Instance of one class to contain arbitrary number of instances of another class in Python

Question

I'm trying to see if there is a better way to design this. I have a class Animal that is inherited by Male and Female classes (Female class has an additional attribute). I also have a class called Habitat, an instance of each would contain any number of Animals including 0.

class Animal:
 def __init__(self, life_span=20, age=0):
 self.__life_span = life_span
 self.__age = age
 def get_life_span(self):
 return self.__life_span
 def get_age(self):
 return self.__age
 def age(self):
 self.__age += 1
class Female(Animal):
 __gender = 'female'
 def __init__(self, pregnant=False):
 self.__pregnant = pregnant
 def impregnate(self):
 self.__pregnant = True
class Male(Animal):
 __gender = 'male'
class Habitat:
 def __init__(self, list_of_animals=[Male(), Female()]): 
 self.__list_of_animals = list_of_animals
 def __add_male(self):
 self.__list_of_animals.append(Male())
 def __add_female(self):
 self.__list_of_animals.append(Female())
 def add_animal(self):
 if random.choice('mf') == 'm':
 self.__add_male()
 else:
 self.__add_female() 
 def get_population(self):
 return self.__list_of_animals.__len__()

Before I add any more functionality, I would like to find out if that's a proper way to design these classes. Maybe there is a design pattern I can use? I'm new to OOP, any other comments/suggestions are appreciated.

• delnan: FWIW, don't call obj.__len__(), use len(obj) instead. Also, no need to use name mangling (double leading underscores): Just use self.age or self._age, especially use the former in favor of trivial getters (you can later replace them with a property, if you need that extra power, without changing the way client code works with your objects).
• Jon Clements: It would appear you're new to OOP in general - what's your experience?
• Martijn Pieters: @tallseth: already flagged as such. OP: if the moderators agree, your question will be automatically migrated for you. Please do not double-post.
• dfo: @JonClements somehow I managed to avoid the whole thing and it's been bugging me for years, so now I'm trying to get a grip of it
• dfo: @MartijnPieters thanks, can I do it myself without waiting for moderators?

Answer 1

In Habitat.__init__, you've committed a classic beginner's fallacy: using a list as a default argument.

In Python, every call to a function with a default argument will use the same default object. If that object is mutable (e.g. a list) any mutations to that object (e.g. .append) will affect that one object. In effect, all your Habitats will end up using the exact same list unless you specify a non-default argument.

Instead, use None as the default argument, and test for it:

def __init__(self, animals=None):
 if animals is None:
 animals = [Male(), Female()]