Snippet from https://github.com/apache/airflow/blob/master/airflow/configuration.py

configuration.py

    def get(self, section, key, **kwargs):
        section = str(section).lower()
        key = str(key).lower()

        deprecated_name = self.deprecated_options.get(section, {}).get(key, None)

        # first check environment variables
        option = self._get_env_var_option(section, key)
        if option is not None:
            return option
        if deprecated_name:
            option = self._get_env_var_option(section, deprecated_name)
            if option is not None:
                self._warn_deprecate(section, key, deprecated_name)
                return option

        # ...then the config file
        if super(AirflowConfigParser, self).has_option(section, key):
            # Use the parent's methods to get the actual config here to be able to
            # separate the config from default config.
            return expand_env_var(
                super(AirflowConfigParser, self).get(section, key, **kwargs))
        if deprecated_name:
            if super(AirflowConfigParser, self).has_option(section, deprecated_name):
                self._warn_deprecate(section, key, deprecated_name)
                return expand_env_var(super(AirflowConfigParser, self).get(
                    section,
                    deprecated_name,
                    **kwargs
                ))

        # ...then commands
        option = self._get_cmd_option(section, key)
        if option:
            return option
        if deprecated_name:
            option = self._get_cmd_option(section, deprecated_name)
            if option:
                self._warn_deprecate(section, key, deprecated_name)
                return option

        # ...then the default config
        if self.airflow_defaults.has_option(section, key) or 'fallback' in kwargs:
            return expand_env_var(
                self.airflow_defaults.get(section, key, **kwargs))

        else:
            log.warning(
                "section/key [{section}/{key}] not found in config".format(**locals())
            )

            raise AirflowConfigException(
                "section/key [{section}/{key}] not found "
                "in config".format(**locals()))