Class Config with descriptor and validator

"""
https://docs.python.org/3.9/howto/descriptor.html#complete-practical-example
"""

from abc import ABC, abstractmethod
import collections
from http import HTTPStatus
import logging
import os
from urllib import request

logging.basicConfig(level=logging.INFO)

class ConfigParameter():

    def __set_name__(self, owner, name):
        """Called at the time the owning class "owner" is created.
        The descriptor has been assigned to "name".

        https://docs.python.org/3/reference/datamodel.html#object.__set_name__
        """
        self.public_name = name
        self.private_name = f'_{name}'

    def __get__(self, obj, objtype=None):
        """
        """
        value = getattr(obj, self.private_name)
        logging.info('Accessing %r giving %r', self.public_name, value)
        return value

    def __set__(self, obj, value):
        logging.info('Updating %r to %r', self.public_name, value)
        setattr(obj, self.private_name, value)


class ConfigValidParameter(ABC):

    def __set_name__(self, owner, name):
        """Called at the time the owning class "owner" is created.
        The descriptor has been assigned to "name".

        https://docs.python.org/3/reference/datamodel.html#object.__set_name__
        """
        self.public_name = name
        self.private_name = f'_{name}'

    def __get__(self, obj, objtype=None):
        """
        """
        value = getattr(obj, self.private_name)
        logging.info('Accessing %r giving %r', self.public_name, value)
        return value

    def __set__(self, obj, value):
        logging.info('Updating %r to %r', self.public_name, value)
        self.validate(value)
        setattr(obj, self.private_name, value)

    @abstractmethod
    def validate(self, value):
        pass


class OneOf(ConfigValidParameter):

    def __init__(self, options):
        self.options = set(options)

    def validate(self, value):
        # Do we accept None value ?
        if value is not in self.options:
            raise ValueError(f'Expected {value!r} to be one of {self.options!r}')


class ValidURL(ConfigValidParameter):
    
    def validate(self, value):
        # Do we accept None value ?
        response = request.urlopen(value)
        if response.status != HTTPStatus.OK:
            raise ValueError(f'Expected {value} should be a valid URL {response.status}')


class Config:

    name = ConfigParameter()
    mode = ConfigParameter()
    url = ValidURL()
    delay = ConfigParameter()
    # format is not a Python keyword, https://docs.python.org/3.8/reference/lexical_analysis.html#keywords
    format = OneOf('json', 'csv', 'xml')
    maxitems = ConfigParameter()

    def __init__(self, defaults):
        self.name = os.environ.get(defaults['name'].envvar, defaults['name'].default)
        self.mode = os.environ.get(defaults['mode'].envvar, defaults['mode'].default)
        self.url = os.environ.get(defaults['url'].envvar, defaults['url'].default)
        self.delay = os.environ.get(defaults['delay'].envvar, defaults['delay'].default)
        self.format = os.environ.get(defaults['format'].envvar, defaults['format'].default)
        self.maxitems = os.environ.get(defaults['maxitems'].envvar, defaults['maxitems'].default)


if __name__ == "__main__":
    # Using namedtuple to better manipulated defaults
    # https://docs.python.org/3.8/library/collections.html#collections.namedtuple
    # ---------------------------------------------------------------------------
    fparam = collections.namedtuple('FetcherParameters', ['name', 'envvar', 'description', 'default'])

    fetcher_defaults = {
            'name': fparam(name='name', envvar='HTTP_FETCHER_NAME', description='Name of the acquisition', default=None),
            'mode': fparam(name='mode', envvar='HTTP_FETCHER_MODE', description='http method', default='GET'),
            'url': fparam(name='url', envvar='HTTP_FETCHER_URL', description='URL to fetch', default=None),
            'delay': fparam(name='delay', envvar='HTTP_FETCHER_DELAY', description='Delay between requests in seconds', default=3600),
            'format': fparam(name='format', envvar='HTTP_FETCHER_FORMAT', description='File format', default='json'),
            'maxitems': fparam(name='maxitems', envvar='HTTP_FETCHER_MAXITEMS', description='Max number of fetches', default=-1)
            }

    print(f"vars(Config) = {vars(Config)}")

    c = Config(fetcher_defaults)

    print(vars(c))

    c.format = 'xml'