Python class - attribute inheritance

Question

I am trying to implement a project using class inheritance, where parent class is:

class radio:
 def __init__(self, artist=None, track=None, genre=None):
 self.artist = artist 
 self.track = track
 self.genre = genre

then I create methods for each attribute (non working examples):

 def seed_artist(self):
 results = api.search(q=self.artist, type='artist')
 return results 
 def seed_track(self):
 results = api.search(q=self.track, type='track')
 return results
 def seed_genre(self):
 results = api.search(q=self.genre, type='genre')
 return results

the user is going to pick a seed above (only one), being it either artist, track, genre or mood, and that's why I initialize all arguments with None, leaving the argument value to be inserted at inheritance level.

the inherited class is:

class playlist(radio):
 def __init__(self,user):
 radio.__init__(self, artist, track, genre)

lets say user inputs at command line a track seed, and my script ends up starting with a global variable:

track = 'karma police'

this way, all other attributes (artist, genre) remain None.

when I create an instance, say:

jeff = playlist('Jeff Smith')

It will throw an error saying that genre and artist are not defined, and I would have to inherit only trackattribute, like so:

radio.__init__(self, track)

I know I could start the script with global variables defined:

track = None
genre = None
artist = None

and this would allow me to have:

radio.__init__(self, artist, track, genre)

but this seems to me rather redundant...

Is there a workaround this, with no need for initial values of global variables set to None, keeping all the code within class scope definitions?

• BrenBarn: I gave an answer below, but it's hard to say what the alternative for the global variables is, because you haven't shown or even described how your code gets from user input to creating a global variable called tracks. One possibility is to make playlist also accept track, artist, etc., and have your input-parsing code pass a dict to it in a manner similar to what I described in my answer.
• 8-Bit Borges: @BrenBarn I've edited the question for you, in case you might want to edit your answer.

Answer 1

Instead of using a separate global variable for each field, use a single dictionary for all fields. Then you can pass the dictionary as keyword arguments.

That is, instead of creating a global variable called artist, create a dictionary called radio_fields or something. When the user inputs their query, set a key on this dict:

radio_fields = {}
...
# when you get the user's command
radio_fields['track'] = 'Karma Police'

Now you can call radio.__init__(self, **radio_fields). This will pass as arguments whatever keys are in radio_fields. If there is only one, only that one will be passed, and the others will take their default values as you defined them when you defined radio.__init__.

There might be a better way to structure your overall program, but it's hard to say based on what you've shown here. For instance, it's unclear why playlist only accepts one user argument. If playlist accepted all the field arguments that radio accepted, it could pass them on to radio without any problem.

Answer 2

You don't have those variables to pass, so don't try and pass them. The parent class already has defaults for them, so just pass the one you need:

radio.__init__(self, artist=artist)

• BrenBarn: The problem is then he would have to write a bunch of separate calls depending on which variables are defined, along with a tangle of if statements to decide which call to use.
• 8-Bit Borges: but I don't know the one I need... That's the user's call.