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.

Evan Fosmark · Answer 1 · 2009-06-19T22:26:09.390

25

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)

edited Jun 19 '09 at 22:26

answered Jun 19 '09 at 22:18

Evan Fosmark

98,895
36
105
117

1

awesome, this does exactly what i want it to, and its not a humongous hack. still trying to wrap my head around this whole dynamic thing, haha – pedlar Jun 19 '09 at 22:27

score 9 · Accepted Answer · answered Jun 19 '09 at 23:15

9

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.

answered Jun 19 '09 at 23:15

Miles

31,360
7
64
74

1

Great that you mentioned Google App Engine as well! +1 – Gergely Orosz May 29 '10 at 12:54
1

In Google App Engine - DB has this, but NDB does not. To see how to do this in NDB look here: http://stackoverflow.com/questions/3531936/selfreferenceproperty-question – Brad Dec 27 '13 at 22:03

score 2 · Answer 3 · answered Jun 19 '09 at 22:23

2

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 (e.g. scope).

answered Jun 19 '09 at 22:23

Piotr Findeisen

19,480
2
52
82

score 1 · Answer 4 · answered Nov 21 '19 at 07:29

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())

score 0 · Answer 5 · answered Jun 19 '09 at 22:20

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.

Self Referencing Class Definition in python

5 Answers5

Linked

Related