Inversion of Control in Python (DI)

IoC.py

import os

class FeatureBroker:

    def __init__(self, allowReplace=False):
        self.providers = {}
        self.allowReplace = allowReplace

    def Provide(self, feature, provider, *args, **kwargs):
        if not self.allowReplace:
            assert feature not in self.providers, \
                "Duplicate feature: {}".format(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 {}".format(feature))
        return provider()

features = FeatureBroker()


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


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"


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)


class SimpleConsole(Component):
    def WriteLine(self, s):
        print(s)


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('')


def GetCurrentUser():
    return os.getenv('USERNAME') or 'Some User'


if __name__ == '__main__':
    print('\n*** IoC Demo ***')
    features.Provide('AppTitle', 'Inversion of Control ...\n\nThe Python Way!')
    features.Provide('CurrentUser', GetCurrentUser)
    features.Provide('Console', BetterConsole, prefix='-->')
    # features.Provide('Console', BetterConsole(prefix='-->'))

    bar = Bar()
    bar.PrintYourself()