Tkinter - How to trace expanding list of variables - python-3.x

Question

What I am trying to do track when any values in a list of StringVar change, even when the list is expanding. Any additions to the list before the trace statement will result in the callback. But any additions afterward, such as when pressing a button, will not cause any callback.
import tkinter as tk
root = tk.Tk()
frame = tk.Frame(root)
frame.grid(row=0)
L = []
def add_entry(event):
L.append(tk.StringVar())
tk.Entry(frame,textvariable=L[len(L)-1]).grid(row=len(L),padx=(10,10),pady=(5,5))
add = tk.Button(frame,text='add Entry',command='buttonpressed')
add.grid(row=0)
add.bind('<Button-1>',add_entry)
for i in range(2):
L.append(tk.StringVar())
tk.Entry(frame,textvariable=L[len(L)-1]).grid(row=len(L),padx=(10,10),pady=(5,5))
for i in L:
i.trace('w',lambda *arg:print('Modified'))
root.mainloop()
Modifying the first two Entry's prints out Modified, but any Entry's after the trace is run, such as the ones produced when a button is pressed, will not.
How do I make it so that trace method will run the callback for the entire list of variables even if the list is expanded?

Answer 1

Simple suggestion, change your add_entry function to something like this:
def add_entry(event):
L.append(tk.StringVar())
tk.Entry(frame,textvariable=L[len(L)-1]).grid(row=len(L),padx=(10,10),pady=(5,5))
L[len(L)-1].trace('w',lambda *arg:print('Modified'))
Extra suggestions:
This add = tk.Button(frame,text='add Entry',command='buttonpressed') is assigning a string to command option, means it will try to execute that string when button is clicked(which will do nothing). Instead, you can assign your function add_entry to command option and it will call that function when button is clicked and you can avoid binding Mouse Button1 click to your Button(Note: No need to use argument event in function when using like this). Read more here
Python supports negative indexing of List, so you can call L[-1] to retrieve the last element in the list instead of calling L[len(L)-1]).
Once you change your add_entry function as suggested, you can reduce your code to
import tkinter as tk
root = tk.Tk()
frame = tk.Frame(root)
frame.grid(row=0)
L = []
def add_entry():
global L
L.append(tk.StringVar())
tk.Entry(frame,textvariable=L[-1]).grid(row=len(L),padx=(10,10),pady=(5,5))
L[-1].trace('w',lambda *arg:print('Modified'))
add = tk.Button(frame,text='add Entry',command=add_entry)
add.grid(row=0)
for i in range(2):
add_entry()
root.mainloop()