Python why object is recognized as str?

Question

Python

Have a function

def get_zreport(idstore, zdate):
    сonn = get_connect()
    cursor = сonn.cursor()
    qt = DBQ.ZReport(cursor, idstore, zdate)
    qt.open()
    qt.fetchall()
    result = {'zreport' : []}
    while(qt.read()):
        shifts = result['zreport']
        idx = len(shifts)
        if idx == 0 or shifts[idx-1]['idshift'] != qt.idshift:
            sh = {'idshift' : qt.idshift,
                 'iddevice' : qt.iddevice,
                 'zdate' : zdate,
                 'position' : [] }
            shifts.append(sh)
        
        position = {'idproduct' :  qt.idproduct,
                    'quantity' : qt.quantity}
        
        sh['position'].append(position)

        qt.next()
    сonn.close()    
    return result

There is a class

class ZReport(Query):
    def __init__(self, cursor, idstore, zdate):
        sql = '''select s.idshift, cv.iddevice, s.open, s.close, c.idproduct, sum(c.ammount) as quantity from shift s
                    left join cashvoucher cv on cv.uuidsession = s.uuid and cv.idstore = s.uuidstore 
                    inner join cost c on c.idcashvoucher = cv.idcashvoucher
                where s.close::date = %(zdate)s::date
                  and s.uuidstore = %(idstore)s
                group by s.idshift, cv.iddevice, c.idproduct
                order by s.open'''
        param = {'idstore' : idstore, 'zdate' : zdate }

        super().__init__(cursor, sql, param)  

        self.idshift = -1
        self.iddevice = ''
        self.open = ''
        self.close = ''
        self.idproduct = -1
        self.quantity = 0

I get this error
File "C:\service\clientrequest.py", line 52, in get_zreport
qt.open()
TypeError: 'str' object is not callable
Other Query descendants work fine under similar conditions.
What is the reason for this strange behaviour?