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