KeyError: '_OrderedDict__root?

Question

Hi I have following code snippet which gives KeyError. I have checked other links specifying make __init__ call to Ordered Dict which I have done. But still no luck.

from collections import OrderedDict

class BaseExcelNode(OrderedDict):
    def __init__(self):
        super(BaseExcelNode, self).__init__()
        self.start_row = -1
        self.end_row = -1
        self.col_no = -1

    def __getattr__(self, name):
        return self[name]

    def __setattr__(self, name, value):
        self[name] = value
BaseExcelNode()

Error:

Traceback (most recent call last):
  File "CIMParser.py", line 29, in <module>
    BaseExcelNode()
  File "CIMParser.py", line 9, in __init__
    super(BaseExcelNode, self).__init__()
  File "C:\Python27\lib\collections.py", line 64, in __init__
    self.__root
  File "CIMParser.py", line 15, in __getattr__
    return self[name]
KeyError: '_OrderedDict__root'

Answer 1

Using monkey patching method:

from collections import OrderedDict

class BaseExcelNode(OrderedDict):
    def __init__(self):
        super(BaseExcelNode, self).__init__()
        self.start_row = -1
        self.end_row = -1
        self.col_no = -1

    def __getattr__(self, name):
        if not name.startswith('_'):
            return self[name]
        super(BaseExcelNode, self).__getattr__(name)

    def __setattr__(self, name, value):
        if not name.startswith('_'):
            self[name] = value
        else:
            super(BaseExcelNode, self).__setattr__(name, value)

b = BaseExcelNode()

Answer 2

OrderedDict is implemented under the assumption that attribute access works by the default mechanisms, and in particular, that attribute access is not equivalent to indexing.

When you subclass it and change how attribute access works, you break one of the deepest assumptions of the OrderedDict implementation, and everything goes to hell.