Self Referencing Class Definition in python

Question

is there any way to reference a class name from within the class declaration? an example follows:

class Plan(SiloBase):
    cost = DataField(int)
    start = DataField(System.DateTime)
    name = DataField(str)
    items = DataCollection(int)
    subPlan = ReferenceField(Plan)

i've got a metaclass that reads this information and does some setup, and the base class implements some common saving stuff. i would love to be able to create recursive definitions like this, but so far in my experimentation i have been unable to get the effect i desire, usually running into a "Plan is not defined" error. I understand what is happening, the name of the class isn't in scope inside the class.

Answer 1

Try this:

class Plan(SiloBase):
    cost = DataField(int)
    start = DataField(System.DateTime)
    name = DataField(str)
    items = DataCollection(int)

Plan.subPlan = ReferenceField(Plan)

OR use __new__ like this:

class Plan(SiloBase):

    def __new__(cls, *args, **kwargs):
        cls.cost = DataField(int)
        cls.start = DataField(System.DateTime)
        cls.name = DataField(str)
        cls.items = DataCollection(int)
        cls.subPlan = ReferenceField(cls)
        return object.__new__(cls, *args, **kwargs)

Answer 2

i've got a metaclass that reads this information and does some setup

Most frameworks that use metaclasses provide a way to resolve this. For instance, Django :

subplan = ForeignKey('self')

Google App Engine :

subplan = SelfReferenceProperty()

The problem with solutions like tacking an additional property on later or using __new__ is that most ORM metaclasses expect the class properties to exist at the time when the class is created.

Answer 3

I understand what is happening, the name of the class isn't in scope inside the class.

Not exactly. The name of the class is not yet defined when defining it's contents (eg scope).

Answer 4

Sine Python 3.7 and PEP 563 there's a way to do that.

Add the import

from __future__ import annotations

and the following code will work

from __future__ import annotations
from typing import List

class Refer(object):
    def __init__(self, x: Plan):
        self.x: Plan = x

class Plan(object):
    def __init__(self):
        pass
    subPlan: Refer(Plan())

Answer 5

No, you can't do that. Think about what would happen if you did this:

 OtherPlan = Plan
 other_plan = OtherPlan()

At instantiation of other_plan , what is the name of the class?

Anyway, this sort of thing is best done in the __new__ method, which takes a cls parameter referring to the class.

Answer 6

One can do this with the following code in python3. We use cached_property to only evaluate the Player.enemyPlayer once and then return the cached result. Because our value comes from a function, it is not evaluated when the class is first loaded.

class cached_property(object):
    # this caches the result of the function call for fn with no inputs
    # use this as a decorator on function methods that you want converted
    # into cached properties
    def __init__(self, fn):
        self._fn = fn

    def __set_name__(self, owner, name):
        # only works in python >= 3.6
        self.name = name
        self._cache_key = "_" + self.name

    def __get__(self, instance, cls=None):
        if self._cache_key in vars(self):
            return vars(self)[self._cache_key]
        else:
            result = self._fn()
            setattr(self, self._cache_key, result)
            return result


class Player:
    @cached_property
    def enemyPlayer():
        return Player