pull-lp-source: how to override package-wide configuration variables?

I am using version 0.188 and I am trying to override the default config variables defined in "config.py" to:

        'MIRROR_FALLBACK': False,
        'UBUNTU_MIRROR': '',
        'UBUNTU_PORTS_MIRROR': '',
        'UBUNTU_INTERNAL_MIRROR': '',

so that the pull-lp-source script would download files from Launchpad only and not use any master mirror.

I tried changing the master mirrors to null using environment variables as well as the config files "/etc/devscripts.conf" and "~/.devscripts" like so:

       PULL_LP_SOURCE_UBUNTU_MIRROR=' ' OR
       PULL_LP_SOURCE_UBUNTU_MIRROR=
      (I did the same for PORTS_MIRROR and INTERNAL_MIRROR)

However, none of the approaches worked and pull-lp-source would still fetch files from a master mirror.

Moreover, when I try debugging the "UDTConfig" class, I notice that the "parse_devscripts_config()" and "get_value()" methods do not get called, which explains why the default config doesn't get overridden.

Could you please mention what I could be missing?

Thank you.