Skip to main content
Version: 2.17 (deprecated)

Advanced plugin concepts

Learning advanced concepts for writing plugins.

Introduction​

In this tutorial, we continue from where we've left in the previous tutorial. Having now a complete goal with a custom target, we are ready to make certain improvements and learn more advanced concepts that you would likely find useful when working on your own plugins.

Adding a custom source field​

In the first tutorial, to keep things simple, we used the default SingleSourceField class for our source field where we provided the path to the VERSION file. We could have added a custom field to provide a file path, however, when using the source field, you get a few features for free such as setting the default value and expected_file_extensions. Furthermore, with the source field, thanks to the unmatched_build_file_globs option, you won't need to provide custom logic to handle errors when path globs do not expand to any files in your repository.

Let's modify our myapp/BUILD file:

version_file(
    name="main-project-version",
    source="non-existing-file",
)

and run the project-version goal:

$ pants project-version myapp:
...
[WARN] Unmatched glob from myapp:main-project-version's `source` field: "myapp/non-existing-file"
[ERROR] 1 Exception encountered:

  InvalidFieldException: The 'source' field in target myapp:main-project-version must have 1 file, but it had 0 files.
...

It is possible to adjust how Pants handle unmatched globs to prevent this type of issue:

$ PANTS_UNMATCHED_BUILD_FILE_GLOBS=error pants project-version myapp:
[ERROR] 1 Exception encountered:
  Exception: Unmatched glob from myapp:main-project-version's `source` field: "myapp/non-existing-file"

We would likely want to use the same name for the version file (VERSION) throughout the repo for consistency, so we should probably set a default value for the target to reduce the amount of boilerplate in the BUILD files. To change a default value, we have to subclass the original field. Visit customizing fields through subclassing to learn more.

pants-plugins/project_version/targets.py
from pants.engine.target import COMMON_TARGET_FIELDS, SingleSourceField, Target

class ProjectVersionSourceField(SingleSourceField):
    help = "Path to the file with the project version."
    default = "VERSION"
    required = False

class ProjectVersionTarget(Target):
    alias = "version_file"
    core_fields = (*COMMON_TARGET_FIELDS, ProjectVersionSourceField)
    help = "A project version target representing the VERSION file."

You may have noticed that we have decided to override the help property to show more relevant information than the default help message:

$ pants help version_file
`version_file` target
---------------------

A project version target representing the VERSION file.

Activated by project_version
Valid fields:

...
source
    type: str | None
    default: 'VERSION'

    Path to the file with the project version.
...

Having a dedicated source field will let us filter the targets based on the fact that they have a ProjectVersionSourceField field instead of checking what their alias is. This means we can refactor how we collect the relevant targets from:

targets = [tgt for tgt in targets if tgt.alias == ProjectVersionTarget.alias]

to

targets = [tgt for tgt in targets if tgt.has_field(ProjectVersionSourceField)]

Using own classes via subclassing will also help with refactoring if you decide to deprecate the target alias in order to rename it. In a more advanced scenario, other plugins may import the ProjectVersionSourceField field and use it in their own custom targets, so that project-version specific behavior would still apply to those targets as well.

Ensuring a version follows a semver convention​

With the current implementation, we have simply returned the contents of the file as is. We may want to add some validation, for instance, to check that a version string follows a semver convention. Let's learn how to bring a 3rd party Python package, namely, packaging, into our plugin to do that!

To start depending on the packaging package in our in-repo plugin, we must extend the pants.toml file:

[GLOBAL]
plugins = ["packaging==22.0"]

Now, let's raise an exception if it isn't possible to construct an instance of the Version class:

from packaging.version import Version, InvalidVersion
from project_version.target_types import ProjectVersionTarget, ProjectVersionSourceField

class InvalidProjectVersionString(ValueError):
    pass

@goal_rule
async def goal_show_project_version(targets: Targets) -> ProjectVersionGoal:
    targets = [tgt for tgt in targets if tgt.has_field(ProjectVersionSourceField)]
    results = await MultiGet(
        Get(ProjectVersionFileView, ProjectVersionTarget, target) for target in targets
    )
    for result in results:
        try:
           _ = Version(result.version)
        except InvalidVersion:
            raise InvalidProjectVersionString(f"Invalid version string '{result.version}' from '{result.path}'")
    ...

To test this behavior, let's set a bogus version and see our goal in action!

$ cat myapp/VERSION
x.y.z

$ pants project-version myapp:
[ERROR] 1 Exception encountered:

    InvalidProjectVersionString: Invalid version string 'x.y.z' from 'myapp/VERSION'

Exploring caching​

When you have run the goal a few times, you may have noticed that sometimes the command takes a few seconds to complete, and sometimes it completes immediately. If that's the case, then you have just seen Pants caching working! Because we use Pants engine to read the VERSION file, it copies it into the cache. Pants knows that when the command is re-run, if there are no changes to the Python source code or the VERSION file, there's no need to re-run the code because the result is guaranteed to stay the same.

If your plugin uses 3rd party Python packages dependencies, it can be worth checking whether the package has any side effects such as reading from the filesystem since this won't let you take full advantage of the Pants engine's caching mechanism. Keep in mind that the commands you run via Pants may be cancelled or retried any number of times, so ideally any side effects should be idempotent. That is, it should not matter if it is run once or several times.

You can confirm that cache is being used by adding log statements. When run for the first time, the logging messages will show up; on subsequent runs, they won't because the code of the rules won't be executed.

Showing output as JSON​

We have so far shown the version string as part of the ProjectVersionFileView class:

$ pants project-version myapp:
ProjectVersionFileView(path='myapp/VERSION', version='0.0.1')

To be able to pipe the output of our command, it may make sense to emit the format in a parseable structure instead of plain text. Pants goals come with lots of options that can adjust their behavior, and this is true for custom goals as well. Let's add a new option for our goal, so that the version information would be shown as a JSON object.

Adding a new option is trivial and is done in the subsystem:

class ProjectVersionSubsystem(GoalSubsystem):
    name = "project-version"
    help = "Show representation of the project version from the `VERSION` file."

    as_json = BoolOption(
        default=False,
        help="Show project version information as JSON.",
    )

To use a subsystem in the goal rule (where we show the version in the console), we need to request it as a parameter:

import json

@goal_rule
async def goal_show_project_version(
    console: Console, project_version_subsystem: ProjectVersionSubsystem
) -> ProjectVersionGoal:
    ...
    if project_version_subsystem.as_json:
        console.print_stdout(json.dumps(dataclasses.asdict(result)))
    else:
        console.print_stdout(str(result))

Let's run our goal with the new --as-json flag:

$ pants project-version --as-json myapp: | jq
{
  "path": "myapp/VERSION",
  "version": "0.0.1"
}

Automating generation of project_version targets​

Pants provides a way to automate generation of standard targets using the tailor goal. If a monorepository has many projects, each containing a VERSION file, it might be useful to generate version_file targets in every directory where the relevant files are found. This is what Pants does, for instance, when Docker backend is enabled, and you have Dockerfile files in the codebase. To make this work for our use case, however, we need to introduce the tailor goal to the VERSION files.

We've reached the moment when the documentation won't be of help: there are no instructions on how to extend the tailor goal. In a situation like this, it may be worth exploring the Pants codebase to see how this was done in other plugins that are part of Pants. Once you find a piece of code that looks like it does what you want, you can copy it and tweak it to better suit your needs. For our use case, the code used in generation of C++ source targets may get handy. After making a few changes, we have a new rule we can place in a new file:

pants-plugins/project_version/tailor.py
from __future__ import annotations

from dataclasses import dataclass

from pants.core.goals.tailor import (
    AllOwnedSources,
    PutativeTarget,
    PutativeTargets,
    PutativeTargetsRequest,
)
from pants.util.dirutil import group_by_dir
from pants.engine.fs import PathGlobs, Paths
from pants.engine.internals.selectors import Get
from pants.engine.rules import collect_rules, rule
from pants.engine.unions import UnionRule
from project_version.target_types import ProjectVersionTarget


@dataclass(frozen=True)
class PutativeProjectVersionTargetsRequest(PutativeTargetsRequest):
    pass


@rule(desc="Determine candidate project_version targets to create")
async def find_putative_targets(
    req: PutativeProjectVersionTargetsRequest,
    all_owned_sources: AllOwnedSources,
) -> PutativeTargets:
    all_project_version_files = await Get(Paths, PathGlobs, req.path_globs("VERSION"))
    unowned_project_version_files = set(all_project_version_files.files) - set(
        all_owned_sources
    )
    classified_unowned_project_version_files = {
        ProjectVersionTarget: unowned_project_version_files
    }

    putative_targets = []
    for tgt_type, paths in classified_unowned_project_version_files.items():
        for dirname, filenames in group_by_dir(paths).items():
            putative_targets.append(
                PutativeTarget.for_target_type(
                    ProjectVersionTarget,
                    path=dirname,
                    name="project-version-file",
                    triggering_sources=sorted(filenames),
                )
            )

    return PutativeTargets(putative_targets)


def rules():
    return [
        *collect_rules(),
        UnionRule(PutativeTargetsRequest, PutativeProjectVersionTargetsRequest),
    ]

In this file, we use an advanced feature of Pants, union rules:

def rules():
    return [
        *collect_rules(),
        UnionRule(PutativeTargetsRequest, PutativeProjectVersionTargetsRequest),
    ]

When the tailor goal is run, the build graph is analyzed to see when PutativeTargetsRequest is needed, i.e. to find out if there are any files (yet unknown to Pants) that look like they could potentially be made targets. For instance, if there is a requirements.txt file, a python_requirement target is created and when there is a Python test_ module, a python_test target is created. To be able to customize the tailor goal (to allow generation of custom targets), we need to "extend" the build graph. That is, we ask Pants to also run our rule when searching for files that maybe should have a target created.

We also have to make sure that the new rule is collected:

pants-plugins/project_version/register.py
...
def rules():
    return [*project_version_rules.rules(), *tailor_rules.rules()]

Let's remove existing version_file target from the myapp/BUILD file and run the tailor goal:

$ pants tailor ::
Created myapp/BUILD:
  - Add version_file target project-version-file

If you have multiple projects, being able to generate the targets automatically may save time. You would also likely want to run the tailor goal in the check mode to confirm that new projects created have a version_file target. Remove the version_file target from the myapp/BUILD file and re-run the tailor goal:

$ pants tailor --check ::
Would create myapp/BUILD:
  - Add version_file target project-version-file

To fix `tailor` failures, run `pants tailor`.

Running system tools​

Pants lets you run system applications your plugin may need. For our use case, we can assume that Git is installed and can be run from the /usr/bin/git. If there's a VERSION file in the root of the repository representing the final artifact version (in case of a monolith), we could use Git to confirm that the version string matches the latest tag the repository was tagged with.

We can create a new rule:

class GitTagVersion(str):
    pass

@rule
async def get_git_repo_version(buildroot: BuildRoot) -> GitTagVersion:
    git_paths = await Get(
        BinaryPaths,
        BinaryPathRequest(
            binary_name="git",
            search_path=["/usr/bin", "/bin"],
        ),
    )
    git_bin = git_paths.first_path
    if git_bin is None:
        raise OSError("Could not find 'git'.")
    git_describe = await Get(
        ProcessResult,
        Process(
            argv=[git_bin.path, "-C", buildroot.path, "describe", "--tags"],
            description="git describe --tags",
        ),
    )
    return GitTagVersion(git_describe.stdout.decode().strip())

and then use this rule in the main goal rule:

class ProjectVersionGitTagMismatch(ValueError):
    pass

@goal_rule
async def goal_show_project_version(...) -> ProjectVersionGoal:
    ...
    git_repo_version = await Get(GitTagVersion)
    ...
    if git_repo_version != result.version:
        raise ProjectVersionGitTagMismatch(
            f"Project version string '{result.version}' from '{result.path}' "
            f"doesn't match latest Git tag '{git_repo_version}'"
        )

Let's modify our VERSION file to have a version different from what we have tagged our repository with:

$ git tag 0.0.1
$ git describe --tags
0.0.1
$ cat myapp/VERSION
0.0.2

$ pants project-version --as-json myapp:
12:40:17.02 [INFO] Initializing scheduler...
12:40:17.14 [INFO] Scheduler initialized.
12:40:17.18 [ERROR] 1 Exception encountered:

  ProjectVersionGitTagMismatch: Project version string '0.0.2' from 'myapp/VERSION' doesn't match latest Git tag '0.0.1'

Now, let's tag our repository with another tag and update our VERSION file:

$ git tag --delete 0.0.1
Deleted tag '0.0.1' (was 006f320)
$ git tag 0.0.2
$ git describe --tags
0.0.2
$ cat myapp/VERSION
0.0.1

$ pants project-version --as-json myapp:
{"path": "myapp/VERSION", "version": "0.0.1"}

Pants is happy, but clearly something is wrong as our Git tag version doesn't match the myapp/VERSION version! If you update your myapp/VERSION with another version, say, 0.0.3, we get an error, but this time, the shown Git tag is wrong:

$ cat myapp/VERSION
0.0.3

$ pants project-version --as-json myapp:
[ERROR] 1 Exception encountered:

  ProjectVersionGitTagMismatch: Project version string '0.0.3' from 'myapp/VERSION' doesn't match latest Git tag '0.0.1'

This happens because of how the Pants cache works. Modifying our repository tags doesn't qualify for the changes that should invalidate the cache. It is not safe to cache the Process runs since we know that Git will access the repository (that is outside the sandbox), we should change its cacheability using the ProcessCacheScope parameter so that our Git call would run once per run of Pants.

git_describe = await Get(
    ProcessResult,
    Process(
        argv=[git_bin.path, "-C", buildroot.path, "describe", "--tags"],
        description="git describe --tags",
        cache_scope=ProcessCacheScope.PER_SESSION,
    ),
)

Let's add another option so that we can control whether Git tag should be retrieved:

class ProjectVersionSubsystem(GoalSubsystem):
    name = "project-version"
    help = "Show representation of the project version from the `VERSION` file."

    ...
    match_git = BoolOption(
        default=False,
        help="Check Git tag of the repository matches the project version.",
    )

Keep in mind that once you've declared custom options in the plugin's subsystem, they can be set in the pants.toml file just like any standard Pants options.

If you know that your Git tag may be different from the project version stored in the VERSION file and that you would always want the output to be in the JSON format, you can set these options in the pants.toml file for visibility (and to avoid setting them via command line flags):

[project-version]
as_json = true
match_git = false

Putting it all together​

We have now extended the plugin with extra functionality:

$ pants project-version myapp:
[INFO] Initializing scheduler...
[INFO] Scheduler initialized.
{"path": "myapp/VERSION", "version": "0.0.1"}

Let's get all of this code in one place:

from typing import Iterable

import project_version.rules as project_version_rules
import project_version.tailor as tailor_rules
from pants.engine.target import Target
from project_version.target_types import ProjectVersionTarget


def target_types() -> Iterable[type[Target]]:
    return [ProjectVersionTarget]


def rules():
    return [*project_version_rules.rules(), *tailor_rules.rules()]

There are a few more things left to do, for example, we haven't written any tests yet. This is what we'll do in the next tutorial!