They can! Numpy exposes a C API to other Python programs [0]. It's not hard to write a Cython library that uses the Numpy C API directly and does not cross into

They can! Numpy exposes a C API to other Python programs [0]. It's not hard to write a Cython library that uses the Numpy C API directly and does not cross into Python [1]. [0]: https://numpy.org/doc/stable/reference/c-api/index.html https://numpy.org/doc/stable/reference/c-api/index.html [1]: https://github.com/kylebarron/pymartini/blob/4774549ffa2051c4ddaa254aca191bedcc4d30c5/pymartini/martini.pyx#L2 https://github.com/kylebarron/pymartini/blob/4774549ffa2051c...