Python projects grow quickly by code reuse and building automation based on third-party libraries. However, the version constraints associated with these libraries are prone to mal-configuration, and this forms a major obstacle to correct project building (known as dependency-conflict (DC) building failure). Our empirical findings suggest that such mal-configured version constraints were mainly prepared manually, and could essentially be refined for better quality to improve the chance of successful project building. We propose a LooCo approach to refining Python projects’ library version constraints by automatically loosening them to maximize their solutions, while keeping the libraries to observe their original behaviors. Our experimental results with real-life Python projects report that LooCo could efficiently refine library version constraints (0.4s per version loosening) by effective loosening (5.5 new versions expanded on average) automatically, and transform 54.8% originally unsolvable cases into solvable ones (i.e., successful building) and significantly increase solutions (21 more on average) for originally solvable cases.