Add and initilalize an enum class variable in Python

Question

I'm implementing an enum class and want to retrieve an aggregate information about the enum members. Each enum represents a breakout board with different number of pins, and I want to get the maximum number of pins over all boards. My approach was to add a class attribute _max_pins which is adjusted during __init__ of each member.

Problem is, it is not possible to define _max_pins ahead if the members as it would become a member, too. It does not help to define it after the members as then the members cannot access it during their __init__ I've looked at Declare a static variable in an enum class but the table can be set up after __init__ - that would be possible but would need to scan again all members after their initialization.

class IgelTyp(Enum):
    LED_1 = (24, 1)
    LED_2 = (24, 2)
    LED_3 = (16, 4)

    _max_pin = -1

    def __init__(self, pins, groups):
        if _max_pin < pins//groups:     # gives error
            _max_pin = pins//groups

    @classmethod
    def get_max_pins(cls):
        return cls._max_pin

Above code produces UnboundLocalError: local variable '_max_pin' referenced before assignment

When I move the assignment of _max_pin in front of the member definition it tells me that TypeError: __init__() missing 2 required positional arguments: ...

Edit 1 Actually, the TypeError is raised regardless where I put the assignment within the class. And when I use IgelTyp._max_pin = -1 I get a NameError: name 'IgelTyp' is not defined

Anyone has an efficient and readable solution?

I tried, but does not work while initialization of IgelTyp is not done (see edit 1). — Walter
– Walter, Commented Jun 24, 2019 at 11:24
Im not 100% certain, but shouldn't it be self._max_pin ... — Magellan88
– Magellan88, Commented Jun 24, 2019 at 11:38

Ethan Furman · Accepted Answer · 2019-06-27 14:43:19Z

3

One-off solution:

Change your __init__ to directly access the class's dictionary:

def __init__(self, pins, groups):
    max_pin = self.__class__.__dict__.get('max_pin', 0)
    self.__class__.max_pin = max(max_pin, pins//groups)

Interestingly, you could easily have each LED member store its own max_pin by adding this line at the end of __init__:

    self.max_pin = pins//groups

so:

>>> IgelTyp.max_pin
24

but:

>>> IgelType.LED_2.max_pin
12

Reusable solution

Create your own class attribute descriptor, and use that to shield max_pin from becoming an IgelTyp member:

class ClassVar:              # add (object) if using Python 2
    "a class variable"

    def __init__(self, value):
        # store initial value
        self.value = value

    def __get__(self, *args):
        # get value in ClassVar instance (only doable because all instances
        # share same value)
        return self.value

    def __set__(self, _, value):
        # save value in ClassVar instance (only doable because all instances
        # share same value)
        self.value = value

Descriptors, such as property usually store the value on the instance itself so that each instance can have it's own value (such as 24, 12, and 4 from your example); however, since you want to know the maximum number of pins across all instances, we just save that single value on the ClassVar instance.

The changes to IgelTyp:

class IgelTyp(Enum):

    LED_1 = (24, 1)
    LED_2 = (24, 2)
    LED_3 = (16, 4)

    max_pin = ClassVar(0)

    def __init__(self, pins, groups):
        self.max_pin = max(self.max_pin, pins//groups)

and in use:

>>> IgelTyp.max_pin
24
>>> IgelTyp.LED_2.max_pin
24

edited Jun 27, 2019 at 14:43

answered Jun 24, 2019 at 17:11

Ethan Furman

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3 Comments

Walter Over a year ago

Thank you for that answer @Ethan Furman. While it seems a proper and reusable solution, it is overshooting for my one-time occasion of use.

Ethan Furman Over a year ago

@Walter: I added a one-off solution at the top. Hope that works better for you.

Walter Over a year ago

Thank you for the addition - the one-off solution might not be as clear and straight as the Reusable solution, but it is what I was looking for!

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