How Can I Pass Loop Variables Correctly to Tkinter Button Commands?

Passing Loop Variables to Tkinter Button Commands

In Tkinter, creating buttons in a loop can be straightforward. However, passing specific arguments to each button's command can become tricky.

Consider the following scenario where you attempt to create three buttons with titles "Game 1" to "Game 3." You intend to pass the corresponding numeric value to the command argument so that when a button is pressed, you can identify which button triggered the action.

def createGameURLs(self):
    self.button = []
    for i in range(3):
        self.button.append(Button(self, text='Game '+str(i+1),
                                  command=lambda: self.open_this(i)))
        self.button[i].grid(column=4, row=i+1, sticky=W)

def open_this(self, myNum):
    print(myNum)

Unfortunately, this code doesn't work as intended. When any button is pressed, the printed value is always 2, the last iteration of the loop. The problem arises because the lambda function uses the value of i at the end of the loop, not its value at the creation of each button.

The Closure Miracle

To solve this issue, you need to create a closure around each button's command. This can be achieved by using the syntax lambda i=i: self.open_this(i).

def createGameURLs(self):
    self.button = []
    for i in range(3):
        self.button.append(Button(self, text='Game '+str(i+1),
                                  command=lambda i=i: self.open_this(i)))
        self.button[i].grid(column=4, row=i+1, sticky=W)

With this modification, each button's command captures the specific value of i at the time of its creation. When a button is pressed, the closure ensures that the correct value of i is passed to the open_this function.

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