Prevent channel/dependency reordering when rendering the environment

[wolny]

Keeps the order of channels/pip dependencies as defined in environment.devenv.yml (fixes #117).

In the current version the rendering of the following:

name: test-env

channels:
  - pytorch
  - nvidia
  - conda-forge

dependencies:
  - pytorch
  - numpy
  - scikit-image
  - pip:
    - "-e c-package"
    - "-e b-package"
    - "-e D-package"

is incorrect:

# generated by conda-devenv, do not modify and do not commit to VCS
channels:
- pytorch
- nvidia
- conda-forge
dependencies:
- numpy
- pytorch
- scikit-image
- pip:
  - -e D-package
  - -e b-package
  - -e c-package
environment: {}
name: test-env

This PR prevents the reordering and generate the correct outcome:

# generated by conda-devenv, do not modify and do not commit to VCS
channels:
- pytorch
- nvidia
- conda-forge
dependencies:
- pytorch
- numpy
- scikit-image
- pip:
  - -e c-package
  - -e b-package
  - -e D-package
environment: {}
name: test-env

[nicoddemus]

Thanks @wolny,

Can you please come up with a test for this, to avoid a regression in the future? Thanks

[sdvillal]

Testing should include screaming out loud when there are conflicting orderings of channels or pip dependencies. We probably should allow opting-in for this to happen and give a best order when this would not matter (e.g., when using newer pip versions with dependency resolution or when disabling channel priority in conda).

[wolny]

Hi @nicoddemus, I've added a single test which checks the ordering of channels and pip dependencies. Cheers!

[nicoddemus]

Question, does this keeps the order for all dependencies, or just channels/pip dependencies?

[wolny]

this change in fact prevents reordering of dependencies: https://github.com/ESSS/conda-devenv/blob/master/src/conda_devenv/devenv.py#L500 this line was responsible for reordering dependencies and I've change the merge_dependencies_version_specifications to prevent this. Channel ordering seems to be preserved in the current version, but since it's more important to prevent channel reordering I've added a test for it as well.

[nicoddemus]

I see, thanks for clarifying.

Technically we should not need to preserve the order of the normal/conda dependencies, and sorting them actually has the benefit of making the diff smaller in case someone changes the order in their devenv files for any reason -- but anyways the generated environment.yml file is not supposed to be committed, so it really it does not matter in the end.

Thanks for the PR!

I will leave this open for a few days to given others the chance to review it.

[wolny]

@nicoddemus yes, the order of dependencies does not matter for conda, since it computes the resolution on the whole set of dependencies. However the order of channels and pip dependencies does matter as mentioned in #117 and should be preserved. Channel ordering is already preserved by conda-devenv (which was the main concern), but the pip dependency ordering is not and this PR fixes it.

[prusse-martin]

Thank you.