Why Your tkinter Global Variable Isn't Updating with multiprocessing and How to Fix It

Discover why your global variable in `tkinter` doesn't retain its value with `multiprocessing` and learn the best solution to make it work seamlessly.
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Understanding the Issue: Global Variables in tkinter and multiprocessing

When working with Python's tkinter for GUI applications and the multiprocessing package for handling separate processes, you may encounter a frustrating problem: global variables not holding their values across different contexts. This issue typically presents itself when you attempt to update a global variable through a tkinter button, yet see it revert to its original state. In this guide, we will go through the solutions step-by-step to ensure your global variable retains the value set by the tkinter interface.

The Problem Explained

In the sample code provided, the user is attempting to monitor a global variable var_a by using a separate process created with multiprocessing. This works as follows:

A GUI is set up with a text entry field and a button.

When the button is pressed, it sets var_a to the value entered in the text field.

A separate process is initiated to continuously print the value of var_a.

However, due to the nature of how multiprocessing works, the new process runs in its own memory space. This means changes made to var_a in the main GUI process do not reflect in the child process, hence it appears that the value of var_a is not updating.

Solution: Use threading Instead of multiprocessing

To resolve this issue, we can switch from using a separate process to using a thread. Unlike processes, threads share the same memory space within the application, allowing changes to global variables to be reflected immediately across both the GUI and the monitoring function.

Here’s how to implement this change effectively:

Step 1: Replace the Process with a Thread

In the original code, find the line that creates the process:

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

Simply replace it with the following line to use a thread instead:

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

Step 2: Full Example Code

Here's how your updated code will look after the modification:

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

Step 3: Running the Application

Input a number in the text entry field.

Click on the "Set Var" button.

Observe that the printed output in the console now updates correctly with the value entered, thanks to the use of threading instead of multiprocessing.

Conclusion

By utilizing threading instead of multiprocessing for functions that need to share global variables with a tkinter interface, you can ensure that your application's variables update as expected. This small yet crucial change can save you from running into unexpected bugs and enhance the functionality of your application.

Feel free to reach out with any further questions or share your experiences below! Happy coding!