""" An alternate implementation of bound inner classes. http://code.activestate.com/recipes/577623-bound-inner-classes-using-an-alternate-approach/ See also, the original approach: http://code.activestate.com/recipes/577070-bound-inner-classes/ Copyright (C) 2011 by Larry Hastings Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions: The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software. THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE. """ __all__ = ["BindingOuterClass", "BoundInnerClass", "UnboundInnerClass"] import weakref class BindingOuterClass(object): def __getattribute__(self, name): attr = super(BindingOuterClass, self).__getattribute__(name) bind_request = getattr(attr, '_binding_class', False) suitable = isinstance(attr, type) and bind_request if not suitable: return attr wrapper_bases = [attr] # iterate over attr's bases and look in self to see # if any have bound inner classes in self. # if so, multiply inherit from the bound inner version(s). multiply_inherit = False for base in attr.__bases__: # if we can't find a bound inner base, # add the original unbound base instead. # this is harmless but helps preserve the original MRO. inherit_from = base # if we inherit from a boundinnerclass from another outer class, # we might have the same name as a legitimate base class. # but if we look it up, we'll call __get__ recursively... forever. # if the name is the same as our own name, there's no way it's a # bound inner class we need to inherit from, so just skip it. if base.__name__ != name: bound_inner_base = getattr(self, base.__name__, None) if bound_inner_base: bases = getattr(bound_inner_base, "__bases__", (None,)) # the unbound class is always the first base of # the bound inner class. if bases[0] == base: inherit_from = bound_inner_base multiply_inherit = True wrapper_bases.append(inherit_from) Wrapper = attr._binding_class(attr, self, wrapper_bases[0]) Wrapper.__name__ = name # Assigning to __bases__ is startling, but it's the only way to get # this code working simultaneously in both Python 2 and Python 3. if multiply_inherit: Wrapper.__bases__ = tuple(wrapper_bases) # cache in self setattr(self, name, Wrapper) return Wrapper class BoundInnerClass(object): @staticmethod def _binding_class(attr, outer, base): assert outer outer_weakref = weakref.ref(outer) class Wrapper(base): # occlude the original _binding_class! # otherwise we'll recurse forever. _binding_class = None def __init__(self, *args, **kwargs): attr.__init__(self, outer_weakref(), *args, **kwargs) # give the bound inner class a nice repr # (but only if it doesn't already have a custom repr) if attr.__repr__ is object.__repr__: def __repr__(self): return "".join([ "<", self.__module__, ".", self.__class__.__name__, " object bound to ", repr(outer_weakref()), " at ", hex(id(self)), ">"]) return Wrapper class UnboundInnerClass(object): @staticmethod def _binding_class(attr, outer, base): class Wrapper(base): # occlude the original _binding_class! # otherwise we'll recurse forever. _binding_class = None return Wrapper # The code in this "if" statement will only execute if you run the module # directly; it won't run if you "import" this code into your own programs. if __name__ == "__main__": class Outer(BindingOuterClass): class Inner(BoundInnerClass): def __init__(self, outer): self.outer = outer class SubclassOfInner(Inner): def __init__(self, outer): super(Outer.SubclassOfInner, self).__init__() assert self.outer == outer class SubsubclassOfInner(SubclassOfInner): def __init__(self, outer): super(Outer.SubsubclassOfInner, self).__init__() assert self.outer == outer class Subclass2OfInner(Inner): def __init__(self, outer): super(Outer.Subclass2OfInner, self).__init__() assert self.outer == outer class RandomUnboundInner(object): def __init__(self): super(Outer.RandomUnboundInner, self).__init__() pass class MultipleInheritanceTest(SubclassOfInner, RandomUnboundInner, Subclass2OfInner): def __init__(self, outer): super(Outer.MultipleInheritanceTest, self).__init__() assert self.outer == outer class UnboundSubclassOfInner(UnboundInnerClass, Inner): pass def tests(): assert outer.Inner == outer.Inner assert isinstance(inner, outer.Inner) assert isinstance(inner, Outer.Inner) assert isinstance(subclass, Outer.SubclassOfInner) assert isinstance(subclass, outer.SubclassOfInner) assert isinstance(subclass, Outer.Inner) assert isinstance(subclass, outer.Inner) assert isinstance(subsubclass, Outer.SubsubclassOfInner) assert isinstance(subsubclass, outer.SubsubclassOfInner) assert isinstance(subsubclass, Outer.SubclassOfInner) assert isinstance(subsubclass, outer.SubclassOfInner) assert isinstance(subsubclass, Outer.Inner) assert isinstance(subsubclass, outer.Inner) import itertools for order in itertools.permutations([1, 2, 3]): outer = Outer() # This strange "for" statement lets us test every possible order of # initialization for the "inner" / "subclass" / "subsubclass" objects. for which in order: if which == 1: inner = outer.Inner() elif which == 2: subclass = outer.SubclassOfInner() elif which == 3: subsubclass = outer.SubsubclassOfInner() tests() multiple_inheritance_test = outer.MultipleInheritanceTest() assert outer.MultipleInheritanceTest.mro() == [ # bound inner class, notice lowercase-o "outer" outer.MultipleInheritanceTest, # unbound inner class, notice uppercase-o "Outer" Outer.MultipleInheritanceTest, outer.SubclassOfInner, # bound Outer.SubclassOfInner, # unbound Outer.RandomUnboundInner, # etc. outer.Subclass2OfInner, Outer.Subclass2OfInner, outer.Inner, Outer.Inner, BoundInnerClass, object ] unbound = outer.UnboundSubclassOfInner() assert outer.UnboundSubclassOfInner.mro() == [ outer.UnboundSubclassOfInner, Outer.UnboundSubclassOfInner, UnboundInnerClass, outer.Inner, Outer.Inner, BoundInnerClass, object ] class InnerChild(outer.Inner): pass inner_child = InnerChild() isinstance(inner_child, Outer.Inner) isinstance(inner_child, InnerChild) isinstance(inner_child, outer.Inner)