Programming pattern for combining factory functions

Question

Say you have a bunch of factory functions, each of which does two things:

1. Modify or add arguments to the initialization of a class
2. Does something with the class instance afterwards

E.g.

class Dog:
  def __init__(self, **very_many_kwargs):
    pass

def create_police_dog(department, **dog_kwargs):
  dog_kwargs['race'] = 'pitbull_terrier'
  dog = Dog(**dog_kwargs)
  police_academy = PoliceAcademy()
  police_academy.train(dog)
  return dog

def create_scary_dog(**dog_kwargs):
  dog_kwargs['teeth_size'] = 'MAX'
  dog_kwargs['eye_color'] = fetch_angry_eye_colors('https://dogs.com')
  dog = Dog(**dog_kwargs)
  dog.experience_unhappy_childhood()
  return dog

How to combine multiple of such functions in series?

Answer 1

Decorators almost work but since you want your modifications to occur both before and after instantiation, they won't chain properly. Instead define a custom system of generic modifiers that can be chained together at creation time:

from abc import ABC, abstractmethod                                                                  
                                                                                                     
class DogModifier(ABC):                                                                              
    @abstractmethod                                                                                  
    def mod_kwargs(self, **kwargs):                                                                  
        pass                                                                                         
                                                                                                     
    @abstractmethod                                                                                  
    def post_init(self, dog):                                                                        
        pass                                                                                         
                                                                                                     
class PoliceDog(DogModifier):                                                                        
    def __init__(self, department):                                                                  
        self._dept = department                                                                      
                                                                                                     
    def mod_kwargs(self, **kwargs):                                                                 
        kwargs['race'] = 'pitbull_terrier'                                                          
                                                                                                    
    def post_init(self, dog):                                                                       
        PoliceAcademy(self._dept).train(dog)                                                        
                                                                                                        
class ScaryDog(DogModifier):                                                                        
    def mod_kwargs(self, **kwargs):                                                                 
        kwargs['teeth_size'] = 'MAX'                                                                
        kwargs['eye_color'] = fetch_angry_eye_color('https://dogs.com')                             
                                                                                                    
    def post_init(self, dog):                                                                       
        dog.experience_unhappy_childhood()                                                          
                                                                                                    
def create_dog(*modifiers, **dog_kwargs):                                                               
    for m in modifiers:                                                                             
        m.mod_kwargs(**dog_kwargs)                                                                      
                                                                                                    
    dog = Dog(**dog_kwargs)                                                                         
                                                                                                    
    for m in modifiers:                                                                             
        m.post_init(dog)                                                                            
                                                                                                    
    return dog                                                                                      
                                                                                                    
# ...                                                                                               
                                                                                                    
police_dog = create_dog(PoliceDog('bomb squad'), kw1='a', kw2='b')                                  
scary_dog = create_dog(ScaryDog(), kw1='x', kw2='y')                                                
scary_police_dog = create_dog(PoliceDog('bomb squad'), ScaryDog(), kw1='z')

*code shown as example only - bugfixes left as an exercise for the reader