How to overwrite data in text file in tkinter

Question

I am creating a program with tkinter which comes with a default name and password stored in a text file. After login you need to open the Toplevel window and type in the name and password you want to use in your subsequent logins. I have defined my variables but if I want to overwrite the text file I receive the below:

Error "NameError: name 'e1' is not defined"

Which I know I have defined.

import sys
from tkinter import messagebox
from tkinter import *


now = open("pass.txt","w+")
now.write("user\n")
now.write("python3")
now.close()


def login_in():
    with open("pass.txt") as f:
        new = f.readlines()
        name = new[0].rstrip()
        password = new[1].rstrip()
    if entry1.get() == name and entry2.get() == password:
        root.deiconify()
        log.destroy()
    else:
        messagebox.showerror("error","login Failed")


def change_login():
    ch = Toplevel(root)
    ch.geometry('300x300')
    e1 = Entry(ch, width=20).pack()
    e2 = Entry(ch, width=20).pack()
    sb = Button(ch, text="save", command=save_changes).pack()

def save_changes():  # function to change data in the txt file
    data = e1.get() + "\n " + e2.get()
    with open("pass.txt", "w") as f:
        f.writelines(data)


root= Tk()
log = Toplevel()


root.geometry("350x350")
log.geometry("200x200")

entry1 = Entry(log)
entry2 = Entry(log)
button1 = Button(log, text="Login", command=login_in) #Login button

entry1.pack()
entry2.pack()
button1.pack()

label = Label(root, text="welcome").pack()
butt = Button(root, text="change data in file", command=change_login).pack()

root.withdraw()
root.mainloop()      

Answer 1

So you have a few options here but in general you have 2 major issues.

The first issue is the use of .pack() after the creation of your e1 and e2 entry fields. This is a problem for the get() method as the geometry manager will return None if you pack this way. The correct method is to create the widget first with e1 = Entry(ch, width=20) and then pack it on the next line with e1.pack() this will allow get() to work on e1.

The second issue is the matter of local variables vs global variables. You have created e1 and e2 inside of the function change_login() and without telling python that e1 and e2 are global variables it will automatically assume you want them as local variables. This means the variables are only accessible from within the function they are created in.

You have a few options and I will break them out for you.

1) The quick and dirty option is to assign e1 and e2 as global variables. This can be done by using global var_name, var2_name, and_so_on so in this case add this line to the top of your change_login(): and save_changes() functions:

global e1, e2

This well tell python to add e1 and e2 to the global name space and it will allow save_changes() to work with the variables in the global name space.

2) Another method is to pass the 2 variables to save_changes() using the button command. We will need to use lambda for this and we can accomplish this by adding:

command = lambda e1=e1, e2=e2: save_changes(e1, e2)

to the button created in change_login() and adding the 2 arguments to save_changes() like this:

save_changes(e1, e2)

This will work just as well as the first option and it also avoids the use of global variables.

3) A third option is to create the entire program as a class and to use class attributes to allow the variables to work with all methods within the class.

Below is an example of your code using the 1st option:

import sys
from tkinter import messagebox
from tkinter import *

now = open("pass.txt","w+")
now.write("user\n")
now.write("python3")
now.close()

def login_in():
    with open("pass.txt") as f:
        new = f.readlines()
        name = new[0].rstrip()
        password = new[1].rstrip()
    if entry1.get() == name and entry2.get() == password:
        root.deiconify()
        log.destroy()
    else:
        messagebox.showerror("error","login Failed")

def change_login():
    global e1, e2
    ch = Toplevel(root)
    ch.geometry('300x300')
    e1 = Entry(ch, width=20)
    e1.pack()
    e2 = Entry(ch, width=20)
    e2.pack()
    sb = Button(ch, text="save", command=save_changes).pack()

def save_changes():  # function to change data in the txt file
    global e1, e2
    data = e1.get() + "\n" + e2.get() # removed space after \n
    with open("pass.txt", "w") as f:
        f.writelines(data)

root= Tk()
log = Toplevel()
root.geometry("350x350")
log.geometry("200x200")

entry1 = Entry(log)
entry2 = Entry(log)
button1 = Button(log, text="Login", command=login_in) #Login button

entry1.pack()
entry2.pack()
button1.pack()

label = Label(root, text="welcome").pack()
butt = Button(root, text="change data in file", command=change_login).pack()

root.withdraw()
root.mainloop()

Below is an example of your code using the 2nd option:

import sys
from tkinter import messagebox
from tkinter import *

now = open("pass.txt","w+")
now.write("user\n")
now.write("python3")
now.close()

def login_in():
    with open("pass.txt") as f:
        new = f.readlines()
        name = new[0].rstrip()
        password = new[1].rstrip()
    if entry1.get() == name and entry2.get() == password:
        root.deiconify()
        log.destroy()
    else:
        messagebox.showerror("error","login Failed")

def change_login():
    ch = Toplevel(root)
    ch.geometry('300x300')
    e1 = Entry(ch, width=20)
    e1.pack()
    e2 = Entry(ch, width=20)
    e2.pack()
    Button(ch, text="save", command=lambda e1=e1, e2=e2: save_changes(e1, e2)).pack()

def save_changes(e1, e2):  # function to change data in the txt file
    data = e1.get() + "\n" + e2.get() # removed space after \n
    with open("pass.txt", "w") as f:
        f.writelines(data)

root= Tk()
log = Toplevel()
root.geometry("350x350")
log.geometry("200x200")

entry1 = Entry(log)
entry2 = Entry(log)
button1 = Button(log, text="Login", command=login_in) #Login button

entry1.pack()
entry2.pack()
button1.pack()

label = Label(root, text="welcome").pack()
butt = Button(root, text="change data in file", command=change_login).pack()

root.withdraw()
root.mainloop()

Below is an example of your code converted to a class and using class attributes to avoid the use of global variables.

import sys
from tkinter import messagebox
from tkinter import *


class MyApp(Frame):

    def __init__(self, master, *args, **kwargs):
        Frame.__init__(self, master, *args, **kwargs)

        self.master = master
        self.master.withdraw()

        self.log = Toplevel()
        self.master.geometry("350x350")
        self.log.geometry("200x200")

        self.entry1 = Entry(self.log)
        self.entry2 = Entry(self.log)
        self.button1 = Button(self.log, text="Login", command=self.login_in)

        self.entry1.pack()
        self.entry2.pack()
        self.button1.pack()

        Label(root, text="welcome").pack()
        Button(root, text="change data in file", command=self.change_login).pack()

    def login_in(self):
        with open("pass.txt") as f:
            new = f.readlines()
            name = new[0].rstrip()
            password = new[1].rstrip()
        if self.entry1.get() == name and self.entry2.get() == password:
            self.master.deiconify()
            self.log.destroy()
        else:
            messagebox.showerror("error","login Failed")

    def change_login(self):
        ch = Toplevel(self.master)
        ch.geometry('300x300')
        self.e1 = Entry(ch, width=20)
        self.e1.pack()
        self.e2 = Entry(ch, width=20)
        self.e2.pack()
        Button(ch, text="save", command=self.save_changes).pack()

    def save_changes(self):
        data = "{}\n{}".format(self.e1.get(), self.e2.get())
        with open("pass.txt", "w") as f:
            f.writelines(data)


if __name__ == "__main__":
    now = open("pass.txt","w+")
    now.write("user\n")
    now.write("python3")
    now.close()

    root = Tk()
    app = MyApp(root)
    root.mainloop()