di-context.py

# dumb example object with no dependencies:
class Printer(object):
  def println(self, message):
    print message


# dumb example object with dependencies:
class Greeter(object):
  
  # can't use it without deciding on a printer
  # (forced dependency inversion)
  def __init__(self, printer):
    self.printer = printer
  
  def say(self, msg):
    self.printer.println(msg)


printer = Printer()

# explicitly resolve dependencies through the constructor:
greeter = Greeter(printer=printer)
test_greeter = Greeter(printer=Mock())

# If you are at the point that doing it by hand in production code is cumbersome
# or results in a duplicated knowledge - assuming a refactoring isn't the right solution
# to that problem - I like to use an explicit container that doesn't do any magic:

class Context(object):
  objects = {}
  
  @classmethod
  def register(cls, name, object):
    cls.objects[name] = object
  
  @classmethod
  def get(cls, name):
    return cls.objects[name]


Context.register('printer', printer)
Context.register('greeter', greeter)

greeter_with_default_deps = Context.get('greeter')

di-factory.py

class Context(object):
  """Factory for getting objects with default deps."""
  
  @property
  def printer(self):
    return Printer()
  
  @property
  def greeter(self, **kwargs):
    kwargs.setdefault('printer', self.printer)
    return Greeter(**kwargs)