Robin Wilson: Py6S v1.9.0 released – plus code cleanup and how I got M1 support for the underlying 6S model

Last week I released version 1.9.0 of Py6S – my Python interface to the 6S radiative transfer model.

It’s been the first non-bugfix release for quite a while (mainly because I’ve been busy with paid work), and so I just wanted to highlight a few aspects of the release.

The full release notes are available here, and they include a couple of minor breaking changes (with very specific exceptions being raised if you run into them). Specifically, the pysolar dependency has been updated to the latest version (old versions are not supported as the pysolar API changed significantly between versions), and ambiguous dates in Geometry.from_time_and_location has been dealt with by requiring date-times to be passed in ISO 8601 format. We also no longer require the matplotlib dependency, which makes Py6S a lot easier (and quicker) to install if graphical output is not required.

Code cleanup

More importantly, from my perspective, the Py6S code has had a significant clean-up. The code was relatively high-quality in terms of its operation, but it wasn’t formatted very well, and didn’t use various Python best practices. My excuse for this is that it was first written a long time ago, and I haven’t had time to update it…until now!

As part of this release, I ran the black formatter on the code-base, and used isort to sort the imports. I then added a pre-commit configuration to run black and isort every time I commit. The following is the contents of my .pre-commit-config.yaml file:

repos:
  - repo: https://github.com/python/black
    rev: 20.8b1
    hooks:
      - id: black
        language_version: python3.7
  - repo: https://github.com/asottile/seed-isort-config
    rev: v2.2.0
    hooks:
    -   id: seed-isort-config
  - repo: https://github.com/timothycrosley/isort
    rev: 5.5.2  # pick the isort version you'd like to use from https://github.com/timothycrosley/isort/releases
    hooks:
    -   id: isort

This will run black and isort on every commit, and also includes an isort ‘seed’ step that seeds a list of third-party modules that are used, which isort uses internally for some of its operations.

When writing Py6S I’d used a lot of * imports – like from Py6S import *. This is not best practice (and wasn’t even best practice when I wrote it!) and so I wanted to remove these – and I found a handy tool to do this: removestar. Simply run this on your source files, and it automatically replaces * imports with imports of the actual objects that you use from that module.

I also went through and fixed all flake8 errors across the whole project.

M1 support for 6S

Another recent change has been the addition of support for running the underlying 6S model on the Apple M1 processor (sometimes referred to as osx-arm64). I was expecting Apple to announce new MacBook Pros with M1 (or similar) processors at WWDC last week – so wanted to make sure that I could run 6S on a new machine (as I was intending to purchase one). Unfortunately, these laptops weren’t announced – but it’s good to get this working for other people anyway.

The recommended way to install Py6S involves installing 6S using conda, with the package hosted on conda-forge – and conveniently conda-forge provide instructions for setting up M1 builds for conda-forge projects. Building 6S on the M1 is a little difficult, as there is no Fortran compiler available for the M1’s at the moment – so a complex cross-compilation step is required.

I’m glad to say that the instructions provided by conda-forge worked really easily. As 6S is not a Python package, I could ignore the long list of additional build dependencies to add, and simply had to add

build_platform:
  osx_arm64: osx_64
test_on_native_only: true

to the conda-forge.yml file and then make sure that ${CMAKE_ARGS} was used in the call to cmake in build.sh. You can see the full PR for these changes here (there are lots of changes to other files caused by ‘re-rendering’ the feedstock to use the latest conda-forge configurations).

As I don’t have a M1 Mac yet, I can’t test it directly – but I am assured by a correspondent on Twitter that it works – hooray!

I’m glad I had chance to do some of these changes to Py6S, and hopefully I will have chance to update it more often in the future.

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