Question

In the code below I intend to have two buttons, and when each is pressed ‘0’ and ‘1’ are to be printed to stdout, respectively. However when the program is run, they both print ‘1’, which is the last value i had in the for iteration. Why?

import Tkinter as tk
import sys

root = tk.Tk()

for i in range(0,2):
    cmd = lambda: sys.stdout.write(str(i))
    tk.Button(text="print '%d'" % i,command=cmd).pack()

root.mainloop()

Answer 1

The i is not captured in the lambda when you create it (as you wanted). Instead, both functions refer back to the i in the external for loop, which changes after the function is created and before it is run. To capture it, you can use a default value:

for i in range(0,2):
    cmd = lambda i=i: sys.stdout.write(str(i))
    tk.Button(text="print '%d'" % i,command=cmd).pack()