How to Avoid Unexpected Behavior When Binding Commands to Buttons Created in a Tkinter Loop?

Tkinter Loop Button Creation with Command Argument Binding

In Tkinter, creating buttons within a for loop can present challenges when attempting to bind each button's command with a unique argument.

Consider the following scenario:

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)

When executing this code, you might encounter a strange behavior where pressing any button always prints the same value, usually the last iteration value. This occurs because each lambda function created within the loop references the same variable i, which is updated in each loop iteration.

Solution: Utilizing Closures

To address this issue, you can leverage Python's closures by assigning i to a new variable within the lambda function, effectively creating a unique scope for each button. Here's the modified code:

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)

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

By incorporating closures, each lambda function captures the current i value at the time of definition, ensuring the correct button behavior.

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