I make the type in database in string i dont know if i have to change it, tell me ?

I dont know what is wrong, thanks in advance

the lists :

[('78,38',), ('100,40745',), ('90,75',), ('110,32',), ('89,6',)]
['78,38', '100,40745', '90,75', '110,32', '89,6']

the error :

ValueError: could not convert string to float: '78,38'

            searchrol = "Isolation Thermique par l'Extérieur"
            cursor = db.cursor()
            result_5 = cursor.execute("SELECT prix FROM PU WHERE desig_pu= ?  ", (str(searchrol),))
            row_5 = result_5.fetchall()
            print(row_5)
            liste = []
            my_list = [] 
            for tupl in row_5: # initialisation de liste
                for i in tupl: 
                    liste.append(i)  
            print (liste) # afficher la liste
            my_list = [float(i) for i in liste]
            #my_list = list(map(float, liste))
            #print(type(liste[0]))
            print (my_list)
            BT = 113.2
            BT0 = 113.4
            row_actualise = []
            for x in liste:
                y = x*(BT/BT0)
                row_actualise.append(str(y))
            print(row_actualise)
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