some time after posting my `Linkdict recipe`__ to aspn__ -- basically, a dictionary with run-time delegational lookup, but this is not important here -- i thought gee that would be fun to make such a customized dictionary thingie an instance dictionary, and get some custom namespace behavior out of that.
.. __: http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/465748 .. __: http://aspn.activestate.com/ here is a simplified example: first, the customized dictionary class and a test:: class CustomDict( dict ): defaultValue = 'THIS ITEM NOT AVAILABLE' def __getitem__( self, 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 print '----------------------------------------' cd = CustomDict( foo = 'bar' ) print cd[ 'foo' ] print cd[ 'bar' ] print 'bar' in cd print cd.has_key( 'bar' ) this gives us:: ---------------------------------------- bar THIS ITEM NOT AVAILABLE True True so it appears to work. note that we may have failed to implement all the conceivable ways to test for membership (such as searching through ``keys()``) or to retrieve a value for a given key. more on that below. now for the class to utilize this definition:: class X( object ): def __init__( self ): self.__dict__ = CustomDict( foo = 'bar' ) and the test code for that:: print '----------------------------------------' x = X() print x.__dict__[ 'foo' ] print x.__dict__[ 'bar' ] print x.foo print x.bar which yields:: ---------------------------------------- bar THIS ITEM NOT AVAILABLE bar Traceback (most recent call last): File "C:\home\projects\__svn__\sundry\#.py", line 39, in ? print x.bar AttributeError: 'X' object has no attribute 'bar' ok. so the custom dict *basically* works as expected, since it does successfully make ``x.foo`` available -- no surprise here. unfortunately, looking up ``x.bar``, which should really return the default value string, causes an ``AttributeError`` to be raised. now of course given the short definition of ``CustomDict``, perhaps there is an essential lookup method that has not been overwritten but that is internally used for attribute lookup. however, i carefully tested my actual class (from the recipe mentioned above) and also compared the methods defined there against the standard ``dict()`` interface, and nothing of importance appeared to be missing. i also tried to bind the dictionary to the instance earlier, in ``__new__``, to no avail. am i missing something here? _wolfgang -- http://mail.python.org/mailman/listinfo/python-list
