###################################################################### ## ## Feature Broker ## ###################################################################### class FeatureBroker: def __init__(self, allowReplace=False): self.providers = {} self.allowReplace = allowReplace def Provide(self, feature, provider, *args, **kwargs): if not self.allowReplace: assert not self.providers.has_key(feature), "Duplicate feature: %r" % feature if callable(provider): def call(): return provider(*args, **kwargs) else: def call(): return provider self.providers[feature] = call def __getitem__(self, feature): try: provider = self.providers[feature] except KeyError: raise KeyError, "Unknown feature named %r" % feature return provider() features = FeatureBroker() ###################################################################### ## ## Representation of Required Features and Feature Assertions ## ###################################################################### # # Some basic assertions to test the suitability of injected features # def NoAssertion(obj): return True def IsInstanceOf(*classes): def test(obj): return isinstance(obj, classes) return test def HasAttributes(*attributes): def test(obj): for each in attributes: if not hasattr(obj, each): return False return True return test def HasMethods(*methods): def test(obj): for each in methods: try: attr = getattr(obj, each) except AttributeError: return False if not callable(attr): return False return True return test # # An attribute descriptor to "declare" required features # class RequiredFeature(object): def __init__(self, feature, assertion=NoAssertion): self.feature = feature self.assertion = assertion def __get__(self, obj, T): return self.result # <-- will request the feature upon first call def __getattr__(self, name): assert name == 'result', "Unexpected attribute request other then 'result'" self.result = self.Request() return self.result def Request(self): obj = features[self.feature] assert self.assertion(obj), \ "The value %r of %r does not match the specified criteria" \ % (obj, self.feature) return obj class Component(object): "Symbolic base class for components" ###################################################################### ## ## DEMO ## ###################################################################### # --------------------------------------------------------------------------------- # Some python module defines a Bar component and states the dependencies # We will assume that # - Console denotes an object with a method WriteLine(string) # - AppTitle denotes a string that represents the current application name # - CurrentUser denotes a string that represents the current user name # class Bar(Component): con = RequiredFeature('Console', HasMethods('WriteLine')) title = RequiredFeature('AppTitle', IsInstanceOf(str)) user = RequiredFeature('CurrentUser', IsInstanceOf(str)) def __init__(self): self.X = 0 def PrintYourself(self): self.con.WriteLine('-- Bar instance --') self.con.WriteLine('Title: %s' % self.title) self.con.WriteLine('User: %s' % self.user) self.con.WriteLine('X: %d' % self.X) # --------------------------------------------------------------------------------- # Some other python module defines a basic Console component # class SimpleConsole(Component): def WriteLine(self, s): print s # --------------------------------------------------------------------------------- # Yet another python module defines a better Console component # class BetterConsole(Component): def __init__(self, prefix=''): self.prefix = prefix def WriteLine(self, s): lines = s.split('\n') for line in lines: if line: print self.prefix, line else: print # --------------------------------------------------------------------------------- # Some third python module knows how to discover the current user's name # def GetCurrentUser(): return 'Some User' # --------------------------------------------------------------------------------- # Finally, the main python script specifies the application name, # decides which components/values to use for what feature, # and creates an instance of Bar to work with # if __name__ == '__main__': print '\n*** IoC Demo ***' features.Provide('AppTitle', 'Inversion of Control ...\n\n... The Python Way') features.Provide('CurrentUser', GetCurrentUser) features.Provide('Console', BetterConsole, prefix='-->') # <-- transient lifestyle ##features.Provide('Console', BetterConsole(prefix='-->')) # <-- singleton lifestyle bar = Bar() bar.PrintYourself() # # Evidently, none of the used components needed to know about each other # => Loose coupling goal achieved # ---------------------------------------------------------------------------------