from configparser import ConfigParser


class MyException(Exception):
    pass


class MyConfig(ConfigParser):
    def __init__(self, config_file):
        super(MyConfig, self).__init__()

        self.read(config_file)
        self.validate_config()

    def validate_config(self):
        required_values = {
            'server': {
                'timeout': None,
                'gc_mode': ('none', 'aggressive', 'smart', 'auto'),
                'mode': ('master')
            },
            'client': {
                'fallback': ('none', 'polling', 'auto'),
                'mode': ('master', 'slave')
            }
        }
        """
        Notice the different mode validations for global mode setting: we can
        enforce different value sets for different sections
        """

        for section, keys in required_values.items():
            if section not in self:
                raise MyException(
                    'Missing section %s in the config file' % section)

            for key, values in keys.items():
                if key not in self[section] or self[section][key] == '':
                    raise MyException((
                        'Missing value for %s under section %s in ' +
                        'the config file') % (key, section))

                if values:
                    if self[section][key] not in values:
                        raise MyException((
                            'Invalid value for %s under section %s in ' +
                            'the config file') % (key, section))

cfg = {}

try:
    # The example config file has an invalid value so cfg will stay empty first
    cfg = MyConfig('config.ini')
except MyException as e:
    # Initially you'll see this due to the invalid value
    print(e)
else:
    # Once you fix the config file you'll see this
    print(cfg['client']['fallback'])