[Numpy-discussion] NumPy's BLAS library on macOS?

[Jerry Morrison]

Would someone please answer installation questions about NumPy's BLAS on
macOS? I'm not finding the answers in the release notes
<https://github.com/numpy/numpy/releases>, the PR
<https://github.com/numpy/numpy/pull/18874> source, the docs
<https://numpy.org/doc/1.21/>, or Stack Overflow
<https://stackoverflow.com/search?tab=newest&q=%5bnumpy%5d%20openblas>.


Q1. The NumPy 1.21.0 release note
<https://github.com/numpy/numpy/releases/tag/v1.21.0> says "This change
enables the Accelerate Framework as an option on macOS." How to set
that option on/off?

Q2. How to determine if NumPy uses Accelerate vs. its internal copy of
OpenBLAS?
After installing a wheel, `numpy.show_config()` shows the openblas_info
library_dirs et al as '/usr/local/lib'. Neither '/usr/local/lib/' nor
'site-packages/numpy/' contains a *blas*.so library (for Python 3.8.* on
macOS 10.14.6) but the doc <https://numpy.org/install/> says "The OpenBLAS
libraries are included in the wheel."

Q3. How to pip install NumPy 1.21.0 in a way that ensures it uses its
embedded OpenBLAS on macOS as on Linux? I'm aiming for as portable results
as possible. Or should we link NumPy to an external OpenBLAS via `pip
install numpy --no-binary numpy==1.21.0` with `~/.numpy-site.cfg`? (Ditto
for SciPy.)

Q4. Can the new NPY_* environment variables select specific BLAS & LAPACK
libraries through pip install, and perhaps install faster than building
NumPy, SciPy, etc. from source? How to do that?

Q5. Is NumPy's embedded OpenBLAS compiled by gcc or clang? Is that
controllable via `pip install`?

Thank you!
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