Extending existing targets

Adding new fields to target types.

When to add new fields?

Adding new fields is useful when you are already using a target type, but need to store additional metadata for your plugin.

For example, if you're writing a codegen plugin to convert a protobuf_library into Java source files, you may want to add a jdk_version field to protobuf_library.

If you are instead adding support for a new language, create a new target type.

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

How to add new fields

First, define the field. Then, register it by using OriginalTarget.register_plugin_field(CustomField), like this:

from pants.backend.codegen.protobuf.target_types import ProtobufLibrary
from pants.engine.target import BoolField

class ProtobufJdkVersion(IntField):
    alias = "jdk_version"
    default = 11
    help = "Which JDK protobuf should target."

def rules():
    return [ProtobufLibrary.register_plugin_field(ProtobufJdkVersion)]

To confirm this worked, run ./pants help protobuf_library, and you should see your new field.

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