Conda build v3¶

Conda build version 3 has lots of nice features that will make managing packages in Bioconda much easier. However there are some changes that you will need to be aware of, especially if you’re used to making recipes using the old conda-build v2 way.

This page documents each change and is intended to serve as a reference for the transition.

The new host section¶

Summary:

• Previously, build-time dependencies were listed in the requirements:build: section.
• Conda-build 3 now has an additional requirements:host: section. It is required if using compilers; otherwise old recipes can remain as-is and can be gradually ported over. New recipes should use the new host section as described below.

Due to the improved way compilers are now being handled (see Compiler tools), the old build section is now split into two sections, build and host. This change is largely to support cross-compiling, which we are not doing yet. However if a recipe uses one of the new {{ compiler() }} methods described in Compiler tools, the host section is required.

The new build section should have things like compilers, git, automake, make, cmake, and other build tools. If there are no compilers or other build tools, there should be no build: section.

The new host section should have everything else.

There are many existing recipes that only have a build: section. They will work for now; they just won’t work if we ever try to cross-compile them. However new recipes should have the host: section.

The run section remains the same.

Before:

package:
name: example
version: 0.1
requirements:
build:
- python
run:
- python


After:

package:
name: example
version: 0.1
requirements:
host:
- python
run:
- python


Compiler tools¶

Summary:

• Previously we used - gcc #[linux] and - llvm # [osx] for compilers
• Instead, we should use the syntax {{ compiler('c') }}, {{ compiler('cxx') }}, and/or {{ compiler('fortran') }}. These should go in the build section, and all other build dependencies should go in the host section.

Anaconda now provides platform-specific compilers that are automatically determined. The string {{ compiler('c') }} will resolve to gcc on Linux, but clang on macOS. This should greatly simplify recipes, as we no longer need to have separate lines for linux and osx.

Note that previously we typically would also add - libgcc #[linux] as a run dependency, but this is now taken care of by the compiler tools (see the global pinning section below for more on this).

Conda-build 3 also now has the ability to cross-compile, making it now possible to compile packages for macOS while running on Linux. To support this, recipes must now make a distinction between dependencies that should be specific to the building machine and dependencies that should be specific to the running machine.

Dependencies specific to the building machine go in build; dependencies specific to the running machine go in host (see The new host section).

Before:

package:
name: example
version: 0.1
requirements:
build:
- python
- gcc  # [linux]
- llvm # [osx]
run:
- python
- libgcc  # [linux]


After:

package:
name: example
version: 0.1
requirements:
build:
- {{ compiler('c') }}
host:
- python
run:
- python


Warning

These compilers are only available in the defaults channel. Until now we have not had this channel as a dependency, so be sure to add the channel when setting up bioconda (see 2. Set up channels).

Global pinning¶

Summary:

• Previously we pinned packages using the syntax - zlib {{ CONDA_ZLIB }}* in both the build and run dependencies.
• Instead, we should now specify only package names in the host and run sections e.g., as simply zlib. They are pinned automatically.

Global pinning is the idea of making sure all recipes use the same versions of common libraries. Problems arise when the build-time version does not match the install-time version. Furthermore, all packages installed into the same environment should have been built using the same version so that they can co-exist. For example, many bioinformatics tools have zlib as a dependency. The version of zlib used when building the package should be the same as the version used when installing the package into a new environment. This implies that we need to specify the zlib version in one place and have all recipes use that version.

Previously we maintained a global, bioconda-specific pinning file (see scripts/env_matrix.yaml). For zlib, that file defined the variable CONDA_ZLIB and that variable was made available to the recipes as a jinja2 variable. One problem with this is that we did not often synchronize our pinned versions with conda-forge’s pinned versions, and this disconnect could cause problems.

There are two major advances in conda-build 3 to address these problems. First is the concept of “variants”. Variants are a generalized way of specifying one or more specific versions, and they come with many weird and wonderful ways to specify constraints. Specifying variants generally takes the form of writing a YAML file. We have adopted the variants defined by conda-forge by installing their conda-forge-pinning conda package in our build environment. Technically, that package unpacks the config YAML into our conda environment so that it can be used for building all recipes. You can see this file at conda_build_config.yaml

The second major advance in conda-build 3 is the the concept of “run exports”. The idea here is to specify that any time a dependency (zlib, in our running example) is used as a build dependency, it should also be automatically be installed as a run dependency without having to explicitly add it as such in the recipe. This specification is done in the zlib recipe itself (which is hosted by conda-forge), so in general bioconda collaborators can just add zlib as a build dependency.

Note that we don’t have to specify the version of zlib in the recipe – it is pinned in that conda_build_config.yaml file we share with conda-forge.

In a similar fashion, the reason that we no longer have to specify libgcc as a run dependency (as described above in the compilers section) is that {{ compiler('c') }} automatically export libgcc as a run dependency.

Before:

package:
name: example
version: 0.1
requirements:
build:
- python
- gcc  # [linux]
- llvm  # [osx]
- zlib {{ CONDA_ZLIB }}*
run:
- python
- libgcc  # [linux]
- zlib {{ CONDA_ZLIB }}*


After:

package:
name: example
version: 0.1
requirements:
build:
- {{ compiler('c') }}
host:
- python
- zlib
run:
- python
- zlib


Bio-specific packages additionally pinned by bioconda can be found at bioconda_utils-conda_build_config.yaml in the bioconda-utils source.