Format check on date of birth in Python

Try something like this:

def validate(self):    dateofbirth = self.DOBEntry.get()    try:        datetime.datetime.strptime(dateofbirth, '%D/%M/%Y')        return True # returns a boolean True    except ValueError:         tkinter.messagebox.showerror("Error","Please enter the date of birth in the correct format DD-MM-YYYY")        return Falsedef AddCustomer(self):    # Same code    if self.validate(): # If true is returned, then...also same as if self.validate()==True        try:            with sqlite3.connect("LeeOpt.db") as db:                cursor = db.cursor()                add_customer = ('''INSERT INTO Customer(CustomerID, BranchID, Name, Surname, DateOfBirth, Town, Postcode, EmailAddress, TelephoneNo, MedicalConditions)                VALUES (?,?,?,?,?,?,?,?,?,?)''')                cursor.execute(add_customer, (customerid,branchid,name,surname,dateofbirth,town,postcode,email,telephone,medical))                tkinter.messagebox.showinfo("Notification","Customer added successfully")                self.ClearEntries()        except (sqlite3.IntegrityError):             tkinter.messagebox.showerror("Error","This CustomerID is already taken, please try another.")            self.ClearID()

Here True will be returned only if there is no error in the line, else False will be returned, so based on that, you make an if inside AddCustomer(). I also changed your parameters for execute onto a tuple rather than a list as it is convention.