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Package code

How to add a new implementation to the package goal.

The package goal bundles all the relevant code and third-party dependencies into a single asset, such as a JAR, PEX, or zip file.

Often, the asset is executable, but it does not need to be.


Example repository

This guide walks through adding a simple package implementation for Bash that simply puts all the relevant source files into a .zip file.

This duplicates the archive target type, and is solely implemented for instructional purposes. See here for the final implementation.

1. Set up a package target type (recommended)

Usually, you will want to add a new target type for your implementation, such as pex_binary or python_distribution.

The fields depend on what makes sense for the package format you're adding support for. For example, when wrapping a binary format like Pex or PyInstaller, you may want a field corresponding to each of the tool's option, like zip_safe and ignore_errors. Often, you will want a field for the entry point.

Usually, you should include OutputPathField from pants.core.goals.package in your target's fields, which will allow the user to change where the package is built to.

See Creating new targets for a guide on how to define new target types.

from pants.core.goals.package import OutputPathField
from pants.engine.target import COMMON_TARGET_FIELDS, Dependencies, Sources, Target

 class BashBinary(Target):
     """A Bash file that may be directly run."""

     alias = "bash_binary"
     core_fields = (*COMMON_TARGET_FIELDS, OutputPathField, Dependencies, BashBinarySources)


Binary targets and the sources field

We've found that it often works best for targets used by the package goal to not have a sources field. Instead, use a "library" target to describe the source code, and add the library as a dependency of the binary target. For example, a pex_binary target may depend on some python_library targets.

Why do we recommend not having a sources field? It can be helpful with modeling to have a clear separation between targets describing first-party code vs. artifacts you want to build. For example, this allows you to use a default value for the sources field of your library target without worrying that a user unintentionally set their binary's sources to overlap with the library's (things like dependency inference do not work as well when >1 target refer to the same source file.)

However, sometimes it does make sense to have a sources field, such as a dockerfile target type. Likewise, this guide uses a sources field for simplicity.

Warning: If you do have a sources field, set expected_num_files to 1 or range(0, 2). Because Pants operates on a file-level, it would try to create one distinct package for each source file belonging to your target, even though you probably only wanted a single package built.

2. Set up a subclass of PackageFieldSet

As described in Rules and the Target API, a FieldSet is a way to tell Pants which Fields you care about targets having for your plugin to work.

Create a new dataclass that subclasses PackageFieldSet. Set the class property required_fields to the fields your target must have registered to work. Usually, this is a field like BashBinarySources or BashBinaryEntryPoint.

from dataclasses import dataclass

from pants.core.goals.package import OutputPathField, PackageFieldSet

class BashBinaryFieldSet(PackageFieldSet):
    required_fields = (BashBinarySources,)

    sources: BashBinarySources
    output_path: OutputPathField

Then, register your new PackageFieldSet with a UnionRule so that Pants knows your binary implementation exists:

from pants.engine.rules import collect_rules
from pants.engine.unions import UnionRule


def rules():
    return [
        UnionRule(PackageFieldSet, BashBinaryFieldSet),

3. Create a rule for your logic

Your rule should take as a parameter the PackageFieldSet from Step 2. It should return BuiltPackage, which has the fields digest: Digest and artifacts: Tuple[BuiltPackageArtifact, ...], where each BuiltPackageArtifact has the field relpath: str and optional extra_log_lines: Tuple[str, ...].

Your package rule can have whatever logic you'd like to create a package. All that Pants cares about is that you return a valid BuiltPackage object.

In this example, we simply create a .zip file with the bash_binary and all of its dependencies.

from dataclasses import dataclass

from pants.core.goals.package import (
from pants.core.util_rules.source_files import SourceFiles, SourceFilesRequest
from pants.engine.addresses import Addresses
from pants.engine.process import BinaryPathRequest, BinaryPaths, Process, ProcessResult
from pants.engine.rules import Get, rule
from pants.engine.target import TransitiveTargets
from pants.util.logging import LogLevel

from examples.bash.target_types import BashBinarySources, BashSources


async def package_bash_binary(field_set: BashBinaryFieldSet) -> BuiltPckage:
    zip_program_paths = await Get(
        BinaryPathRequest(binary_name="zip", search_path=["/bin", "/usr/bin"]),
    if not zip_program_paths.first_path:
        raise ValueError(
            "Could not find the `zip` program on `/bin` or `/usr/bin`, so cannot create a package "
            f"for {field_set.address}."

    transitive_targets = await Get(TransitiveTargets, Addresses([field_set.address]))
    sources = await Get(
            for tgt in transitive_targets.closure
            if tgt.has_field(BashSources)

    output_filename = field_set.output_path.value_or_default(
        field_set.address, file_ending="zip"
    result = await Get(
            description=f"Zip {field_set.address} and its dependencies.",
    return BuiltPackage(
        result.output_digest, artifacts=(BuiltPackageArtifact(output_filename),)

Note that we use field_set.output_path.value_or_default to determine the output filename, which will use the output_path field if defined, and will default to an unambiguous value otherwise.

Finally, update your plugin's register.py to activate this file's rules.

from bash import package_binary

def rules():
    return [*package_binary.rules()]

Now, when you run ./pants package ::, Pants should create packages for all your package target types in the --pants-distdir (defaults to dist/).

4. Add tests (optional)

Refer to Testing rules.

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