#20925 `__defaults__(all=...)` is a future-compatibility hazard for adding new field Issue created by huonw Describe the bug The

all

functionality for

__default__

is somewhat of a future-compatibility hazard: 1. In a code base using Pants 2.34, there's a target

t1

with a field called

f

(this could be in any backend, even a private in-repo one), and a second built-in target called

t2

without that field. 2. Someone has an existing BUILD file like:

__defaults__(all=dict(f="abc"))
    t1(name="one") # after applying __defaults__: t1(name="one", f="abc")
    t2(name="two") # after applying __defaults__: t2(name="two")

3. In Pants 2.35, we now add a new field to

t2

also called

f

. 4. When the code above upgrades to Pants 2.35, the

t2

target will now become

t2(name="two", f="abc")

after applying

__defaults__

, and this could either: 1. silently change behaviour 2. fail with confusing error messages (if

abc

doesn't make sense for `t2`'s

f

field) With Pants' current behaviour, if we wanted to be strict about not breaking existing code, this means we could never add new fields, because someone could have an existing field of the same name, and use it in

__defaults__(all=...)

. Not ever adding new fields is clearly untenable, so either we have to: 1. allow breaking existing user code (but we could do things like improve diagnostics, or have a convention of calling it out in release notes or something) 2. change how

all

works (e.g. deprecate and remove it(!), have some notion of "all fields as of a particular release", so people need to bump that ) I think we should make an explicit policy decision (even if it's just "users just have to tolerate any breakage due to `all`"). * * * For instance, in #20894 (comment), a user had:

__defaults__(
    extend=True,
    all=dict(
        # This will set PYTHON_IMAGE build arg in all docker images by default.
        extra_build_args=["PYTHON_IMAGE=python:3.11"],
        # This will tell pants that all python sources need to use python 3.11 by default.
        interpreter_constraints=["==3.11.*"],
    ),
)

In 2.20 and earlier (before #20737), the only built-in target with the

extra_build_args

field was

docker_image

, and so this

all

was applying that docker-formatted build arg to all. In 2.21 (which includes #20737),

pex_binary

also has this field. It has completely different meaning (a list of CLI args to pass to a

pex

invocation). Thus, that

__default__

configuration starts setting

pex_binary(..., extra_build_args=["PYTHON_IMAGE=python:3.11"])

and understandably an invocation like

pex ... PYTHON_IMAGE=python:3.11

doesn't work. The fix the user identified is to move the field from

all

to `docker_image`:

__defaults__(
    {
        (docker_image,): dict(
            # This will set PYTHON_IMAGE build arg in all docker images by default.
            extra_build_args=["PYTHON_IMAGE=python:3.11"],
        )
    },
    all=dict(
        # This will tell pants that all python sources need to use python 3.11 by default.
        interpreter_constraints=["==3.11.*"],
    ),
    extend=True,
)

Pants version All, although specific example is in 2.21 OS All Additional info N/A pantsbuild/pants