Developers heavily rely on Application Programming Interfaces (APIs) from libraries to build their projects. However, libraries might become obsolete, or new libraries with better APIs might become available. In such cases, developers replace the used libraries with alternative libraries, a process known as library migration. Since manually migrating between libraries is tedious and error prone, there has been a lot of effort towards automated library migration. However, most of the current research on automated library migration focuses on Java libraries, and even more so on version migrations of the same library. Despite the increasing popularity of Python, limited research has investigated migration between Python libraries. To provide the necessary data for advancing the development of Python library migration tools, this paper contributes PyMigBench, a benchmark of real Python library migrations. PyMigBench contains 59 analogous library pairs and 75 real migrations with migration-related code changes in 160 Python files across 57 client repositories.