Why Do All Buttons Print the Same Index When Using Lambda Functions in a Loop?

Lambda Closures in Python

In Python, lambda functions create closures, allowing access to variables outside their scope. However, when used within a loop, this behavior can lead to unexpected results.

Problem Statement

Consider the following code, where five buttons are created using a loop. Each button binds a command to print its index:

<code class="python">for i in range(5):
    make_button = Tkinter.Button(frame, text ="make!", 
                                 command= lambda: makeId(i))</code>

Behavior

When clicked, all buttons print the same index, which is always 5. This occurs because variable resolution in lambdas happens at execution time. By the time a button is clicked, the loop has concluded, and i is equal to 5 for all buttons.

Resolution

To fix this issue, we need to create local variables within each lambda that capture the correct value of i. This can be achieved by passing i as an argument to lambda, like so:

<code class="python">make_button = Tkinter.Button(frame, text ="make!", 
                              command= lambda i=i: makeId(i))</code>

This creates a local i variable for each lambda, which retains the value of i from the loop iteration. By doing this, we ensure that each button prints the expected index.

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