Python 3.3 : I do not understand Buttons and their lambdas in Tkinter()

Question

I am having some trouble with the button widgets.

Here's my code. Basically, what I would like to do is to print "0,0" when I press the first button, print "0,1" when I press the second button and so on. But what happens is that it always prints "1,1", which are last values in my for loops. How could I fix that?

Thanks a lot for helping

from tkinter import *

def show(x,y):

    print(x,y)

root = Tk()

a = 0

for i in range(2):

    for j in range(2):
        Button(root, text=a, command=lambda:show(i,j)).grid(column=j, row=i)
        a += 1

root.mainloop()

The `i, j` is looked up when the button is clicked, not when the `lambda` is defined, as they are closures. — Martijn Pieters, Nov 24 '13 at 02:51

score 0 · Accepted Answer · answered Nov 24 '13 at 02:51

It is because of the closure property of Python. To fix this, change

Button(root, text=a, command=lambda:show(i,j)).grid(column=j, row=i)

to

def inner(i=i, j=j):
    Button(root, text=a, command=lambda:show(i,j)).grid(column=j, row=i)
inner()

We are just wrapping the values of i and j with a function. Since the function show is not executed immediately, it just accepts the values of the two variables i and j. So, all the buttons now, will have the same i and j and they will get the values which are at the end of the loop.

Now, by defining a new function, we are getting a default parameter of the same names i and j and we get the values of i and j at the particular moment. So, the current values will be retained.

Your answer might be better if you give the other ways to solve the problem in the same manner. For example, instead of creating `inner`, you can bind the values at the time you create the lambda like so: `command = lambda i=i, j=j: show(i,j)` — Bryan Oakley, Nov 24 '13 at 13:24

Python 3.3 : I do not understand Buttons and their lambdas in Tkinter()

1 Answers1