Why do all my buttons print the same index when using a lambda in a loop?

Understanding Closure in Lambdas

A common issue arises when attempting to bind a command to a button using a lambda within a loop. Instead of printing the expected index, it consistently prints the final value of the loop variable. This occurs due to the closure's variable resolution mechanism.

In the provided example:

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

When the lambda is executed, it resolves the variable i to its value at that instant. Since the loop has finished by then, i has incremented to 5, causing all buttons to print the same index.

To rectify this, a local variable can be created within the lambda using the syntax command= lambda i=i:. This assigns the current value of i to a local variable that is captured by the lambda closure.

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

Now, the lambda will execute with the correct index value for each button. Note that the local variable can be assigned any name, ensuring it remains distinct from the loop variable.

The above is the detailed content of Why do all my buttons print the same index when using a lambda in a loop?. For more information, please follow other related articles on the PHP Chinese website!