Check unique value when define concrete class for abstract variable in python

Question

Suppose that I have this architecture for my classes:

# abstracts.py
import abc
class AbstractReader(metaclass=abc.ABCMeta):
  
    @classmethod
    def get_reader_name(cl):
        return cls._READER_NAME

    @classmethod
    @property
    @abc.abstractmethod
    def _READER_NAME(cls):
        raise NotImplementedError

# concretes.py
from .abstracts import AbstractReader
class ReaderConcreteNumber1(AbstractReader):
    _READER_NAME = "NAME1"

class ReaderConcreteNumber2(AbstractReader):
    _READER_NAME = "NAME2"

Also I have a manager classes that find concrete classes by _READER_NAME variable. So I need to define unique _READER_NAME for each of my concrete classes.

how do I check that NAME1 and NAME2 are unique when concrete classes are going to define?

Answer 1

You can create a metaclass with a constructor that uses a set to keep track of the name of each instantiating class and raises an exception if a given name already exists in the set:

class UniqueName(type):
    names = set()

    def __new__(metacls, cls, bases, classdict):
        name = classdict['_READER_NAME']
        if name in metacls.names:
            raise ValueError(f"Class with name '{name}' already exists.")
        metacls.names.add(name)
        return super().__new__(metacls, cls, bases, classdict)

And make it the metaclass of your AbstractReader class. since Python does not allow a class to have multiple metaclasses, you would need to make AbstractReader inherit from abc.ABCMeta instead of having it as a metaclass:

class AbstractReader(abc.ABCMeta, metaclass=UniqueName):
    ... # your original code here

Or if you want to use ABCMeta as metaclass in your AbstractReader, just override ABCMeta class and set child ABC as metaclass in AbstractReader:

class BaseABCMeta(abc.ABCMeta):
    """
    Check unique name for _READER_NAME variable
    """
    _readers_name = set()

    def __new__(mcls, name, bases, namespace, **kwargs):
        reader_name = namespace['_READER_NAME']
        if reader_name in mcls._readers_name:
            raise ValueError(f"Class with name '{reader_name}' already exists. ")
        mcls._readers_name.add(reader_name)

        return super().__new__(mcls, name, bases, namespace, **kwargs)

class AbstractReader(metaclass=BaseABCMeta):
    # Your codes ...

So that:

class ReaderConcreteNumber1(AbstractReader):
    _READER_NAME = "NAME1"

class ReaderConcreteNumber2(AbstractReader):
    _READER_NAME = "NAME1"

would produce:

ValueError: Class with name 'NAME1' already exists.

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