BUG: 'PYTHON' is undefined for multi-output builds if some outputs are generic

[ytausch]

Checklist

• I added a descriptive title
• I searched open reports and couldn't find a duplicate

What happened?

Consider the following recipe:

{% set version = "0.14.0" %}
{% set min_python = "python >=3.9" %}

package:
  name: ariadne-codegen-split
  version: {{ version }}

source:
  - folder: dist
    url: https://pypi.io/packages/source/a/ariadne-codegen/ariadne_codegen-{{ version }}.tar.gz
    sha256: d5b15470195c3858b88a2bae855b5c04602ffae40228077b6cfb26a57d3f14f6
  - folder: src
    url: https://github.com/mirumee/ariadne-codegen/archive/refs/tags/{{ version }}.tar.gz
    sha256: 86d79970c8260fa8292e40ada695babcf47560d6ac599611d039daedaae8543c

build:
  noarch: python
  number: 0

requirements:
  host:
    - {{ min_python }}
  run:
    - {{ min_python }}

outputs:
  - name: ariadne-codegen
    build:
      noarch: python
      script: cd dist && {{ PYTHON }} -m pip install . -vv --no-deps --no-build-isolation
      entry_points:
        - ariadne-codegen = ariadne_codegen.main:main
    requirements:
      host:
        - {{ min_python }}
        - hatchling
        - pip
      run:
        - {{ min_python }}
        - autoflake
        - black
        - click >=8.1,<9.0
        - graphql-core >=3.2.0,<3.3
        - httpx >=0.23,<1.0
        - isort
        - pydantic >=2.0.0,<3.0.0
        - toml >=0.10,<1.0
      run_constrained:
        - websockets >=11.0,<12.0
    test:
      requires:
        - pip
      imports:
        - ariadne_codegen
      commands:
        - pip check
        - ariadne-codegen --help

  - name: ariadne-codegen-with-websockets
    build:
      noarch: generic
    requirements:
      run:
        - {{ pin_subpackage("ariadne-codegen", exact=True) }}
        - websockets

I don't see why this recipe should not be able to be built.

However, conda build fails with Error: Failed to render jinja template in /home/conda/recipe_root/meta.yaml: 'PYTHON' is undefined

Apparently, this happens because the entire jinja template is rendered for every output but PYTHON is only set if the currently-processed output has Python as a host dependency.

I think a better behavior would be to strip out details about other outputs including the variables they reference before rendering with patched metadata dependent on individual outputs.

Traceback (most recent call last):
  File "/Users/ytausch/autotickbot/conda-build/conda_build/metadata.py", line 2010, in _get_contents
    rendered = template.render(environment=env)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/Caskroom/miniforge/base/envs/cf-scripts/envs/cf-scripts/lib/python3.11/site-packages/jinja2/environment.py", line 1301, in render
    self.environment.handle_exception()
  File "/opt/homebrew/Caskroom/miniforge/base/envs/cf-scripts/envs/cf-scripts/lib/python3.11/site-packages/jinja2/environment.py", line 936, in handle_exception
    raise rewrite_traceback_stack(source=source)
  File "<template>", line 7, in top-level template code
jinja2.exceptions.UndefinedError: 'PYTHON' is undefined

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/ytausch/autotickbot/conda-build/conda_build/build.py", line 3709, in build_tree
    packages_from_this = build(
                         ^^^^^^
  File "/Users/ytausch/autotickbot/conda-build/conda_build/build.py", line 2335, in build
    output_metas = expand_outputs([(m, need_source_download, need_reparse_in_env)])
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ytausch/autotickbot/conda-build/conda_build/render.py", line 916, in expand_outputs
    for output_dict, m in deepcopy(_m).get_output_metadata_set(
                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ytausch/autotickbot/conda-build/conda_build/metadata.py", line 2658, in get_output_metadata_set
    conda_packages = finalize_outputs_pass(
                     ^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ytausch/autotickbot/conda-build/conda_build/metadata.py", line 1025, in finalize_outputs_pass
    fm = finalize_metadata(
         ^^^^^^^^^^^^^^^^^^
  File "/Users/ytausch/autotickbot/conda-build/conda_build/render.py", line 649, in finalize_metadata
    m = parent_metadata.get_output_metadata(m.get_rendered_output(m.name()))
                                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ytausch/autotickbot/conda-build/conda_build/metadata.py", line 2743, in get_rendered_output
    for output_ in self.get_rendered_outputs_section(
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ytausch/autotickbot/conda-build/conda_build/metadata.py", line 2725, in get_rendered_outputs_section
    self._get_contents(
  File "/Users/ytausch/autotickbot/conda-build/conda_build/metadata.py", line 2020, in _get_contents
    raise CondaBuildUserError(
conda_build.exceptions.CondaBuildUserError: Failed to render jinja template in :
'PYTHON' is undefined

Conda Info

No response

Conda Config

No response

Conda list

No response

Additional Context

No response

[jamesmyatt]

What's the workaround for this?

[ytausch]

Apparently, this works.