Creating new targets

When to create a new target type?

Adding new target types is most helpful when you are adding support for a new language.

If you instead want to reduce boilerplate in BUILD files, such as changing default values, use macros .

If you are already using a target type, but need to store additional metadata for your plugin, add a new field to the target type.

Step 1: Define the target type

To define a new target:

1. Subclass pants.engine.target.Target.
2. Define the class property alias. This is the symbol that people use in BUILD files.
3. Define the class property core_fields.
4. Define the class property help. This is used by ./pants help.

For core_fields, we recommend including COMMON_TARGET_FIELDS to add the useful tags and description fields. You will also often want to add Dependencies, and either SingleSourceField or MultipleSourcesField.

from pants.engine.target import (
    COMMON_TARGET_FIELDS,
    Dependencies,
    SingleSourceField,
    StringField,
    Target,
)


class CustomField(StringField):
    alias = "custom_field"
    help = "A custom field."


class CustomTarget(Target):
    alias = "custom_target"
    core_fields = (*COMMON_TARGET_FIELDS, Dependencies, SingleSourceField, CustomField)
    help = (
      "A custom target to demo the Target API.\n\n"
      "This docstring will be used in the output of "
      "`./pants help $target_type`."
    )

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Tip: subclass SingleSourceField or MultipleSourcesField

Use SingleSourceField for source: str and MultipleSourcesField for sources: Iterable[str].

You will often want to subclass either of these fields to give custom functionality:

• set the default
• set expected_file_extensions, e.g. to (".json", ".txt")
• set expected_num_files, e.g. to 1 or range(0, 5) (i.e. 0 to 4 files)

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Using the fields of an existing target type

Sometimes, you may want to create a new target type that behaves similarly to one that already exists, except for some small changes.

For example, you might like how pex_binary behaves in general, but you have a Django application and keep writing entry_point="manage.py". Normally, you should write a macro to set this default value; but, here, you also want to add new Django-specific fields, so you decide to create a new target type.

Rather than subclassing the original target type, use this pattern:

Python

from pants.backend.python.target_types import PexBinaryTarget, PexEntryPointField
from pants.engine.target import Target
from pants.util.ordered_set import FrozenOrderedSet

class DjangoEntryPointField(PexEntryPointField):
   default = "manage.py"


class DjangoManagePyTarget(Target):
   alias = "django_manage_py"
   core_fields = (
       *(FrozenOrderedSet(PexBinaryTarget.core_fields) - {PexEntryPoint}),
       DjangoEntryPointField,
   )

In this example, we register all of the fields of PexBinaryTarget, except for the field PexEntryPoint . We instead register our custom field DjangoEntryPointField .

Step 2: Register the target type in register.py

Now, in your register.py, add the target type to the def target_types() entry point.

from plugins.target_types import CustomTarget

def target_types():
    return [CustomTarget]

You can confirm this works by running ./pants help custom_target.