Fix case when using multiple platforms but pip contains a single platform

[basnijholt]

This came up in basnijholt/unidep#23. This UniDep is a package that unifies Conda and Pip dependency specification and integrates with conda-lock. I think conda-lock folks might like it 😄

About this fix, it prevents:

  File "/Users/bas.nijholt/micromamba/lib/python3.11/site-packages/conda_lock/src_parser/environment_yaml.py", line 58, in _parse_environment_file_for_platform
    for spec in mapping_spec["pip"]:
TypeError: 'NoneType' object is not iterable

Which occurs when locking:

name: example
channels:
  - conda-forge
dependencies:
  - tomli
  - pip:
    - psutil  # [linux64]
platforms:
  - linux-64
  - osx-arm64

Which becomes:

env_yaml_data = {'name': 'test-pip-with-platform-selector', 'channels': ['conda-forge'], 'dependencies': ['tomli', {'pip': None}], 'platforms': ['linux-64', 'osx-arm64']}

For osx-arm64.

This fix skips {"pip": None}.

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[maresb]

Ah, took me a bit to figure out exactly what this is about. Here's my summary:

For the osx-arm64 platform, after filtering

  - pip:
    - psutil  # [linux64]

becomes

  - pip:

then we parse this like

yaml.safe_load("pip:")  # {'pip': None}

so we must deal with the edge case mapping_spec["pip"] = None and not just mapping_spec["pip"] = [].

This looks great! Only thing is that this introduces an orphaned environment.yml in the tests directory. Would you be able to write a test to check that your environment.yml parses correctly and that psutil is present only for linux64?

---

And wow, UniDep looks awesome!!! It would be great if we could collaborate. Apart from getting fresh eyes on the codebase to clean up stuff like this, I really wish that conda-lock could output more intermediate representations along the lines of your requirements.yaml.

[maresb]

Thanks @basnijholt!!!