## 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:
Subclass `
pants.engine.target.Target`.Define the class property `
alias`. This is the symbol that people use in BUILD files.Define the class property `
core_fields`.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`.
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)
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:
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.
You can confirm this works by running `./pants help custom_target`.