python类属性

Question

我试图找到扩展类变量的最佳方法。希望迄今为止我提出的方法的一个例子将清楚地说明这一点。

class A(object): 
    foo = ['thing', 'another thing'] 

class B(A): 
    foo = A.foo + ['stuff', 'more stuff'] 

所以我试图让子类继承和扩展父类变量。上面的方法工作，但似乎有点kludgey。我愿意接受任何建议，包括使用完全不同的方法完成类似的事情。

显然我可以继续使用这种方法，如果需要的话，但如果有更好的方法，我想找到它。

Answer 1

可以使用元类：

class AutoExtendingFoo(type): 

    def __new__(cls, name, bases, attrs): 
     foo = [] 
     for base in bases: 
      try: 
       foo.extend(getattr(base, 'foo')) 
      except AttributeError: 
       pass 
     try: 
      foo.extend(attrs.pop('foo_additions')) 
     except KeyError: 
      pass 
     attrs['foo'] = foo 
     return type.__new__(cls, name, bases, attrs) 

class A(object): 
    __metaclass__ = AutoExtendingFoo 
    foo_additions = ['thing1', 'thing2'] 
    # will have A.foo = ['thing1', 'thing2'] 

class B(A): 
    foo_additions = ['thing3', 'thing4'] 
    # will have B.foo = ['thing1', 'thing2', 'thing3', 'thing4'] 

class C(A): 
    pass 
    # will have C.foo = ['thing1', 'thing2'] 

class D(B): 
    pass 
    # will have D.foo = ['thing1', 'thing2', 'thing3', 'thing4'] 

Answer 2

我明确地会去的实例属性。 （如果我是正确的，他们不必然是静态的你的情况？！）

>>> class A: 
...  @property 
...  def foo(self): 
...   return ['thin', 'another thing'] 
... 
>>> class B(A): 
...  @property 
...  def foo(self): 
...   return super().foo + ['stuff', 'thing 3'] 
... 
>>> B().foo 
['thin', 'another thing', 'stuff', 'thing 3']