Got it!

Summary:
* Installing a numpy wheel (e.g. `pip install numpy==1.21.0`) uses its embedded OpenBLAS on every platform that has a wheel.
  That OpenBLAS is always compiled with gcc/gfortran.
  In this case, `np.show_config()` reports `library_dirs = ['/usr/local/lib']` even though there's no libblas in that directory.

* Installing numpy from source (e.g. `pip install numpy==1.21.0 --no-binary numpy)` looks for BLAS & LAPACK libraries at build time as influenced by the environment vars NPY_BLAS_ORDER/NPY_LAPACK_ORDER or by the file ~/.numpy-site.cfg.
  On macOS, 'accelerate' is in the default search order after 'openblas'.
  On macOS < 11.3, importing numpy that's linked to Accelerate will detect an Accelerate bug and raise a RuntimeError.

On Wed, Jul 7, 2021 at 1:32 PM Ralf Gommers <ralf.gommers@gmail.com> wrote:


On Wed, Jul 7, 2021 at 9:56 PM Jerry Morrison <jerry.morrison+numpy@gmail.com> wrote:
Would someone please answer installation questions about NumPy's BLAS on macOS? I'm not finding the answers in the release notes, the PR source, the docs, or Stack Overflow.


Q1. The NumPy 1.21.0 release note says "This change enables the Accelerate Framework as an option on macOS." How to set that option on/off?

It's autodetected at build time. If you have no other BLAS installed, it will be used. Or explicitly select it with NPY_BLAS_ORDER/NPY_LAPACK_ORDER


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 says "The OpenBLAS libraries are included in the wheel."

It's a build-time option, you cannot select it at runtime.


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.)

If you install a wheel, you will always get the bundled OpenBLAS on every platform for which we have binary wheels.
 

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?

This question seems a little bit confused. Those env vars just select the BLAS/LAPACK library. It will not affect build time - we're never building BLAS or LAPACK itself from source.


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

gcc/gfortran. and no, you cannot control it through pip

Cheers,
Ralf


Thank you!
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