Python's Class Variables and Inheritance

Question

I have here some code for a unit conversion program; it throws a NameError, because of Python's inheritance order.

class _Units :
    _metric_unit_names   = {'metric'}
    _standard_unit_names = {'standard'}

class TemperatureUnits (_Units) :
    _metric_unit_names.update({'celsius', 'c'})
    _standard_unit_names.update({'fahrenheit', 'f'})

TemperatureUnits()

I was wondering what the "best" technique for this situation would be. I could make _metric_unit_names and _standard_unit_names instance variables, but to make a new set on each instantiation seems wasteful. Also having a shared behavior seems optimal in this particular situation.

What makes you believe you need multiple classes and even inheritance here, when multiple instances of the same class would probably be sufficient? Is there some behaviour in the subclasses that you didn't show here? — Niklas B.
– Niklas B., Commented Feb 26, 2013 at 0:04

Demian Brecht · Accepted Answer · 2013-02-25 23:43:49Z

The best course of action is to not define the attributes as static attributes of the class. What you're after is something like this:

class _Units :
    def __init__(self):
        self._metric_unit_names   = {'metric'}
        self._standard_unit_names = {'standard'}

class TemperatureUnits (_Units) :
    def __init__(self):
        _Units.__init__(self)
        self._metric_unit_names.update({'celsius', 'c'})
        self._standard_unit_names.update({'fahrenheit', 'f'})

TemperatureUnits()

Defining attributes outside of __init__ cause them to be static members of the class (i.e. _Units._metric_unit_names). Defining them within init cause them to be attributes of a class instance (i.e. my_units_instance._metric_unit_names).

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