Почему issubclass в python работает не так как ожидалось?

Question

[Любитель поиграть в самолёты]

Привет, делаю миграции в peewee по статье с хабра - https://habr.com/ru/post/262697/

Есть абстрактный класс мигратор

file: ./migrator.py

class Migrator(object):
    """
    Migration interface
    """

    __metaclass__ = abc.ABCMeta

    db_connection = PostgresqlDatabase(config.DB_NAME,
                                       user=config.USERNAME,
                                       password=config.PASSWORD,
                                       host=config.HOST,
                                       port=config.PORT,
                                       autorollback=True)

    # db_connection is a Borg instance
    connection = db_connection.connection
    migrator = PostgresqlMigrator(db_connection.connection)

    @abc.abstractproperty
    def migrations(self):
        """
        List of the migrations dictionaries
        :param self: class instance
        :return: list
        """
        return [
            {'statement': 1 != 2, 'migration': ['list', 'of', 'migration', 'options'],
             'migration_kwargs': {}, 'pre_migrations': list(), 'post_migrations': list()}
        ]          # Just an example

    def migrate(self):
        """
        Run migrations
        """
        for migration in self.migrations:
            if migration['statement']:
                # Run scripts before the migration
                pre_migrations = migration.get('pre_migrations', list())
                for pre_m in pre_migrations:
                    pre_m()
                # Migrate
                with self.db_connection.connection.transaction():
                    migration_kwargs = migration.get('migration_kwargs', {})
                    migrate(*migration['migration'], **migration_kwargs)
                # Run scripts after the migration
                post_migrations = migration.get('post_migrations', list())
                for post_m in post_migrations:
                    post_m()

Также в этом файле есть функции которые ищут модули в папке ./migrations/migrations

file: ./migrator.py

def get_migration_modules(packages=None):
    """
    Get python modules with migrations
    :param packages: iterable - list or tuple with packages names for the searching
    :return: list - ('module.path', 'module_name')
    """
    if packages is None:
        packages = []
    # List of the modules to migrate
    migration_modules = []
    for pack in packages:
        migration_module = __import__(
            pack, globals(), locals(), fromlist=['migrations'])

        with contextlib.suppress(AttributeError):
            # Check, that imported object is module
            if inspect.ismodule(migration_module.migrations):
                # Find submodules inside the module
                migration_modules.extend((migration_module.migrations.__name__, modname) for importer, modname, ispkg in pkgutil.iter_modules(
                    migration_module.migrations.__path__) if re.match(r'^\d{3,}_migration_[\d\w_]+$', modname) and not ispkg)

            # Unregister module
            sys.modules.pop(migration_module.__name__)
    return migration_modules


def get_migration_classes(migration_modules):
    """
    Get list of the migration classes
    :type migration_modules: iterable
    :param migration_modules: array with a migration modules
    :return: list
    """
    migration_classes = []
    for mig_mod, m in migration_modules:
        mig = __import__(mig_mod, globals(), locals(), fromlist=[m])
        with contextlib.suppress(AttributeError):
            target_module = mig.__getattribute__(m)
            # Check, that imported object is module
            if inspect.ismodule(target_module):
                migration_classes.extend(obj for name, obj in inspect.getmembers(target_module) if inspect.isclass(obj) and issubclass(obj, Migrator) and obj != Migrator)

            # Remove imported module from the stack
            sys.modules.pop(mig.__name__)
    return migration_classes

И также запуск поиска и авто запуска миграций

file: ./migrator.py

if __name__ == '__main__':
    # Get modules with migrations
    m_mods = get_migration_modules(packages=['migrations'])
    # Get migration classes
    m_classes = get_migration_classes(m_mods)
    print(m_classes)

    # Execute migrations
    for m_class in m_classes:
        mig = m_class()
        mig.migrate()

Пока что в папке ./migrations/migrations лежит одна миграция 001_migration_add_new_field.py

Вот она:

file: ./migrations/migrations/001_migration_add_new_field.py

from migrator import Migrator
from utils.database import db as mig_db


class AddFieldInPoll(Migrator):
    table_name = mig_db.Poll._meta.table_name

    def __field_not_exists(self):
        """
        Check, that new field does not exists
        :return: bool
        """
        q = 'SELECT 1 from information_schema.columns where column_name = \'group_for_send\' and table_name = \'{0}\''.format(
            self.table_name)
        cursor = self.connection.execute_sql(q)
        result = int(cursor.fetchone()[0])
        return result == 0

    @property
    def migrations(self):
        print('001_migration')
        return [
            # add group_for_send column
            {
                'statement': self.__field_not_exists(),
                'migration': [self.migrator.add_column(self.table_name, 'group_for_send', mig_db.Poll.group_for_send)],
            }
        ]

Но миграция не находится так как в этом моменте не очень понятно работает issubclass

Момент:

file: ./migrator.py

migration_classes.extend(obj for name, obj in inspect.getmembers(target_module) if inspect.isclass(obj) and issubclass(obj, Migrator) and obj != Migrator)

В этом цикле в самой первой итерации

obj = <class 'migrations.migrations.001_migration_add_new_field.AddFieldInPoll'>

Первое условие выдаёт True: inspect.isclass(obj)
Третье выдаёт True: obj != Migrator
А вот второе выдаёт False: issubclass(obj, Migrator)

Хотя родитель класса AddFieldInPoll это класс Migrator. Почему так и как это исправить?