Hi, PEP 518 introduces build requirements that pip installs before calling setup.py. As it currently appears to be, pip installs them to a temporary environment used during the build, with "pip install --ignore-installed ...", so that packages given in pyproject.toml are reinstalled temporarily even if they are already available in the system. Better behavior could be to reinstall build-requirements only if the requirement is not already satisfied by system packages. Pip issue: https://github.com/pypa/pip/issues/4582 Numpy is a common build requirement for several packages providing C extensions that deal with numpy arrays. The current preinstallation behavior causes the following problems: 1) Numpy C extension ABI is backward compatible, but not forward compatible. As a consequence, C extensions built using a newer version of Numpy than installed on the system may simply segfault. (If there's a way to mark the version requirement in the built wheel, the end result is incompatible with the system numpy version.) This can easily happen if unsuspecting people put 'requires=numpy' to pyproject.toml. 2) It wastes time and resources unnecessarily: numpy rebuild can be 10 min per pop, plus downloading stuff from internet. -- Pauli Virtanen