def _isurl(filename):
    """
    Determine if filename is a URL. Valid URLs start with
        'http://'
        'https://'
        'ftp://'
    """
    if filename[0:7].lower() == 'http://':
        return True
    elif filename[0:8].lower() == 'https://':
        return True
    elif filename[0:6].lower() == 'ftp://':
        return True
    else:
        return False