@dataclass
class CounterData:
    counter_id: str
    period_start: datetime
    period_end: datetime
    counter_name: str = ""
    enter_count: int = 0
    exit_count: int = 0
class SensorData:
    def __init__(self, sensor_type: str, sensor_serial: str, counter_data: list[CounterData], sensor_name: str = ""):
        self.sensor_serial = sensor_serial
        self.sensor_name = sensor_name
        self.sensor_type = sensor_type
        self.counters = counter_data
    def __str__(self):
        txt = f'SensorType={self.sensor_type}, SerialNumber={self.sensor_serial}, SensorName={self.sensor_name}'
        if len(self.counters) == 0:
            txt += '\t No counters data.'
        else:
            for counter in self.counters:
                txt += f'\n\tcounter_id={counter.counter_id}, counter_name={counter.counter_name}' \
                       f'\n\t\tperiod= {counter.period_start} - {counter.period_end}' \
                       f'\n\t\tentered= {counter.enter_count} exited= {counter.exit_count}'
        return txt
    def add_counter_data(self, counter_data: CounterData):
        self.counters.append(counter_data)

 def parse_xml(file_name_full: str) -> SensorData | None:
    try:
        xml_tree = ElementTree.parse(file_name_full)
    except ElementTree.ParseError as err:
        print(repr(err))
        return None
    xml_root = xml_tree.getroot()
    namespace = {'ns': 'http://www.tralivali.com/api/v2'}
    s_type = '3Dv2'
    s_serial = xml_root.find('ns:sensor-info/ns:serial-number', namespace).text
    if not s_serial:
        return None
    s_name = file_name_full
    sensor_data = SensorData(sensor_serial=s_serial, sensor_name=s_name, sensor_type=s_type)
    count_lines = xml_root.findall('ns:content/ns:elements/ns:element', namespace)
    for count_line in count_lines:
        line_measurements = count_line.findall('ns:measurements/ns:measurement', namespace)
        for measurement in line_measurements:
            enter_count = int(measurement.find("ns:values/ns:value[@label='fw']", namespace).text)
            exit_count = int(measurement.find("ns:values/ns:value[@label='fw']", namespace).text)
            line_count_data = CounterData(
                counter_id=count_line.find("ns:element-id", namespace).text,
                period_start=datetime.fromisoformat(count_line.find("ns:from", namespace).text),
                period_end=datetime.fromisoformat(count_line.find("ns:to", namespace).text),
                counter_name=count_line.find("ns:element-name", namespace).text,
                enter_count=enter_count,
                exit_count=exit_count
            )
            sensor_data.add_counter_data(line_count_data)
    if len(sensor_data.counters) == 0:
        return None
    else:
        return sensor_data

 ...
sensor_count_data: Optional[SensorData] = parse_xml(file_name_full)
...

 line_count_data = CounterData(.....

 def __init__(self, sensor_type: str, sensor_serial: str, counter_data: list[CounterData] = [], sensor_name: str = "")

  
>>> def f(n, lst=[]):
...     lst.append(n)
...     return lst
... 
>>> f(1)
[1]
>>> f(2)
[1, 2]
>>> f(3)
[1, 2, 3]
>>> 
>>> f(4, [])
[4]
>>> f(5)
[1, 2, 3, 5]
>>> 
>>> f(6, [])
[6]
>>> f(7, [])
[7]
>>>

  
>>> def f(n):
...     f.x += 10
...     return n
... 
>>> f.x = 0
>>> 
>>> f(1)
1
>>> f(2)
2
>>> f(3)
3
>>> 
>>> f.x
30
>>>