Selectors in recipes

Recipe and variant configuration files can utilize selectors to conditionally add, remove, or modify dependencies, configuration options, or even skip recipe execution based on specific conditions.

Selectors are implemented using a simple if / then / else map, which is a valid YAML dictionary. The condition is evaluated using minijinja and follows the same syntax as a Python expression.

During rendering, several variables are set based on the platform and variant being built. For example, the unix variable is true for macOS and Linux, while win is true for Windows. Consider the following recipe executed on Linux:

requirements:
  host:
    - if: unix
      then: unix-tool
    - if: win
      then: win-tool

This will be evaluated as:

requirements:
  host:
    - unix-tool

The line containing the Windows-specific configuration is removed. Multiple items can also be selected, such as:

host:
  - if: linux
    then:
    - linux-tool-1
    - linux-tool-2
    - linux-tool-3

For Linux, this will result in:

host:
  - linux-tool-1
  - linux-tool-2
  - linux-tool-3

Other examples often found in the wild:

if: build_platform != target_platform ... # true if cross-platform build
if: osx and arm64 ... # true for apple silicon (osx-arm64)
if: linux and (aarch64 or ppc64le)) ... # true for linux ppc64le or linux-aarch64

Available variables

The following variables are available during the initial rendering and afterward:

Variable	Description
target_platform	the configured target_platform for the build
build_platform	the build platform
linux	"true" if target_platform is Linux
osx	"true" if target_platform is OSX / macOS
win	"true" if target_platform is Windows
unix	"true" if target_platform is a Unix (macOS or Linux)
x86_64, x86, arm64, ...	The architecture ("x86_64" for 64 bit, "x86" for 32 bit, otherwise arm64, aarch64, ppc64le, etc.)

After the initial phase, when the variant configuration is selected, the variant values are also available in selectors. For example, if the build uses python: 3.8 as a variant, we can use if: python == "3.8" to enable a dependency for only when the Python version is 3.8.

The cmp function

Inside selectors, one can use a special cmp function to test if the selected variant version has a matching version. For example, if we have again a python: 3.8 variant, we could use the following tests:

- if: cmp(python, "3.8")    # true
  then: mydep
- if: cmp(python, ">=3.8")  # true
  then: mydep
- if: cmp(python, "<3.8")   # false
  then: mydep

This function eliminates the need to implement any Python-specific conda-build selectors (such as py3k, py38, etc.) or the py and npy integers.

Please note that during the initial phase of rendering we do not know the variant, and thus the cmp condition always evaluates to true.