def _validate_session_write_concern(session, write_concern):
    """Validate that an explicit session is not used with an unack'ed write.

    Returns the session to use for the next operation.
    """
    if session:
        if write_concern is not None and not write_concern.acknowledged:
            # For unacknowledged writes without an explicit session,
            # drivers SHOULD NOT use an implicit session. If a driver
            # creates an implicit session for unacknowledged writes
            # without an explicit session, the driver MUST NOT send the
            # session ID.
            if session._implicit:
                return None
            else:
                raise ConfigurationError(
                    'Explicit sessions are incompatible with '
                    'unacknowledged write concern: %r' % (
                        write_concern,))
    return session
Validate that an explicit session is not used with an unack'ed write.

    Returns the session to use for the next operation.
    
stackoverflow/venv/lib/python3.6/site-packages/pymongo/client_session.py