43

I'm trying to create a few Buttons (with a for loop) like so:

def a(self, name):
    print(name)

users = {"Test": "127.0.0.0", "Test2": "128.0.0.0"}
row = 1
for name in users:
    user_button = Tkinter.Button(self.root, text=name,
                                 command=lambda: self.a(name))
    user_button.grid(row=row, column=0)
    row += 1

and for the buttons to each get their own parameter (Test getting "Test" and Test2 getting "Test2"), but when I press the buttons they both print "Test2" which means they are using the same function with the same parameter.

How can I solve this?

martineau
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Shay
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1 Answers1

66

The problem is your lambda expression in the for loop. It is using the name variable, but the name variable gets reassigned each iteration of the for loop, so in the end, all of the buttons get the last value that name was assigned to in the for loop. To avoid this you can use default keyword parameters in your lambda expression like so:

user_button = Tkinter.Button(self.root, text=name,
                             command=lambda name=name: self.a(name))

This binds the current value of the name variable to the lambda's name keyword argument each time through the loop, producing the desired effect.

martineau
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mshildt
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  • It is not a tkinter issue, it is an issue concerning nesting of functions, closures, etc. It as well exists in completely other contexts. Nevertheless +1, as it otherwise describes exactly what happens. – glglgl Oct 25 '13 at 06:20
  • @glglgl You're 100% correct. Not the best choice of words on my part, will revise. – mshildt Oct 28 '13 at 12:43
  • Thank you very much for this answer. This saved me from some more hours of search. – bprasanna May 24 '19 at 14:26
  • what if we want to execute multiple function? self.a and self.b – greendino May 27 '20 at 07:58
  • this worked for me, but I've noticed a lot of strange things like this with lambda. Is there a reason lambda is needed at all? – 1QuickQuestion Feb 28 '21 at 02:17
  • @1QuickQuestion please see [Python argument binders](https://stackoverflow.com/questions/277922) and [Why is my Button's command executed immediately when I create the Button, and not when I click it?](https://stackoverflow.com/questions/5767228). – Karl Knechtel Aug 18 '22 at 03:13