Why Do Lambdas in For Loops Only Capture the Last Value?

Lambda in For Loop Only Captures Last Value

When using a lambda function within a for loop, it's crucial to understand its behavior regarding local variable capturing. By default, a lambda captures the values of local variables at the time the lambda is defined. However, if those variables change within the loop, the lambda will always capture the final value they take on.

Code Sample:

Consider the following code:

options = ["INFO", "WARNING", "DEBUG"]

for i in range(len(options)):
    option = options[i]
    __cMenu.add_command(
        label="{}".format(option),
        command=lambda: self.filter_records(column, option)
    )

This code aims to create multiple lambdas, each capturing a different value of the option variable. However, when these lambdas are executed, they all behave as though option is set to "DEBUG", the last value it takes on in the loop.

Reason:

This behavior occurs because the names in the function bodies of lambdas are evaluated when the function is executed. Therefore, in the provided code, the option variable is evaluated when each lambda is called, resulting in the capture of the final value of option only.

Solution:

To avoid this issue, it's necessary to capture the value of option at the time the lambda is defined. This can be achieved by assigning option to a new variable within the lambda, as seen in the modified code below:

options = ["INFO", "WARNING", "DEBUG"]

for i in range(len(options)):
    option = options[i]
    __cMenu.add_command(
        label="{}".format(option),
        command=lambda opt=option: self.filter_records(column, opt)
    )

By using the opt parameter within the lambda, the value of option is captured at the time the lambda is created, ensuring that each lambda operates with the correct value.

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