def fromEnvironment(cls, environ=None, start=None):
        """
        @param environ: A dictionary-like object to inspect to discover
            inherited descriptors.  By default, L{None}, indicating that the
            real process environment should be inspected.  The default is
            suitable for typical usage.

        @param start: An integer giving the lowest value of an inherited
            descriptor systemd will give us.  By default, L{None}, indicating
            the known correct (that is, in agreement with systemd) value will be
            used.  The default is suitable for typical usage.

        @return: A new instance of C{cls} which can be used to look up the
            descriptors which have been inherited.
        """
        if environ is None:
            from os import environ
        if start is None:
            start = cls._START

        descriptors = []

        try:
            pid = int(environ['LISTEN_PID'])
        except (KeyError, ValueError):
            pass
        else:
            if pid == getpid():
                try:
                    count = int(environ['LISTEN_FDS'])
                except (KeyError, ValueError):
                    pass
                else:
                    descriptors = range(start, start + count)
                    del environ['LISTEN_PID'], environ['LISTEN_FDS']

        return cls(descriptors)

        @param environ: A dictionary-like object to inspect to discover
            inherited descriptors.  By default, L{None}, indicating that the
            real process environment should be inspected.  The default is
            suitable for typical usage.

        @param start: An integer giving the lowest value of an inherited
            descriptor systemd will give us.  By default, L{None}, indicating
            the known correct (that is, in agreement with systemd) value will be
            used.  The default is suitable for typical usage.

        @return: A new instance of C{cls} which can be used to look up the
            descriptors which have been inherited.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/python/systemd.py