'Demonstrate effective use of super()' import collections import logging logging.basicConfig(level='INFO') class LoggingDict(dict): # Simple example of extending a builtin class def __setitem__(self, key, value): logging.info('Setting %r to %r' % (key, value)) return super().__setitem__(key, value) class LoggingOD(LoggingDict, collections.OrderedDict): # Build new functionality by reordering the MRO pass ld = LoggingDict([('red', 1), ('green', 2), ('blue', 3)]) print(ld) ld['red'] = 10 ld = LoggingOD([('red', 1), ('green', 2), ('blue', 3)]) print(ld) ld['red'] = 10 print('-' * 20) # ------- Show the order that the methods are called ------- def show_call_order(cls, methname): 'Utility to show the call chain' classes = [cls for cls in cls.__mro__ if methname in cls.__dict__] print(' ==> '.join('%s.%s' % (cls.__name__, methname) for cls in classes)) show_call_order(LoggingOD, '__setitem__') show_call_order(LoggingOD, '__iter__') # ------- Getting the argument signatures to match ---------- class Shape: def __init__(self, **kwds): self.shapename = kwds.pop('shapename') super().__init__(**kwds) class ColoredShape(Shape): def __init__(self, **kwds): self.color = kwds.pop('color') super().__init__(**kwds) cs = ColoredShape(color='red', shapename='circle') # -------- Making sure a root exists -------------------------- class Root: def draw(self): pass # the delegation chain stops here class Shape(Root): def __init__(self, **kwds): self.shapename = kwds.pop('shapename') super().__init__(**kwds) def draw(self): print('Drawing. Setting shape to:', self.shapename) super().draw() class ColoredShape(Shape): def __init__(self, **kwds): self.color = kwds.pop('color') super().__init__(**kwds) def draw(self): print('Drawing. Setting color to:', self.color) super().draw() ColoredShape(color='blue', shapename='square').draw() # ------- Show how to wrap a non-cooperative class -------- class Moveable: # non-cooperative class that doesn't use super() def __init__(self, x, y): self.x = x self.y = y def draw(self): print('Drawing at position:', self.x, self.y) class MoveableWrapper(Root): # use co def __init__(self, **kwds): x = kwds.pop('x') y = kwds.pop('y') self.moveable = Moveable(x, y) super().__init__(**kwds) def draw(self): self.moveable.draw() super().draw() class MovableColoredShape(ColoredShape, MoveableWrapper): pass # -------- Complete example --------------------------------- from collections import Counter, OrderedDict class OrderedCounter(Counter, OrderedDict): 'Counter that remembers the order elements are first encountered' def __repr__(self): return '%s(%r)' % (self.__class__.__name__, OrderedDict(self)) def __reduce__(self): return self.__class__, (OrderedDict(self),) oc = OrderedCounter('abracadabra') print(oc)