Given multiple executables with the same name in PATH, Subprocess.run selects an arbitrary one. I stumbled upon that with Swiftly picking either Xcode's swift or the one it installed itself.

You can argue, tools should use absolute paths to call binaries. However, relying on the order defined in PATH is also valid.

I'll have a PR ready in a minute. It shows the issue better than any words would be able to explaining it here. Also see swiftlang/swiftly#463.