How and Why Developers Migrate Python Tests

Nowadays, Python developers can rely on two major testing frameworks: unittest and pytest. Due to the benefits of pytest (e.g., fixture reuse), several relevant projects in the Python ecosystem have migrated from unittest to pytest. Despite being performed by the Python community, we are not yet aware of how systems are migrated from unittest to pytest nor the major reasons behind the migration. In this paper, we provide the first empirical study to assess testing framework migration. We analyze how and why developers migrate from unittest to pytest. We mine 100 popular Python systems and assess their migration status. We find that 34% of the systems rely on both testing frameworks and that Python projects are moving to pytest. While some systems have fully migrated, others are still migrating after a long period, suggesting that the migration is not always straightforward. Overall, the migrated test code is smaller than the original one. Furthermore, developers migrate to pytest due to several reasons, such as the easier syntax, interoperability, easier maintenance, and fixture flexibility/reuse, however, the implicit mechanics of pytest is a concern. We conclude by discussing practical implications for practitioners and researchers.