customized instance dictionaries, anyone?

wolfgang.lipp at gmail.com wolfgang.lipp at gmail.com
Wed Jan 25 18:35:50 CET 2006


thx! indeed, it worked -- took me some time to figure out how to
implement the setting of attributes, too. i finally managed to get that
done using super:

custom dictionary, unchanged::

    class CustomDict( dict ):

        defaultValue = 'THIS ITEM NOT AVAILABLE'

        def __getitem__( self, name ):
            #print 'CustomDict.__getitem__( %r )' % ( name, )
            try:
                return super( CustomDict, self ).__getitem__( name )
            except KeyError:
                return self.defaultValue

        def __contains__( self, name ):
            return True

        def has_key( self, name ):
            return True

dictionary user class::

    class X( object ):

        def __init__( self ):
            #print 'X.__init__()'
            self._dict = CustomDict( foo = 'bar' )

        @property
        def __dict__( self ):
            #print 'X.__dict__ ( get() )'
            return self._dict

        def __getattr__( self, name ):
            #print 'X.__getattr__( %r )' % ( name, )
            return self.__dict__[ name ]

        def __setattr__( self, name, value ):
            #print 'X.__setattr__( %r, %r )' % ( name, value, )
            if name == '_dict':
                return super( X, self ).__setattr__( name, value )
            self._dict[ name ] = value

the ``~.__setattr__()`` method tests for the special name ``_dict`` and
defers execution to the super of the ``X`` instance, ``object``. other
stuff is handled by the instance itself.
testing that with ::

    x = X()
    print x.__dict__[ 'foo' ]
    print x.__dict__[ 'bar' ]
    print x.foo
    print x.bar
    print x.__dict__
    x.oops = 42
    print x.__dict__

yields ::

    bar
    THIS ITEM NOT AVAILABLE
    bar
    THIS ITEM NOT AVAILABLE
    {'foo': 'bar'}
    {'foo': 'bar', 'oops': 42}

as expected.

i tried to reason *why* the usage of a property makes this particular
piece of code work, but i seemingly can't find out. anyone around who
has thoughts on that?

_wolfgang




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