Exception on Invalid References During Table Import

[pasetti]

Consider the following function in convert.py:

def convert_exp_to_db(project, s):
    """
    The argument is a plain reference field in export representation 
    (the reference is represented by the string domain:name). 
    The function converts it to database representation.
    Invalid references raise an exception which must be handled by the caller.
    """
    if s == '':
        return None
    m = pattern_ref_exp.match(s)
    ref = m.group().split(':')
    return SpecItem.objects.exclude(status='OBS').exclude(status='DEL').\
                        get(project_id=project.id, domain=ref[0], name=ref[1])

If I export a table where there are reference fields which are empty, the csv will have a value of 'None' for such cells. If I then import the same table, the function above will throw an exception when processing the cell with the 'None' value.

More generally, I need to modify this function to handle cases where there are invalid references.