Скачать или смотреть Fix the tkinter Button Callback Lambda Unexpected Result: Correctly Pass Key Values

Discover how to effectively pass dynamic key values to button callbacks in your `tkinter` applications to avoid unexpected results.
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This video is based on the question https://stackoverflow.com/q/63763890/ asked by the user 'Robin Andrews' ( https://stackoverflow.com/u/3042018/ ) and on the answer https://stackoverflow.com/a/63764050/ provided by the user 'Ji Wei' ( https://stackoverflow.com/u/8350440/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Fixing the tkinter Button Callback Lambda Unexpected Result

When working with Tkinter, Python's GUI toolkit, developers often encounter unexpected behaviors, particularly when it involves callbacks linked to buttons. One such problem arises when trying to store multiple rows of widgets in a dictionary and accessing the correct keys dynamically. In this guide, we will dissect one such issue - where a button callback always outputs the same value - and provide a clear solution.

The Problem

You have set up multiple rows of Tkinter widgets using a loop and stored them in a dictionary. The intention was to pass a unique key for each button's callback. However, the result is frustrating - no matter which button you press, the output is always the same, specifically the value 'E'. This happens because the variable key retains its last value from the loop when the widgets' creation is complete. Let's take a closer look at the code that leads to this problem:

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As the loop exits, key holds the last assigned value, which is 'E' after the loop finishes running. This is why every button press results in the same output.

The Solution

To solve this issue, you need to ensure that each button's command captures the current value of key at the time of button creation. To achieve that, we can use a default argument in the lambda function, like so:

Corrected Button Command

Replace the faulty lambda command with this corrected version in your loop:

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Here, x=key creates a default argument for the lambda function. This means that each button will correctly retain the value of key at the time it was created, rather than the final value of key after the loop.

Complete Code with Fix

Here's how the adjusted part of your Tkinter code should look:

[[See Video to Reveal this Text or Code Snippet]]

Final Thoughts

With this simple adjustment, your Tkinter application will now work as intended, allowing each button to correctly pass its respective key. Remember, when working with lambdas inside loops, always consider how scopes and closures work to prevent unexpected output. Happy coding!