Custom Extensions

Spack extensions permit you to extend Spack capabilities by deploying your own custom commands or logic in an arbitrary location on your filesystem. This might be extremely useful e.g. to develop and maintain a command whose purpose is too specific to be considered for reintegration into the mainline or to evolve a command through its early stages before starting a discussion to merge it upstream. From Spack’s point of view an extension is any path in your filesystem which respects a prescribed naming and layout for files:

spack-scripting/ # The top level directory must match the format 'spack-{extension_name}'
├── pytest.ini # Optional file if the extension ships its own tests
├── scripting # Folder that may contain modules that are needed for the extension commands
│   └── cmd # Folder containing extension commands
│       └── # A new command that will be available
├── tests # Tests for this extension
│   ├──
│   └──
└── templates # Templates that may be needed by the extension

In the example above the extension named scripting adds an additional command (filter) and unit tests to verify its behavior. The code for this example can be obtained by cloning the corresponding git repository:

$ pwd
$ mkdir tmp && cd tmp
$ git clone
Cloning into 'spack-scripting'...
remote: Counting objects: 11, done.
remote: Compressing objects: 100% (7/7), done.
remote: Total 11 (delta 0), reused 11 (delta 0), pack-reused 0
Receiving objects: 100% (11/11), done.

As you can see by inspecting the sources, Python modules that are part of the extension can import any core Spack module.

Configure Spack to Use Extensions

To make your current Spack instance aware of extensions you should add their root paths to config.yaml. In the case of our example this means ensuring that:

  - /home/user/tmp/spack-scripting

is part of your configuration file. Once this is setup any command that the extension provides will be available from the command line:

$ spack filter --help
usage: spack filter [-h] [--installed | --not-installed]
                    [--explicit | --implicit] [--output OUTPUT]

filter specs based on their properties

positional arguments:
  specs            specs to be filtered

optional arguments:
  -h, --help       show this help message and exit
  --installed      select installed specs
  --not-installed  select specs that are not yet installed
  --explicit       select specs that were installed explicitly
  --implicit       select specs that are not installed or were installed implicitly
  --output OUTPUT  where to dump the result

The corresponding unit tests can be run giving the appropriate options to spack unit-test:

$ spack unit-test --extension=scripting

============================================================== test session starts ===============================================================
platform linux2 -- Python 2.7.15rc1, pytest-3.2.5, py-1.4.34, pluggy-0.4.0
rootdir: /home/mculpo/tmp/spack-scripting, inifile: pytest.ini
collected 5 items

tests/ ...XX
============================================================ short test summary info =============================================================
XPASS tests/[flags3-specs3-expected3]
XPASS tests/[flags4-specs4-expected4]

=========================================================== slowest 20 test durations ============================================================
3.74s setup    tests/[flags0-specs0-expected0]
0.17s call     tests/[flags3-specs3-expected3]
0.16s call     tests/[flags2-specs2-expected2]
0.15s call     tests/[flags1-specs1-expected1]
0.13s call     tests/[flags4-specs4-expected4]
0.08s call     tests/[flags0-specs0-expected0]
0.04s teardown tests/[flags4-specs4-expected4]
0.00s setup    tests/[flags4-specs4-expected4]
0.00s setup    tests/[flags3-specs3-expected3]
0.00s setup    tests/[flags1-specs1-expected1]
0.00s setup    tests/[flags2-specs2-expected2]
0.00s teardown tests/[flags2-specs2-expected2]
0.00s teardown tests/[flags1-specs1-expected1]
0.00s teardown tests/[flags0-specs0-expected0]
0.00s teardown tests/[flags3-specs3-expected3]
====================================================== 3 passed, 2 xpassed in 4.51 seconds =======================================================