Based on the LLVM documentation [1], Python is used to run the automated test suite. Therefore is it always a dependency for LLVM. However, if build without Python (~python), we limit it to a build time dependency. Note that py-lit is not added as a spack dependency even though it is available as a spack package. This is because it is already included in llvm and llvm is difficult to configure using an external py-lit (several CMake variables to set correctly). Additionally, having py-lit as a spack dependency adds Python as a runtime dependency for llvm even though it is not required at runtime. [1] https://llvm.org/docs/GettingStarted.html#requirements