Updates the given defaults with values from the config files and
        the environ. Does a little special handling for certain types of
        options (lists).

def update_defaults(self, defaults):
        """
        Updates the given defaults with values from the config files and
        the environ. Does a little special handling for certain types of
        options (lists).
        """
        # Then go and look for the other sources of configuration:
        config = {}
        # 1. config files
        config.update(dict(self.get_config_section('virtualenv')))
        # 2. environmental variables
        config.update(dict(self.get_environ_vars()))
        # Then set the options with those values
        for key, val in config.items():
            key = key.replace('_', '-')
            if not key.startswith('--'):
                key = '--%s' % key  # only prefer long opts
            option = self.get_option(key)
            if option is not None:
                # ignore empty values
                if not val:
                    continue
                # handle multiline configs
                if option.action == 'append':
                    val = val.split()
                else:
                    option.nargs = 1
                if option.action == 'store_false':
                    val = not strtobool(val)
                elif option.action in ('store_true', 'count'):
                    val = strtobool(val)
                try:
                    val = option.convert_value(key, val)
                except optparse.OptionValueError:
                    e = sys.exc_info()[1]
                    print("An error occured during configuration: %s" % e)
                    sys.exit(3)
                defaults[option.dest] = val
        return defaults