######################################################################
##
## 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 os.getenv('USERNAME') or 'Some User' # USERNAME is platform-specific
# ---------------------------------------------------------------------------------
# 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
# ---------------------------------------------------------------------------------
