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Jinja#

rattler-build comes with a couple of useful Jinja functions and filters that can be used in the recipe.

Functions#

The compiler function#

The compiler function can be used to put together a compiler that works for the current platform and the compilation "target_platform". The syntax looks like: ${{ compiler('c') }} where 'c' signifies the programming language that is used.

This function evaluates to <compiler>_<target_platform> <compiler_version>. For example, when compiling on linux and to linux-64, this function evaluates to gcc_linux-64.

The values can be influenced by the variant_configuration. The <lang>_compiler and <lang>_compiler_version variables are the keys with influence. See below for an example:

Usage in a recipe#

recipe.yaml
requirements:
  build:
    - ${{ compiler('c') }}

With a corresponding variant_configuration:

variant_configuration.yaml
c_compiler:
- clang
c_compiler_version:
- 9.0

The variables shown above would select the clang compiler in version 9.0. Note that the final output will still contain the target_platform, so that the full compiler will read clang_linux-64 9.0 when compiling with --target-platform linux-64.

rattler-build defines some default compilers for the following languages (inherited from conda-build):

  • c: gcc on Linux, clang on osx and vs2017 on Windows
  • cxx: gxx on Linux, clangxx on osx and vs2017 on Windows
  • fortran: gfortran on Linux, gfortran on osx and vs2017 on Windows
  • rust: rust

The stdlib function#

The stdlib function closely mirrors the compiler function. It can be used to put together a standard library that works for the current platform and the compilation "target_platform".

Usage: ${{ stdlib('c') }}

Results in <stdlib>_<target_platform> <stdlib_version>. And uses the variant variables <lang>_stdlib and <lang>_stdlib_version to influence the output.

Usage in a recipe:#

recipe.yaml
requirements:
  build:
    # these are usually paired!
    - ${{ compiler('c') }}
    - ${{ stdlib('c') }}

With a corresponding variant_configuration:

variant_configuration.yaml
# these are the values `conda-forge` uses in their pinning file
# found at https://github.com/conda-forge/conda-forge-pinning-feedstock/blob/main/recipe/conda_build_config.yaml
c_stdlib:
- sysroot
c_stdlib_version:
- 2.17

The pin functions#

A pin is created based on the version input (from a subpackage or a package resolution).

The pin functions take the following three arguments:

  • lower_bound (default: "x.x.x.x.x.x"): The lower bound pin expression to be used. When set to None, no lower bound is set.
  • upper_bound (default: "x"): The maximum pin to be used. When set to None, no upper bound is set.

The lower bound and upper bound can either be a "pin expression" (only x and . are allowed) or a hard-coded version string.

A "pin expression" is applied to the version input to create the lower and upper bounds. For example, if the version is 3.10.5 with a lower_bound="x.x", upper_bound="x.x.x", the lower bound will be 3.10 and the upper bound will be 3.10.6.0a0. A pin expression for the upper_bound will increment the last selected segment of the version by 1, and append .0a0 to the end to prevent any alpha versions from being selected.

If the last segment of the version contains a letter (e.g. 9e or 1.1.1j), then incrementing the version will set that letter to a, e.g. 9e will become 10a, and 1.1.1j will become 1.1.2a. In this case, also no 0a0 is appended to the end.

Sometimes you want to strongly connect your outputs. This can be achieved with the following input:

  • exact=True (default: False): This will pin the version exactly to the version of the output, incl. the build string.

To override the lower or upper bound with a hard-coded value, you can use the following input:

  • lower_bound (default: None): This will override the lower bound with the given value.
  • upper_bound (default: None): This will override the upper bound with the given value.

Both lower_bound and upper_bound expect a valid version string (e.g. 1.2.3).

The pin_subpackage function#

  • ${{ pin_subpackage("mypkg", lower_bound="x.x", upper_bound="x.x") }} creates a pin to another output in the recipe. With an input of 3.1.5, this would create a pin of mypkg >=3.1,<3.2.0a0.
  • ${{ pin_subpackage("other_output", exact=True) }} creates a pin to another output in the recipe with an exact version.
  • ${{ pin_subpackage("other_output", lower_bound="1.2.3", upper_bound="1.2.4") }} creates a pin to another output in the recipe with a lower bound of 1.2.3 and an upper bound of 1.2.4. This is equivalent to writing other_output >=1.2.3,<1.2.4.

The pin_compatible function#

The pin compatible function works exactly as the pin_subpackage function, but it pins the package in the run requirements based on the resolved package of the host or build section.

  • pin_compatible pins a package in the run requirements based on the resolved package of the host or build section.

The cdt function#

  • ${{ cdt("mypkg") }} creates a cross-dependency to another output in the recipe.

This function helps add Core Dependency Tree packages as dependencies by converting packages as required according to hard-coded logic. See below for an example of how this function can be used:

# on x86_64 system
cdt('package-name') # outputs: package-name-cos6-x86_64
# on aarch64 system
cdt('package-name') # outputs: package-name-cos6-aarch64

The hash variable#

  • ${{ hash }} is the variant hash and is useful in the build string computation.

The version_to_buildstring function#

  • ${{ python | version_to_buildstring }} converts a version from the variant to a build string (it removes the . character and takes only the first two elements of the version).

The env object#

You can use the env object to retrieve environment variables and forward them to your build script. ${{ env.get("MY_ENV_VAR") }} will return the value of the environment variable MY_ENV_VAR or throw an error if it is not set.

To supply a default value when the environment variable is not set, you can use ${{ env.get("MY_ENV_VAR", default="default_value") }}. In this case, if MY_ENV_VAR is not set, the value default_value will be returned (and no error is thrown).

You can also check for the existence of an environment variable:

  • ${{ env.exists("MY_ENV_VAR") }} will return true if the environment variable MY_ENV_VAR is set and false otherwise.

Filters#

A feature of jinja is called "filters". Filters are functions that can be applied to variables in a template expression.

The syntax for a filter is {{ variable | filter_name }}. A filter can also take arguments, such as ... | replace('foo', 'bar').

The following Jinja filters are available, taken from the upstream minijinja library:

  • replace: replace a string with another string (e.g. "{{ 'foo' | replace('oo', 'aa') }}" will return "faa")
  • lower: convert a string to lowercase (e.g. "{{ 'FOO' | lower }}" will return "foo")
  • upper: convert a string to uppercase (e.g. "{{ 'foo' | upper }}" will return "FOO") - int: convert a string to an integer (e.g. "{{ '42' | int }}" will return 42)
  • abs: return the absolute value of a number (e.g. "{{ -42 | abs }}" will return 42)
  • bool: convert a value to a boolean (e.g. "{{ 'foo' | bool }}" will return true)
  • default: return a default value if the value is falsy (e.g. "{{ '' | default('foo') }}" will return "foo")
  • first: return the first element of a list (e.g. "{{ [1, 2, 3] | first }}" will return 1) - last: return the last element of a list (e.g. "{{ [1, 2, 3] | last }}" will return 3)
  • length: return the length of a list (e.g. "{{ [1, 2, 3] | length }}" will return 3)
  • list: convert a string to a list (e.g. "{{ 'foo' | list }}" will return ['f', 'o', 'o'])
  • join: join a list with a separator (e.g. "{{ [1, 2, 3] | join('.') }}" will return "1.2.3")
  • min: return the minimum value of a list (e.g. "{{ [1, 2, 3] | min }}" will return 1)
  • max: return the maximum value of a list (e.g. "{{ [1, 2, 3] | max }}" will return 3)
  • reverse: reverse a list (e.g. "{{ [1, 2, 3] | reverse }}" will return [3, 2, 1])
  • slice: slice a list (e.g. "{{ [1, 2, 3] | slice(1, 2) }}" will return [2])
  • batch: This filter works pretty much like slice just the other way round. It returns a list of lists with the given number of items. If you provide a second parameter this is used to fill up missing items.
  • sort: sort a list (e.g. "{{ [3, 1, 2] | sort }}" will return [1, 2, 3])
  • trim: remove leading and trailing whitespace from a string (e.g. "{{ ' foo ' | trim }}" will return "foo")
  • unique: remove duplicates from a list (e.g. "{{ [1, 2, 1, 3] | unique }}" will return [1, 2, 3])
  • split: split a string into a list (e.g. "{{ '1.2.3' | split('.') }}" will return ['1', '2', '3']). By default, splits on whitespace.
Removed filters

The following filters are removed from the builtins:

  • attr
  • indent
  • select
  • selectattr
  • dictsort
  • reject
  • rejectattr
  • round
  • map
  • title
  • capitalize
  • urlencode
  • escape
  • pprint
  • safe
  • items
  • float
  • tojson

Extra filters for recipes#

The version_to_buildstring filter#

  • ${{ python | version_to_buildstring }} converts a version from the variant to a build string (it removes the . character and takes only the first two elements of the version).

For example the following:

context:
  cuda: "11.2.0"

build:
  string: ${{ hash }}_cuda${{ cuda_version | version_to_buildstring }}

Would evaluate to a abc123_cuda112 (assuming the hash was abc123).

Various remarks#

Inline conditionals with Jinja#

The new recipe format allows for inline conditionals with Jinja. If they are falsey, and no else branch exists, they will render to an empty string (which is, for example in a list or dictionary, equivalent to a YAML null).

When a recipe is rendered, all values that are null must be filtered from the resulting YAML.

requirements:
  host:
    - ${{ "numpy" if cuda == "yes" }}

If cuda is not equal to yes, the first item of the host requirements will be empty (null) and thus filtered from the final list.

This must also work for dictionary values. For example:

build:
  number: ${{ 100 if cuda == "yes" }}
  # or an `else` branch can be used, of course
  number: ${{ 100 if cuda == "yes" else 0 }}