Python format and validate class variable without instantiating it

Question

I'm trying to validate and format a class variable. The class extends a class with ABCMeta as its __metaclass__ and I can't yet instantiate my child class. So when I run this below code it prints property object and not the output I want, which I understand why. But then how do I do it?

from abc import ABCMeta, abstractproperty

class RuleBase(object):
    __metaclass__ = ABCMeta

    id = None
    id = abstractproperty(id)

    @property
    def format_id(self):
        try:
            _id = 'R%02d' % self.id
        except TypeError:
            raise TypeError("ID must be an integer")
        return _id

    @classmethod
    def verbose(cls):
        print cls.format_id

class Rule(RuleBase):
    id = 1

Rule.verbose()  # prints property object and not R01

In my theory, I think this would work.

class FormattingABCMeta(ABCMeta):
    def __new__(mcl, classname, bases, dictionary):
        # Format here, remove format_id property from RuleBase, 
        # and use FormattingABCMeta as metaclass for RuleBase.
        return super(C, mcl).__new__(mcl, classname, bases, dictionary)

But then I feel I'm twisting it too much. So, how to do this? And is my understanding correct? Any help is appreciated.

Properties only work on instances, not on classes. They are simply not bound to a class. Since verbose is a class method, you can't use cls.format_id. You'd have to make format_id a classmethod too and call it. — Martijn Pieters
– Martijn Pieters, Commented May 8, 2016 at 21:58

Padraic Cunningham · Accepted Answer · 2016-05-08 21:53:14Z

2

Not entirely sure what your goal is but you would need to use fget on the property object passing cls to get the _id:

 print cls.format_id.fget(cls)

answered May 8, 2016 at 21:53

Padraic Cunningham

181k30 gold badges264 silver badges327 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Anshu Kumar Over a year ago

Yes that would work. Thank you. However, is this the most pythonic way to do this? Also, does my goal look too skewed? I mean I have certain Rules extending RuleBase and I want their verbosity to be printed. Does doing it without creating their instances sound weird?

Martijn Pieters Over a year ago

This'd work provided the id attribute is always a class attribute.

Padraic Cunningham Over a year ago

@AnshuKumar, as Martijn commented below your question, the other option is to make it a classmethod and print cls.format_id() but this will work fine presuming you are dealing with class attributes which from your question I presumed you were.

Collectives™ on Stack Overflow

Python format and validate class variable without instantiating it

1 Answer 1

3 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related