Question

I have a conceptual Python design dilemma.

Say I have a City class, which represents a city in the database. The City object can be initialized in two ways:

1. An integer (actually, an ID of an existing city in a database)
2. A list of properties (name, country, population, …), which will generate a new city in the database, and retrieve its ID.

This means that the City object will always have an ID – either the initialized ID or a newly-created ID derived from the database.

The classic Java approach would overload the constructor – One constructor would get a single intparameter, and the other would get numerous strongly-typed parameters.

I’ve failed to find an elegant way to do it in Python:

• I can create a base class with a single method get_city_id, and derive CityFromID and
  CityFromNewData from it, but that’s a lot of effort to work around this language lacuna.
• Using class methods seems awkward.
• Using a constructor with a long list of parameters is also awkward: I’d put both city id and the alternatives, and verify within the method that that only a specific subset have values.

Using **kargs seems very inelegant, because the signature of the constructor does not clearly state the required input parameters, and docstrings just ain’t enough:

class City(object):
    def __init__(self, city_id=None, *args, **kargs):
        try:
            if city_id==None:
                self.city_id=city_id
            else:
                self.city_name=kargs['name']
        except:
            error="A city object must be instanciated with a city id or with"+\
            " full city details."
            raise NameError(error)

Is there a Pythonic, elegant solution to constructor overloading?

Adam

Answer 1

How about:

class City(object):
   def __init__(self, name, description, country, populations):
      self.city_name = name
      # etc.

   @classmethod
   def from_id(cls, city_id):
       # initialise from DB 

Then you can do normal object creation:

 >>> c = City('Hollowberg', '', 'Densin', 3)
 >>> c.id
 1233L

 >>> c2 = City.from_id(1233)

~~~~~~

Also you might want to check out SQLAlchemy (and Elixir) for nicer ways to do these things