Conda skeleton does not support environment markers

[evhub]

PEP 508 added support for environment markers in PyPI version specifications. Specifically, environment markers allow you to have versions like (example on PyPI here)

pygments<2.3,>=2.2;python_version>='2.7'
prompt-toolkit<1.1,>=1.0;python_version>='2.7'
futures<3.1,>=3.0;python_version<'3'
argparse<1.5,>=1.4;python_version<'2.7'

where the package on the left is only a dependency if the current Python version meets the requirement on the right. This is a very useful feature for packages that aim to support multiple Python versions.

However, when conda skeleton pypi creates meta.yaml, it does not respect the environment markers, leaving any dependencies with environment markers completely out of the dependency structure. Thus, when one runs conda build on such a skeleton, it fails with (full error at evhub/coconut#246)

RuntimeError: Setuptools downloading is disabled in conda build. Be sure to add all dependencies in the meta.yaml

since setuptools tries to respect the environment markers, but conda won't let it.

This is a major problem for packages that use environment markers in their dependencies. It'd be great if this could be fixed, but until then, do you have any ideas for a suitable workaround?

[msarahan]

I think selectors are the nearest concept in conda-build: https://conda.io/docs/building/meta-yaml.html#preprocessing-selectors

Adding translation of environment markers to selectors to the skeleton generator is definitely desirable, but I don't think I'll be able to get to it any time soon. PRs welcome - and I'd be happy to answer any questions if anyone wants to tackle it.

[github-actions]

Hi there, thank you for your contribution!

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Thanks!