Why Do Lambda Functions in Loops Capture the Final Index Value Instead of the Loop Iteration Value?

Understanding Closure in Python Lambda Functions

Original Question: Lambda Closure Puzzle with Index Printing

In programming, a closure refers to an inner function that captures the variables of its enclosing function even after the enclosing function has returned. This allows the inner function to retain access to the variables, making them persistent within the lifetime of the closure.

A common use case for lambda expressions is to bind a variable to a function. However, if used within a loop, as in the example code provided, lambda functions can exhibit unexpected behavior due to closure.

Code Explanation

In the given code, five buttons are created within a loop. Each button is assigned a lambda function that calls the makeId() function and prints the index i. However, when the user clicks on any button, the makeId() function always prints 5, which is the final value of i in the loop.

Problem and Solution

This issue arises because when the lambda expression is executed, it resolves the variable i to its value at that time. Since the loop has already terminated by then, i is resolved to its final value, which is 5.

To fix this issue, the lambda expression must be modified to explicitly specify the correct value of i using a lambda parameter. By default, lambdas use the enclosing scope's variables, but specifying a parameter ensures that a local variable with the desired value is created within the lambda:

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

With this modification, each button's lambda function captures the correct value of i from the loop, and the makeId() function can correctly print the intended index.

Additional Note

The lambda parameter doesn't need to have the same name as the enclosing variable. It can have any name, ensuring that the correct value is captured within the lambda's scope.

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