Advanced noarch topics

Building noarch packages on other platforms

By default, noarch packages are built on Linux, and all dependencies must be available on Linux. If a noarch package cannot be built on Linux, one or more noarch_platforms can be provided in conda-forge.yml. For example, a package that can only be built on Windows can use the following conda-forge.yml:

conda-forge.yml

noarch_platforms:
- win-64

A more complex example is pywin32-on-windows, which is built both on Linux and Windows, with build_number offsets to create a pair of packages.

noarch packages with OS-specific dependencies

It is possible to build noarch packages with runtime requirements that depend on the target OS (Linux, Windows, MacOS), regardless the architecture (amd64, ARM, PowerPC, etc). This approach relies on three concepts:

1. Virtual packages. Prefixed with a double underscore, they are used by conda to represent system properties as constraints for the solver at install-time. We will use __linux, __win or __osx, which are only present when the running platform is Linux, Windows, or MacOS, respectively. __unix is present in both Linux and MacOS. Note that this feature is only fully available on conda 4.10 or above.
2. conda-forge.yml's noarch_platforms option.

The idea is to generate different variants of a noarch package. While all of these variants are noarch and therefore are visible on all platforms, they feature a dependency on the OS-specific virtual package such as __linux that make them not installable on other platforms. The resolver automatically selects the variant with the compatible virtual package, or throws an error if no compatible package can be found.

Let's say you have a pure Python package, perfectly eligible for noarch: python, but on Windows it requires windows-only-dependency. A platform-specific recipe could look like the following:

v0 (meta.yaml)

recipe/meta.yaml (original)

name: package
source:
  # ...
build:
  number: 0
requirements:
  # ...
  run:
    - python
    - numpy
    - windows-only-dependency  # [win]

v1 (recipe.yaml)

recipe/recipe.yaml (original)

name: package
source:
  # ...
build:
  number: 0
requirements:
  # ...
  run:
    - python
    - numpy
    - if: win
      then: windows-only-dependency

With this recipe, the build matrix will include separate builds for every supported platform, and for every supported Python version on every platform. All the built package will have the same contents, and only win-64 packages will have different dependencies (though they still will be duplicated across all Python versions).

We can get it down to two packages if replace it with this other approach:

v0 (meta.yaml)

recipe/meta.yaml (modified)

name: package
source:
  # ...
build:
  number: 0
  noarch: python
requirements:
  host:
    - python {{ python_min }}.*
    # ...
  run:
    - python >={{ python_min }}
    - numpy
    - __unix  # [unix]
    - __win   # [win]
    - windows-only-dependency  # [win]

v1 (recipe.yaml)

recipe/recipe.yaml (modified)

name: package
source:
  # ...
build:
  number: 0
  noarch: python
requirements:
  host:
    - python {{ python_min }}.*
    # ...
  run:
    - python >={{ python_min }}
    - numpy
    - if: unix
      then:
        - __unix
      else:
        - __win
        - windows-only-dependency

Do not forget to specify the platform virtual packages with their selectors. Otherwise, the solver will not be able to choose the variants correctly.

By default, conda-forge will only build noarch packages on a linux_64 CI runner, so only the unix selector would be true. However, we can change this behaviour using the noarch_platforms option in conda-forge.yml:

conda-forge.yml

noarch_platforms:
  - linux_64
  - win_64

Once rerendered, this will result in two CI jobs.

If you need conditional dependencies on all three operating systems, this is how you do it:

v0 (meta.yaml)

name: package
source:
  # ...
build:
  number: 0
  noarch: python
requirements:
  # ...
  run:
    - python >={{ python_min }}
    - numpy
    - __linux  # [linux]
    - __osx    # [osx]
    - __win    # [win]
    - linux-only-dependency    # [linux]
    - osx-only-dependency      # [osx]
    - windows-only-dependency  # [win]

v1 (recipe.yaml)

name: package
source:
  # ...
build:
  number: 0
  noarch: python
requirements:
  # ...
  run:
    - python >={{ python_min }}
    - numpy
    - if: linux
      then:
        - __linux
        - linux-only-dependency
    - if: osx
      then:
        - __osx
        - osx-only-dependency
    - if: win
      then:
        - __win
        - windows-only-dependency

conda-forge.yml

noarch_platforms:
  - linux_64
  - osx_64
  - win_64

Again, remember to rerender after adding / modifying these files so the changes are applied.