Currently the API for defining different parameters by target platform is not ideal and we need to jump over hoops whilst trying to maintain it. The main difficulties are:

• We would like to specify different pip.parse attributes per target platform, but we cannot, because some of the parameters need to be labels, label lists or other.
• We cannot have overrides for these things easily and the definitions of what those target platforms are are hard-coded in the code.

The idea that I have is to reuse the same recipe from #2578, where we create a builder for the configuration and it allows us easily define:

• default constraint and flag values for the config settings that we create for different platform variants.
• default values for the pip index url, etc
• default values for which platforms we should support.
• default auth configuration for the bazel_downloader.
• The list of python_versions that are supported pip.parse(python_version=) should not be mandatory #1708.
• build the configuration incrementally for the pip.parse invocation allowing per-target-platform configuration of:
  • extra_pip_args. feat: add support for passing extra pip args by platform #2745
  • requirements_lock.
  • constraint_values and flag_values. pip_parse: requirements_by_platform: allow custom defined platforms #2548

What is more the override API could be blended in more easily to provide better support for specifying different, patches, etc.

TODO:

• Design defaults and configure APIs and use that to set the defaults within rules_python for selecting the whls.
• Do the same for index_url setting, at some point rules_python could set the default index_url to default to the new code path.
• Transition the current pip.parse to use pip.configure under the hood.