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Scripting with Spack

This tutorial introduces advanced Spack features related to scripting. Specifically, we will show you how to write scripts using spack find and spack python. Earlier sections of the tutorial demonstrated using spack find to list and search installed packages. The spack python command gives you access to all of Spack's internal APIs, allowing you to write more complex queries, for example.

Since Spack has an extensive API, we'll only scratch the surface here. We'll give you enough information to start writing your own scripts and to find what you need, with a little digging.

Scripting with spack find

The output we've seen from spack find has been for human consumption. But you can take advantage of some advanced options of the command to generate machine-readable output suitable for piping to a script.

spack find --format

The main job of spack find is to show the user a bunch of concrete specs that correspond to installed packages. By default, we display them with some default attributes, like the @version suffix you're used to seeing in the output.

The --format argument allows you to display the specs however you choose, using custom format strings. Format strings let you specify the names of particular parts of the specs you want displayed. Let's examine the first option.

Suppose you only want to display the name, version, and first ten (10) characters of the hash for every package installed in your Spack instance. You can generate that output with the following command:

.. literalinclude:: outputs/scripting/find-format.out
   :language: console
   :emphasize-lines: 1

Note that name, version, and hash are attributes of Spack's internal Spec object and enclosing them in braces ensures they are output according to your format string.

Using spack find --format allows you to retrieve just the information you need to do things like pipe the output to typical UNIX command line tools like sort or uniq.

spack find --json

Alternatively, you can get a serialized version of Spec objects in the JSON format using the --json option. For example, you can get attributes for all installations of zlib-ng by entering:

.. literalinclude:: outputs/scripting/find-json.out
   :language: console
   :emphasize-lines: 1

The spack find --json command gives you everything we know about the specs in a structured format. You can pipe its output to JSON filtering tools like jq to extract just the parts you want.

Check out the basic usage docs for more examples.

Introducing the spack python command

What if we need to perform more advanced queries?

Spack provides the spack python command to launch a python interpreter with Spack's python modules available to import. It uses the underlying python for the rest of its commands. You can write scripts to:

  • run Spack commands;
  • explore abstract and concretized specs; and
  • directly access other internal components of Spack.

Let's launch a Spack-aware python interpreter by entering:

.. literalinclude:: outputs/scripting/spack-python-1.out
   :language: console
   :emphasize-lines: 1,5

As we are in a Python interpreter, use exit() to end the session and return to the terminal.

Accessing the Spec object

Let's take a look at the internal representation of the Spack Spec. As you already know, specs can be either abstract or concrete. The specs you've seen in package.py files (e.g., in the install() method) have been concrete, or fully specified. The specs you've typed on the command line have been abstract. Understanding the differences between the two types is key to using Spack's internal API.

Let's open another python interpreter with spack python, instantiate the zlib spec, and check a few properties of an abstract spec:

.. literalinclude:: outputs/scripting/spack-python-abstract.out
   :language: console
   :emphasize-lines: 1-3,5,11,13

Notice that there are Spec properties and methods that are not accessible to abstract specs; specifically:

  • an exception -- SpecError -- is raised if we try to access its version;
  • there are no associated versions; and
  • the spec's operating system is None.

Without exiting the interpreter, let's concretize the spec and try again:

.. literalinclude:: outputs/scripting/spack-python-concrete.out
   :language: console
   :emphasize-lines: 1-2,4,6,8

Notice that the concretized spec now:

  • has a version;
  • has a single entry in its versions list; and
  • the operating system is now ubuntu22.04.

It is not necessary to store the intermediate abstract spec -- you can use the .concretized() method as shorthand:

.. literalinclude:: outputs/scripting/spack-python-sans-intermediate.out
   :language: console
   :emphasize-lines: 1-2

Querying the Spack database

Even more powerful queries are available when we look at the information stored in the Spack database. The Database object in Spack is in the spack.store.STORE.db variable. We'll interact with it mainly through the query() method. Let's see the documentation available for query() using python's built-in help() function:

.. literalinclude:: outputs/scripting/spack-python-db-query-help.out
   :language: console
   :emphasize-lines: 1-2,9-13

We will primarily make use of the query_spec argument.

Recall that queries using the spack find command are limited to queries of attributes with matching values, not values they do not have. In other words, we cannot use the spack find command for all packages that do not satisfy a certain criterion.

We can use the python interface to write these types of queries. For example, let's find all packages that were compiled with gcc but do not depend on mpich. We can do this by using custom python code and Spack database queries. We will use the spack.cmd.display_specs for output to achieve the same printing functionality as the spack find command:

.. literalinclude:: outputs/scripting/spack-python-db-query-exclude.out
   :language: console
   :emphasize-lines: 1-5

Now we have a powerful query not available through spack find.

Exit the interpreter to return to the command line:

>>> exit()

before generalizing the functionality for reuse.

Using scripts

Now let's parameterize our script to accept arguments on the command line. With a few generalizations to use the include and exclude specs as arguments, we can create a powerful, general-purpose query script.

Open a file called find_exclude.py in your preferred editor and add the following code:

.. literalinclude:: outputs/scripting/0.find_exclude.py.example
   :language: python

We added importing and using the system package (sys) to access the first and second command line arguments.

Now we can run our new script by entering the following:

.. literalinclude:: outputs/scripting/find-exclude-1.out
   :language: console
   :emphasize-lines: 1

This is beneficial for us, as long as we remember to use Spack's python command to run it.

Using the spack-python executable

What if we want to make our script available for others to use without the hassle of having to remember to use spack python?

We can take advantage of the shebang line typically added as the first line of python executable files. There is a catch, as we will soon see.

Open the find_exclude.py script we created above in your preferred editor and add the shebang line with spack python as the arguments to env:

.. literalinclude:: outputs/scripting/1.find_exclude.py.example
   :language: python
   :emphasize-lines: 1

Exit our editor and add execute permissions to the script before running it as follows:

.. literalinclude:: outputs/scripting/find-exclude-2.out
   :language: console
   :emphasize-lines: 1-2

If you are lucky, it worked on your system, but there is no guarantee. Some systems only support a single argument on the shebang line (see here). spack-python, which is a wrapper script for spack python, solves this issue.

Bring up the file in your editor again and change the env argument to spack-python as follows:

.. literalinclude:: outputs/scripting/2.find_exclude.py.example
   :language: python
   :emphasize-lines: 1

Exit your editor and run the script again:

.. literalinclude:: outputs/scripting/find-exclude-3.out
   :language: console
   :emphasize-lines: 1

It will now work on any system with Spack installed.

You now have the basic tools to create your own custom Spack queries and prototype ideas. We encourage you to contribute them back to Spack in the future.