Detecting Test Smells in Python Test Code Generated by LLM: An Empirical Study with GitHub Copilot

Writing unit tests is a time-consuming and labor-intensive development practice. Consequently, various techniques for automatically generating unit tests have been studied. Among them, the use of Large Language Models (LLMs) has recently emerged as a popular approach for automatically generating tests from natural language descriptions. Although many recent studies are dedicated to measuring the ability of LLMs to write valid unit tests, few evaluate the quality of these generated tests. In this context, this study aims to measure the quality of the test codes generated by GitHub Copilot in Python by detecting test smells in the test cases generated. To do this, we used approaches to generating unit tests by LLMs that have already been applied in the literature and collected a sample of 194 unit test cases in 30 Python test files. We then measured them using tools specialized in detecting test smells in Python. Finally, we conducted an evaluation of these test cases with software developers and software quality assurance professionals. Our results indicated that 47.4% of the tests generated by Copilot had at least one test smell detected, with a lack of documentation in the assertions being the most common quality problem. These findings indicate that although GitHub Copilot can generate valid unit tests, quality violations are still frequently found in these codes.