Is there a way to set a goal option or a target field in a plugin dynamically?

You can define functions in BUILD files that generate output based on the

env

object. (

env("PLATFORM")

,

env("FOO_BAR")

), but you need to set it before running

pants

I need a different value for every target when the goal is executed

So you can define it for every target

target(name="foo", option=make_option_foo())
target(name="bar", option=make_option_bar())

If you want to make multiple targets you can do it based on env:

for val in env('FOO').split(' '):
  somehow_make_target(val)

well, it's not scalable, I don't want to create a new env var every time I create a new target

Yeah, but it's the only way I found to do any kind of dispatching. That's why I made a question if there is alternative.

I want to be able to register a new field on a target and then use this field in an existing rule which is already in a pants backend

I suggest you write your own plugin to do something similar to what Bazel does with

config_setting

I'm asking exactly that, is there an API for a plugin to set existing fields to some value

I wouldn't mutate fields, I would do it like Bazel:

target(name="foo", option=select(conditionA="this", conditionB="and that"))

I want to be able to set the value of a field from a plugin (not a BUILD file), it can depend on something else, like other fields of the target

I don't think that can be done, because build evaluation will become non-detereministic and as such non-cacheable. Imagine you could dynamically change

dependencies

field based on output of other targets. The best you can do, if you really need this functionality, is generating BUILD files using double-pass builds (first pass is generation based on Pants itself or some other scripts).

Well, InferDependenciesRequest does exactly that 😄 and it is deterministic because it doesn't allow you to change dependencies based on other dependencies