Using super() in nested classes.

Question

Imagine this:

class A(object):
    class B(object):
        def __init__(self):
            super(B, self).__init__()

This creates an error:

NameError: global name B is not defined.

I've tried A.B, but then it says that A is not defined.

Update:

I've found the problem.

I've had a class like this:

class A(object):
    class B(object):
        def __init__(self):
            super(B, self).__init__()

    someattribute = B()

In that scope, A isn't defined yet.

Answer 1

Since B will likely never be extended itself, this should work:

class A(object):
    class B(object):
        def __init__(self):
            super(self.__class__, self).__init__()

Answer 2

I'm not sure why A.B is not working correctly for you, as it should.. Here's some shell output that works:

>>> class A(object):
...   class B(object):
...     def __init__(self):
...       super(A.B, self).__init__()
...   def getB(self):
...     return A.B()
... 
>>> A().getB()
<__main__.B object at 0x100496410>

Answer 3

If the class A.B is unlikely to participate in any multiple inheritance, then you're better off just hard-coding the constructor call:

class A(object):
    class B(object):
        def __init__(self):
            object.__init__(self)

But if you really do need to have the full power of super, then you can get what you want by defining a custom descriptor that will initialize the B attribute lazily:

class LazyAttribute(object):
    def __init__(self, func, *args, **kwargs):
        self._func = func
        self._args = args
        self._kwargs = kwargs
        self._value = None

    def __get__(self, obj, type=None):
        if self._value is None:
            print 'created', self._value
            self._value = self._func(*self._args, **self._kwargs)
        return self._value

class A(object):
    class B(object):
        def __init__(self):
            super(A.B, self).__init__()

    someattribute = LazyAttribute(B)

This will cause the B attribute to be instantiated the first time it's accessed, and then reused thereafter:

>>> print A.someattribute
created <__main__.B object at 0x00AA8E70>
<__main__.B object at 0x00AA8E90>
>>> print A().someattribute
<__main__.B object at 0x00AA8E90>

For more info on descriptors, see: http://users.rcn.com/python/download/Descriptor.htm