Python: I can't assign a defined list to the class attribute?

You have to explicitly specify the class when accessing class variables:

if category == 'under 20 min': self.clist = Runner.under_twenty_min elif category == 'under 30 min': self.clist = Runner.under_twenty_min elif category == 'under 40 min': self.clist = Runner.under_forty_min elif category == '40 min and over': self.clist = Runner.forty_and_over 

You can use a dict, set each catgory name as a key and the value is a list of all the runners in that category.
here is a simple implementation

from collections import defaultdict #runners is a list of all your runners def get_runners_by_cat(): d = defaultdict(list) for runner in runners: dict[runner.category].append(runner) return d 

There are couple of things you have to modify in your code:

1. The indentation of the init method needs to be indented within the class definition.
2. As @Tamas points out you need to explicitly state that the variables are class variable.
3. As @taoufik mentions, it might be convenient to define a dictionary instead.
4. A init method should not return anything (except None). Since your requirement is to return the runners, you could add an additional method as @taofik suggested (or you can just print them once created as shown below).

Here's an updated version of the code. Hope it helps.

class Runner: """ information of registered runners Attributes: @type email: str email of the registered runner @type category: str the speed they estimate that they can finish the race @type list: clist the list of runners in the same category """ clist = { 'under_twenty_min': [], 'under_thirty_min': [], 'under_forty_min':[], 'forty_and_above': [] } def __init__(self, email, category): """Register the email and the speed estimation of runners @type self: Runner @type email: str @type speed: int @type category:str @type clist: list @rtype: list >>>runner1=Runner('[email protected]','under 40 min') >>>runner1.email '[email protected]' >>>runner1.category 'under 40 min' """ self.email = email self.category = category if category=='under 20 min': Runner.clist['under_twenty_min'].append(self) print Runner.clist['under_twenty_min'] elif category=='under 30 min': Runner.clist['under_thirty_min'].append(self) print Runner.clist['under_thirty_min'] elif category=='under 40 min': Runner.clist['under_forty_min'].append(self) print Runner.clist['under_forty_min'] elif category=='40 min and over': Runner.clist['forty_and_above'].append(self) print Runner.clist['forty_and_above'] 

#get class name, convert it to class object if category=='under 20 min': self.clist= eval(self.__class__.__name__).under_twenty_min elif category=='under 30 min': self.clist = eval(self.__class__.__name__).under_thirty_min elif category=='under 40 min': self.clist = eval(self.__class__.__name__).under_forty_min elif category=='40 min and over': self.clist = eval(self.__class__.__name__).forty_and_above