Colony

import weakref

_model_dependents=weakref.WeakKeyDictionary()

def prchanged(what=None):

    print "Changed:",what

class model(object):

    """

    >>> def prchanged(what=None):

    ...     print "Changed",what

    ...

    >>> m=model()

    >>> m.add_dependent(prchanged)

    >>> m.changed('foo')

    Changed foo

    >>> def prchanged2(what=None):

    ...     print "Changed2",what

    ... 

    >>> m.add_dependent(prchanged2)

    >>> m.changed('foo')

    Changed foo

    Changed2 foo

    >>> m.remove_dependent(prchanged)

    >>> m.changed('foo')

    Changed2 foo

    """

    def __init__(self):

        "No need to call"

        pass

    def dependents(self, create=0):

        "Return a list of my dependents"

        if not _model_dependents.has_key(self): # k in WeakDictionary broken in 2.2.3

            if create:

                _model_dependents[self]=[]

            else:

                return ()

        # If this fails, check self==self to see if compare works

        return _model_dependents[self]

    def changed(self,what=None):

        "Indicate that my data has changed, inform dependents"

        for x in self.dependents(0):

            x(what)

    def add_dependent(self,x):

        "Add a function to call with one argument (what) when I change"

        self.dependents(1).append(x)

    def remove_dependent(self,x):

        "Remove the function"

        self.dependents(1).remove(x)

    def clear_dependents(self):

        dep=self.dependents(0)

        if dep:

            dep[:]=[]