Tkinter dynamically create widgets from button

I'm attempting to make a dynamic GUI where clicking a button causes the creation of a new frame that is placed above the button with 3 entry widgets (user options) inside of it, and I need to be able to read the user input from the 3 entry widgets & possibly alter them. Each time the button is pressed, three new callable entry widgets should appear.

I know that this is wrong because it has been giving me errors, but could something similar to the lists be used to create the widgets dynamically?

from Tkinter import *

app = Tk()

frameNames = []
widgetNames = []

def createwidgets():
    global widgetNames
    global frameNames
    frameNames += (str("g"+str(len(frameNames))))  #why does the letter & number get added as seperate elements?
    widgetNames += [(str("w"+str(len(widgetNames)-1))), 
                    (str("w"+str(len(widgetNames)))),    
                    (str("w"+str(len(widgetNames)+1)))]

    frameNames[len(frameNames) - 1] = Frame(app)
    frameNames[len(frameNames) - 1].pack()

    widgetNames[len(widgetNames) - 3] = Entry(frameNames[len(frameNames) - 1])
    widgetNames[len(widgetNames) - 3].pack()
    widgetNames[len(widgetNames) - 2] = Entry(frameNames[len(frameNames - )- 1])
    widgetNames[len(widgetNames) - 2].pack()
    widgetNames[len(widgetNames) - 1] = Entry(frameNames[len(frameNames) - 1])
    widgetNames[len(widgetNames) - 1].pack()

createWidgetButton = Button(app, text="createWidgets", command=createwidgets())
createWidgetButton.grid(sticky=S)

app.mainloop()

Solution

The main problem is these four lines of code:

frameNames[len(frameNames) - 1] = Frame(app)
frameNames[len(frameNames) - 1].pack()
...
createWidgetButton = Button(app, text="createWidgets", command=createwidgets())
createWidgetButton.grid(sticky=S)

You are creating both the frame and button as a child of app, but you are using grid for one and pack for the other. You must be consistent with all direct descendants of app - they must all use pack or they must all use grid.

The second problem is this line:

frameNames += (str("g"+str(len(frameNames))))  #why does the letter & number get added as seperate elements?

Here, frameNames is a list and you are trying to add it with a string. Adding is not the same as appending. You need to append the new name, or put the new name in a temporary list before adding it.

frameNames.append(str(...))

The third problem is this line:

createWidgetButton = Button(app, text="createWidgets", command=createwidgets())

The above is exactly the same as this:

result = createWidgets()
createWidgetButton = Button(app, text="createWidgets", command=result)

You must pass a reference to a function, not call the function. Change the line to this (notice the lack of parenthesis after createWidgets):

createWidgetButton = Button(app, text="createWidgets", command=createwidgets)

Unrelated to the problem, but your code would be much easier to read if you used temporary variables instead of repeating the pattern (str("w"+str(len(widgetNames)-1). As written, your code is almost impossible to read. Also, you don't want to be storing widget names, you need to store the actual widgets themselves.

And finally, don't do a wildcard import. There is simply no good reason to do it.

Here is how I would rewrite your code:

import Tkinter as tk

app = tk.Tk()

frames = []
widgets = []

def createwidgets():
    global widgetNames
    global frameNames

    frame = tk.Frame(app, borderwidth=2, relief="groove")
    frames.append(frame)

    frame.pack(side="top", fill="x")

    for i in range(3):
        widget = tk.Entry(frame)
        widgets.append(widget)

        widget.pack(side="left")

createWidgetButton = tk.Button(app, text="createWidgets", command=createwidgets)
createWidgetButton.pack(side="bottom", fill="x")

app.mainloop()