dep_injection.py

#!/usr/bin/python

class Cow(object):
    """ 
        the cow needs to have 1 stomach with 4 
        sections!
        (but here we only have one for simplicity)
    """
    def __init__(self,stomach): # <- here we injected out stomach dependency  
                                   #into the cow class
        self.stomach = Stomach() 

    def fart(self):
        self.stomach.make_noise(strength=3)
        self.stomach.exhust_gas()

class Stomach(object):
    """
        this is out dependency 
    """
    def __init__(self):
        pass

    def make_noise(self, strength):
        print("frrrrp "*strength)

    def exhust_gas(self):
        print("releasing green fumes of stinky gas")

if __name__ == "__main__":
    stomach = Stomach()
    cow = Cow(stomach) # the dependency is passed in the main to the cow , the
                        #cow has no idea of how to implement a somach just the 
                        #methods it uses