Warning when new attributes are added to classes at run time

[Matthew Wilson]

I sometimes inadvertently create a new attribute on an object rather
update a value bound to an existing attribute.  For example:

    In [5]: class some_class(object):
       ...:      def __init__(self, a=None):
       ...:          self.a = a
       ...:

    In [6]: c = some_class(a=1)

    In [7]: c.a
    Out[7]: 1

    In [8]: c.A = 2

I meant to update c.a but I created a new c.A.  I make this mistake
probably hourly.

I suspect adding attributes at run time can be a beautiful thing, but in
this particular instance, I'm only using this feature to hurt myself.

I wrote a simple class that will warn me when I make this mistake in the
future:

    import warnings

    class C(object):

        warn_on_new_attributes = True

        standard_attributes = []

        def __setattr__(self, name, value):

            if self.warn_on_new_attributes \
                and name is not 'warn_on_new_attributes' \
                and name not in self.standard_attributes:

                warnings.warn("%s has no standard attribute %s."
                              % (self.__class__.__name__, name))


            self.__dict__[name] = value


    class C1(C):

        standard_attributes = ['a1', 'a2']


    class C2(C):

        warn_on_new_attributes = False

    # Do some simple testing.
    c11 = C1()
    c11.a1 = 1
    c11.a2 = 2
    c11.a3 = 3
    c11.a4 = 4

    # Disable warnings for this instance.
    c12 = C1()
    c12.warn_on_new_attributes = False
    c12.a1 = 1
    c12.a2 = 2
    c12.a3 = 3
    c12.a4 = 4

    c11.a5 = 5

    # Use an object that has warnings disabled by default.
    c2 = C2()
    c2.a1 = 1
    c2.a2 = 2
    c2.a3 = 3
    c2.a4 = 4

    # enable warnings for this object.
    c2.warn_on_new_attributes = True
    c2.a1 = 1
    c2.a5 = 5


All comments are welcome.  Is there a better way of implementing the
above class, OR, is this approach generally wrong-headed?  Am I the only
one that makes this mistake?

TIA

-- 
A better way of running series of SAS programs:
http://overlook.homelinux.net/wilsonwiki/SasAndMakefiles