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Inheriting from OrderedDict causes problem

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  Inheriting from OrderedDict causes problem Bruce Eckel <lists.eckel@gmail.com> - 2012-02-22 08:53 -0800
    Re: Inheriting from OrderedDict causes problem Peter Otten <__peter__@web.de> - 2012-02-22 18:10 +0100
      Re: Inheriting from OrderedDict causes problem Bruce Eckel <lists.eckel@gmail.com> - 2012-02-22 09:33 -0800

#20683 — Inheriting from OrderedDict causes problem

Notice that both classes are identical, except that one inherits from
dict (and works) and the other inherits from OrderedDict and fails.
Has anyone seen this before? Thanks.

import collections

class Y(dict):
    def __init__(self, stuff):
        for k, v in stuff:
            self[k] = v

# This works:
print Y([('a', 'b'), ('c', 'd')])

class X(collections.OrderedDict):
    def __init__(self, stuff):
        for k, v in stuff:
            self[k] = v

# This doesn't:
print X([('a', 'b'), ('c', 'd')])

""" Output:
{'a': 'b', 'c': 'd'}
Traceback (most recent call last):
  File "OrderedDictInheritance.py", line 17, in <module>
    print X([('a', 'b'), ('c', 'd')])
  File "OrderedDictInheritance.py", line 14, in __init__
    self[k] = v
  File "C:\Python27\lib\collections.py", line 58, in __setitem__
    root = self.__root
AttributeError: 'X' object has no attribute '_OrderedDict__root'
"""

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#20684

Bruce Eckel wrote:

> Notice that both classes are identical, except that one inherits from
> dict (and works) and the other inherits from OrderedDict and fails.
> Has anyone seen this before? Thanks.
> 
> import collections
> 
> class Y(dict):
>     def __init__(self, stuff):
>         for k, v in stuff:
>             self[k] = v
> 
> # This works:
> print Y([('a', 'b'), ('c', 'd')])
> 
> class X(collections.OrderedDict):
>     def __init__(self, stuff):
>         for k, v in stuff:
>             self[k] = v
> 
> # This doesn't:
> print X([('a', 'b'), ('c', 'd')])
> 
> """ Output:
> {'a': 'b', 'c': 'd'}
> Traceback (most recent call last):
>   File "OrderedDictInheritance.py", line 17, in <module>
>     print X([('a', 'b'), ('c', 'd')])
>   File "OrderedDictInheritance.py", line 14, in __init__
>     self[k] = v
>   File "C:\Python27\lib\collections.py", line 58, in __setitem__
>     root = self.__root
> AttributeError: 'X' object has no attribute '_OrderedDict__root'
> """

Looks like invoking OrderedDict.__init__() is necessary:

>>> from collections import OrderedDict
>>> class X(OrderedDict):
...     def __init__(self, stuff):
...             super(X, self).__init__()
...             for k, v in stuff:
...                     self[k] = v
...
>>> X([("a", "b"), ("c", "d")])
X([('a', 'b'), ('c', 'd')])

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#20686

On Feb 22, 10:10 am, Peter Otten <__pete...@web.de> wrote:
> Looks like invoking OrderedDict.__init__() is necessary:
>
> >>> from collections import OrderedDict
> >>> class X(OrderedDict):
>
> ...     def __init__(self, stuff):
> ...             super(X, self).__init__()
> ...             for k, v in stuff:
> ...                     self[k] = v
> ...>>> X([("a", "b"), ("c", "d")])
>
> X([('a', 'b'), ('c', 'd')])

Thank you! That worked. Languages. Some of them automatically call the
default base-class constructors, others don't. Apparently.

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