a' # B inherits a
assert C.a == 'c' # C overrides a

它还允许实例覆盖默认值

a = A()
a.a = 'not a'
assert A.a == 'a' and A().a == 'a'
assert a.a == 'not a'

然而，也许你不希望你的默认值是这样可访问的。不用担心。在父类上定义defaults类，并让子类定义子类defaults。如:

class A(object):
    class defaults(object):
        a = 'a'
class B(A):
    class defaults(A.defaults):
        b = 'b'
class C(A):
    class defaults(A.defaults):
        a = 'c'
assert A.defaults.a == 'a' and A().defaults.a == 'a'
assert B.defaults.a == 'a' # B inherits a
assert C.defaults.a == 'c' # C overrides a

或者，如果你真的，真的，想要默认值是一个类似字典的对象，那么就定义一个类似字典的对象，覆盖__getitem__和其他各种你可能关心的方法。

def defaults(**kwargs):
    """decorator to add defaults to a class"""
    def default_setter(cls):
        if not hasattr(cls, "defaults"):
            cls.defaults = DefaultGetter()
        for key, value in kwargs:
            setattr(cls, "_default_"+key, value)
        return cls
    return default_setter
class DefaultGetter(object):
    """dict-like object that retrieves the defaults"""
    def __getitem__(self, name):
        try:
            return getattr(self.owner, "_default_" + name)
        except AttributeError:
            raise KeyError(name) from None
@defaults(a='a')
class A(object):
     pass
assert A.defaults['a'] == 'a'
assert A._default_a == 'a' # actual storage location of a
A.defaults['b'] # raises KeyError