Скачать или смотреть How to Use Global Variables to Update User Data in Paho MQTT on_message Callback

Learn how to efficiently manage your user data with global variables in Paho MQTT's on_message callback. This guide walks through common pitfalls and provides a clear solution for your MQTT applications.
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This video is based on the question https://stackoverflow.com/q/63580034/ asked by the user 'Ivan Hu' ( https://stackoverflow.com/u/11065282/ ) and on the answer https://stackoverflow.com/a/63588964/ provided by the user 'Ivan Hu' ( https://stackoverflow.com/u/11065282/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
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Introduction

When working with Paho MQTT in Python, many developers encounter a common issue: how to manage user data across multiple invocations of the on_message callback. In the case presented, a developer attempted to use the userdata concept to maintain a counter that would increment with each received message. However, the output didn't match expectations, as the counter failed to update after the first message was received.

In this guide, we will explore why the initial approach did not work and ultimately provide a solution using global variables.

Understanding the Problem

Given the following scenario:

You want to track how many messages are received in a specific MQTT topic via the on_message callback.

You're using the userdata parameter to maintain a counter.

The expectation was to see an output like this after publishing messages:

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

However, the actual output returned was:

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

Clearly, the userdata variable did not update as expected. The source of this issue lies in how the userdata behaves within the Paho client context.

What Went Wrong?

The Misuse of userdata

In the original code, userdata was defined at the client instantiation but not updated correctly within the on_message callback. The operation userdata + = 1 does not modify the original variable; it simply creates a new local variable, leaving the original client_userdata unchanged on successive calls.

The Solution: Using Global Variables

Instead of relying on userdata, a better approach is to utilize global variables that can be shared across the functions in your script. Here’s how you can implement it successfully:

Step-by-Step Code Example

Import the Library: Ensure you have the Paho MQTT package imported.

Define Global Variables: Use a global variable to keep track of your message count.

Setup Callbacks: Implement the on_connect and on_message callbacks.

Connect and Loop: Establish connection to the MQTT broker and enter the loop.

Here’s the revised, working script:

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

Expected Output

When running this modified script and publishing messages to the IvanHu topic, the expected results should now correctly reflect increments:

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

Conclusion

By transitioning from userdata to global variables, we can effectively manage shared states in Paho MQTT applications. This subtle shift allows your application to function as intended, reliably incrementing values in each successive execution of the on_message callback.

If you’re ever faced with similar issues, remember to reconsider how your variables are scoped and updated, as this can make all the difference in the functionality of your MQTT applications!