Code

import sys
from PyQt4 import QtGui, uic
import sqlite3 as lite
import csv

loginMenuUi = uic.loadUiType('Menu.ui')[0]
makeacUi = uic.loadUiType('Make Account.ui')[0]
loginUi = uic.loadUiType('Login.ui')[0]
mainMenuUi = uic.loadUiType('MainMenu.ui')[0]
homeworkUi = uic.loadUiType('Homework.ui')[0]
prefUi = uic.loadUi('Preferences.ui')[0]

class LoginMenu(QtGui.QMainWindow, loginMenuUi):
    def __init__(self, parent = None):
        QtGui.QMainWindow.__init__(self, parent)
        self.setupUi(self)
        self.btnLogin.clicked.connect(self.login)
        self.btnSignUp.clicked.connect(self.signUp)

        self.c = d.checkUsers(0)
        if self.c == 0:
            self.btnLogin.setHidden(True)
            self.btnSignUp.setGeometry(90, 100, 201, 71)
        else:
            self.btnLogin.setHidden(False)

    def login(self):
        loginMenu.close()
        self.login = Login(None)
        self.login.show()

    def signUp(self):
        loginMenu.hide()
        self.create = CreateAccount(None)
        self.create.show()


class CreateAccount(QtGui.QMainWindow, makeacUi):
    def __init__(self, parent = None):
        QtGui.QMainWindow.__init__(self, parent)
        self.setupUi(self)
        self.btnCreateAccount.clicked.connect(self.createAccount)

    def createAccount(self):
        self.uValid = False
        self.mValid = False
        self.lValid = False
        self.usValid = False
        self.sValid = False

        if len(self.edtUsername.text()) < 4:
            QtGui.QMessageBox.warning(self, 'Incorrect Length', 'Please enter a username that is at least 4 characters', QtGui.QMessageBox.Ok, QtGui.QMessageBox.Ok)
            self.clear()
            self.uValid = False
        else:
            self.uValid = True

        if ' ' in self.edtUsername.text():
            QtGui.QMessageBox.warning(self, 'Spaces not allowed', 'Spaces are not allowed in usernames', QtGui.QMessageBox.Ok, QtGui.QMessageBox.Ok)
            self.clear()
            self.sValid = False
        else:
            self.sValid = True

        if self.edtPass1.text() != self.edtPass2.text():
            QtGui.QMessageBox.warning(self, 'Password Matches', 'Please ensure your confirmed password matches your entered password', QtGui.QMessageBox.Ok, QtGui.QMessageBox.Ok)
            self.clear()
            self.mValid = False
        else:
            self.mValid = True

        if len(self.edtPass1.text()) < 6 or len(self.edtPass1.text()) > 16:
            QtGui.QMessageBox.warning(self, 'Password length', 'Please ensure your password is between 6 and 16 characters long', QtGui.QMessageBox.Ok, QtGui.QMessageBox.Ok)
            self.clear()
            self.lValid = False
        else:
            self.lValid= True

        if len(d.res) > 0:
            for each in d.res:
                if self.edtUsername.text() in each:
                    QtGui.QMessageBox.warning(self, 'Username taken', 'This username is already in use', QtGui.QMessageBox.Ok, QtGui.QMessageBox.Ok)
                    self.usValid = False
                    self.clear()
                else:
                    self.usValid = True
        else:
            self.usValid = True

        if self.mValid and self.uValid and self.lValid and self.usValid and self.sValid:
            self.addUser()
            self.prefs = d.checkPrefs(0)
            print(type(self.prefs))


    def clear(self):
        self.edtUsername.setText('')
        self.edtPass1.setText('')
        self.edtPass2.setText('')
        self.edtUsername.setFocus()

    def addUser(self):
        with open('Users.csv', 'a', newline = '') as fp:
            a = csv.writer(fp, delimiter = ',')
            data = [[self.edtUsername.text(), self.edtPass1.text()]]
            a.writerows(data)
        d.checkUsers(0)

    def keyPressEvent(self, QKeyEvent):
        if QKeyEvent.key() == 16777220:
            self.createAccount()


class Login(QtGui.QMainWindow, loginUi):
    def __init__(self, parent = None):
        QtGui.QMainWindow.__init__(self, parent)
        self.setupUi(self)
        self.edtUsername.setFocus()
        self.btnLogin.clicked.connect(self.checkLogin)

    def checkLogin(self):
        self.users = d.checkUsers(1)
        self.userFound = False
        self.loginCorrect = False
        self.entered = False
        self.username = self.edtUsername.text()
        self.password = self.edtPassword.text()

        if self.username == '' or self.password == '':
            QtGui.QMessageBox.warning(self, 'Incomplete', 'Please complete all required fields', QtGui.QMessageBox.Ok, QtGui.QMessageBox.Ok)
            self.entered = False
        else:
            self.entered = True

        if self.entered:
            for each in self.users:
                if each[0] == self.username:
                    self.userFound = True
                    if each[1] == self.password:
                        self.loginCorrect = True
                    else:
                        self.loginCorrect = False
                else:
                    self.userFound = False

            if not self.userFound:
                QtGui.QMessageBox.warning(self, 'User not found', 'This username was not found', QtGui.QMessageBox.Ok, QtGui.QMessageBox.Ok)
            elif not self.loginCorrect:
                QtGui.QMessageBox.warning(self, 'Incorrect Password', 'The entered password does not match the one on file', QtGui.QMessageBox.Ok, QtGui.QMessageBox.Ok)

            else:
                loginMenu.login.hide()
                self.mainMen = MainMenu(None)
                self.mainMen.show()


class MainMenu(QtGui.QMainWindow, mainMenuUi):
    def __init__(self, parent = None):
        QtGui.QMainWindow.__init__(self, parent)
        self.setupUi(self)
        self.btnHomework.clicked.connect(self.homework)
        self.btnClasses.clicked.connect(self.classes)

    def homework(self):
        loginMenu.login.mainMen.hide()
        self.hw = Homework(None)
        self.hw.show()

    def classes(self):
        print('Classes')


class Homework(QtGui.QMainWindow, homeworkUi):
    def __init__(self, parent = None):
        QtGui.QMainWindow.__init__(self, parent)
        self.setupUi(self)


class Preferences(QtGui.QMainWindow, prefUi):
    def __init__(self, parent = None):
        QtGui.QMainWindow.__init__(self, parent)
        self.setupUi(self)


class Database:
    def __init__(self):
        self.res = []
        self.userPrefs = []

    def checkUsers(self, type):
        db = 'Users.db'
        con = lite.connect(db)
        cur = con.cursor()
        cur.execute('DROP TABLE IF EXISTS Users')
        cur.execute('''CREATE TABLE 'Users' (
        'UserID' INTEGER,
        'Username' TEXT NOT NULL,
        'Password' TEXT NOT NULL,
        PRIMARY KEY('UserID')
        );''')

        # cur.execute('DROP TABLE IF EXISTS Homework')
        # cur.execute('''CREATE TABLE 'Homework' (
        # 'HomeworkID' INTEGER,
        # 'Task' TEXT NOT NULL,
        # 'Teacher' TEXT NOT NULL,
        # 'Date Due' TEXT NOT NULL,
        # 'Subject' TEXT NOT NULL,
        # PRIMARY KEY('HomeworkID')
        # );''')
        # con.commit()

        filename = 'Users.csv'
        f = open(filename, 'rt')
        contents = f.read()
        rows = contents.split('\n')
        count = 0
        for each in rows:
            if len(each) > 0:
                columns = each.split(',')
                columns.insert(0, count)
                with con:
                    cur.execute('INSERT INTO Users VALUES(?, ?, ?)', columns)
            count += 1

        s = 'SELECT * FROM Users'
        cur.execute(s)
        results = cur.fetchall()
        for each in results:
            self.res.append(each)
        if type == 0:
            if len(self.res) == 0:
                return 0
            else:
                return -1
        elif type == 1:
            return self.res

    def checkPrefs(self, user):
        db = 'Users.db'
        con = lite.connect(db)
        cur = con.cursor()
        cur.execute('DROP TABLE IF EXISTS UserPrefs')
        cur.execute('''CREATE TABLE 'UserPrefs' (
       'Class' TEXT NOT NULL,
       'Teacher' TEXT NOT NULL,
       'Day Taught' TEXT NOT NULL,
       'Period Taught' INTEGER NOT NULL,
       'UserID' INTEGER
       );''')

        filename = 'UserPrefs.csv'
        f = open(filename, 'rt')
        contents = f.read()
        rows = contents.split('\n')
        count = 0
        for each in rows:
            if len(each) > 0:
                columns = each.split(',')
                with con:
                    cur.execute('INSERT INTO UserPrefs VALUES(?, ?, ?, ?, ?)', columns)
            count += 1

        s = 'SELECT * FROM UserPrefs'
        cur.execute(s)
        results = cur.fetchall()
        for each in results:
            self.userPrefs.append(each)

        count = 0
        self.found = False
        for prefs in self.userPrefs:
            if prefs[4] == user:
                self.found = True
                return self.userPrefs[count]
            else:
                count += 1

        if not self.found:
            return 0


app = QtGui.QApplication(sys.argv)
d = Database()
loginMenu = LoginMenu(None)
loginMenu.show()
app.exec_()