How does code style inconsistency affect pull request integration? An exploratory study on 117 GitHub projects

GitHub is a popular code platform that provides infrastructures to facilitate collaborative development. A Pull Request (PR) is one of the key ideas to support collaboration. Developers are encouraged to submit PRs to ask for the integration of their contributions. In practice, not all submitted PRs can be integrated into the codebase by project maintainers. Existing studies have investigated factors affecting PR integration. Nevertheless, the code style of PRs, which is largely considered by project maintainers, has not been deeply studied yet. In this paper, we performed an exploratory analysis on the effect of code style on PR integration in GitHub. We modeled the code style via the inconsistency between a submitted PR and the existing code in its target codebase. Such modeling makes our study not limited by a specific definition of code style. We conducted our experiments on 50,092 closed PRs in 117 Java projects. Our findings show that: (1) There indeed exists code style inconsistency between PRs and the codebase. (2) Several code style criteria on how to use spaces or indents, make comments, and write code lines with a suitable length, tend to show more inconsistency among PRs. (3) A PR that is consistent with the current code style tends to be merged into the codebase more easily. (4) A PR that violates the current code style is likely to take more time to get closed. Our study shows evidence to developers about how to deliver better contributions to facilitate efficient collaboration.