Resolving the locust Unexpected Keyword Argument Error: A Quick Fix

Discover how to fix the `locust` unexpected keyword argument 'user_count' error and ensure your performance tests run smoothly.
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This video is based on the question https://stackoverflow.com/q/69282953/ asked by the user 'Guy' ( https://stackoverflow.com/u/8180615/ ) and on the answer https://stackoverflow.com/a/69283447/ provided by the user 'Guy' ( https://stackoverflow.com/u/8180615/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Resolving the locust Unexpected Keyword Argument Error: A Quick Fix

When running performance tests with Locust, encountering errors can be frustrating, especially when your setup seems to execute successfully but still results in an unexpected termination with an error code. In this post, we will explore a specific issue: an unexpected keyword argument user_count, and how to quickly fix it.

Understanding the Problem

In one recent case, a user experienced a breakdown in their locust test, which ended with the exit code 2. Despite reaching all specified performance metrics, the test log indicated an uncaught exception:

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

This error arises when an event handler within Locust is not correctly defined, causing locust to throw an error during the test run.

Key Observations in the Log

Exit Code: The process terminated with exit code 2, signaling an error.

Error Origin: The traceback indicated that the issue originated from an event handler.

Event Listener: An @ events.spawning_complete.add_listener was specifically mentioned as a potential cause of the issue.

Solution Overview

Upon further inspection and experimentation, the problem was identified as stemming from an improperly defined event listener. The immediate fix was to remove the listener that did not specify any arguments.

Steps to Resolve the Issue

Here’s a clear step-by-step guide to addressing the unexpected keyword argument 'user_count' error in Locust:

Identify the Event Listener: Look at your custom event listeners in your Locust file, particularly those tied to @ events.spawning_complete.

Check Function Definitions: Ensure that any listener functions you have defined match the expected parameters. For spawning_complete, they typically expect parameters like user_count.

Remove Misdefined Listeners: If you have defined any listeners that do not take parameters, such as this ill-defined example:

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

Test Again: After removing or fixing the listener, re-run your Locust tests to ensure that they terminate without throwing an error.

Conclusion

Encountering an error in performance testing with Locust can disrupt workflows, but the key to resolving issues often lies in reviewing event listeners and their definitions. By carefully defining your listeners and ensuring that they accept the correct parameters, you can avoid unexpected errors like the user_count issue.

If you're looking for more insights or have other questions about Locust, feel free to reach out or explore additional resources. Happy testing!