Python Sqlite3 Database Locked, QWidget

Question

I'm getting a sqlite3.OperationalError: database is locked when I try to update the database from a pyqt widget.

The main window shows a list of items in a database. When an item is double clicked a widget will pop up with options to modify a value in the same row of the database.

I have 4 separate .py files; (I have removed most of the code)

DBmanager.py that contains all the functions for interacting with the database.

class DatabaseUtility:
        def __init__(self, databaseFile):
        self.db = databaseFile 
        self.conn = sqlite3.connect(self.db)
        self.c = self.conn.cursor()  

    def ChangeItemQuantity(self, itemName, incramentQuantity):
        try:
            # Change given item quantity in database
            self.c.execute('''
                 SELECT quantity
                 FROM items
                 WHERE itemName=?
                 ''',(itemName,))
            print(itemName)
            print(incramentQuantity)
            current_quantity = self.c.fetchone()
            print(current_quantity[0])
            new_quantity = current_quantity[0] + incramentQuantity
            self.c.execute('''
                 UPDATE items
                 SET quantity = ?
                 WHERE itemName=?
                 ''',(new_quantity, itemName))
            self.conn.commit()
        except Exception as error:
            # Rollback any changes if something goes wrong.
            self.conn.rollback()
            raise error

    def __del__(self):
        """Commit and close database connection when the class is terminated."""
        # pass
        self.conn.commit()
        self.c.close()
        self.conn.close()

StartGui.py that contains the functions for the GUI.

import sys
from PyQt4 import QtCore, QtGui
from guiFormat import Ui_MainWindow
from itemWidget import Ui_itemWidget
from DBmanager import DatabaseUtility


class MainWindow(QtGui.QMainWindow):
    def __init__(self, parent=None):
        QtGui.QWidget.__init__(self, parent)
        self.ui = Ui_MainWindow()
        self.ui.setupUi(self)
        self.db = 'testdb.db'
        self.dbUtil = DatabaseUtility(self.db)
        self.ui.updateButton.clicked.connect(self.populateTable_default)
        self.ui.updateButton.clicked.connect(self.dbUtil.UpdateDatabase)
        self.ui.searchButton.clicked.connect(self.populateTable_search)
        self.ui.tableWidget.doubleClicked.connect(self.open_item_widget)
        self.ui.tableWidget.resizeRowsToContents()

        # Populate table on initial startup
        self.populateTable_default()

    def open_item_widget(self):
        self.item_widget = ItemWidget(self)
        # self.item_widget.show()
        column_count = self.ui.tableWidget.columnCount()
        self.item_widget.ui.tableWidget.setColumnCount(column_count)
        self.item_widget.ui.tableWidget.setRowCount(1)
        column_names = self.dbUtil.GetColumns('items')
        self.item_widget.ui.tableWidget.setHorizontalHeaderLabels(column_names)
        row = self.ui.tableWidget.currentRow()
        for column in range(column_count):
            x = self.ui.tableWidget.item(row, column).text()
            item_data = QtGui.QTableWidgetItem(str(x))
            self.item_widget.ui.tableWidget.setItem(0, column, item_data)
        self.item_widget.show()


class ItemWidget(QtGui.QWidget):
    def __init__(self, parent=None):
        self.db = 'testdb.db'
        self.dbUtil2 = DatabaseUtility(self.db)
        QtGui.QWidget.__init__(self, parent)
        self.ui = Ui_itemWidget()
        self.ui.setupUi(self)
        self.ui.cancelOkButtonBox.rejected.connect(self.close)
        self.ui.cancelOkButtonBox.accepted.connect(self.submit_changes)

    def submit_changes(self):
        # Get item Name.
        item_name = self.ui.tableWidget.item(0,0).text()
        # Get number from increment box.
        quant_increment = self.ui.QuantSpinBox.value()
        alert_incrament = self.ui.alertSpinBox.value()
        print('Changing quantity...', item_name, quant_increment)
        self.dbUtil2.ChangeItemQuantity(item_name, quant_increment)

        self.close()



##======================================================================================================================
##======================================================================================================================
if __name__ == "__main__":
    dbUtil = DatabaseUtility('testdb.db')
    app = QtGui.QApplication(sys.argv)
    mainwindow = MainWindow()
    mainwindow.show()
    # itemwidget = ItemWidget()
    # itemwidget.show()
    sys.exit(app.exec_())

Mainwindow.py is an export from pyqt designer for the main window.

Itemwidget.py is also an export from pyqt designer for the item widget.

Thank you for all your help.

Answer 1

The sqlite3 documentation says:

When a database is accessed by multiple connections, and one of the processes modifies the database, the SQLite database is locked until that transaction is committed.

Since you have two connections, one opened by MainWindow and the other by ItemWidget , one likely cause of your problem is just what the doc says. One connection tries to update the database when changes made by the other connection is not committed. This won't happen with the ChangeItemQuantity method you are showing as the change it makes is immediately committed, but obviously DatabaseUtility has other methods you are not showing, like UpdateDatabase . If they make changes that are not immediately committed, that could be the problem.

Let MainWindow and ItemWidget share one connection and see if the problem goes away. You can do this by making conn a shared attribute of DatabaseUtility instances, with no change to callers:

class DatabaseUtility:

    conn = None

    def __init__(self, databaseFile):
        self.db = databaseFile
        if self.__class__.conn is None:
            self.__class__.conn = sqlite3.connect(self.db)
        self.c = self.conn.cursor()

Also, although this seems less likely to be the problem, if some other process has an open transaction on the same SQLite database, the database will be locked for as long as that transaction is active. Some apps have the bad habit of maintaining an active transaction it seems. Whenever I open a database with SQLite Browser , I can't do anything to the database from other programs (maybe I can turn that off, but I never bothered to figure out how...)