demo: pure-soft hello-world example using the @attr decorator

[coretl]

Closes #398

The first rung of the demo ladder, and the only one with no device behind it. Every value lives in the controller object, so this module runs with no simulator, no socket and no external process — which is the point: it shows the declarative spelling on its own, with nothing else to read past.

class HelloWorldController(Controller):
    def __init__(self, subject: str = "world") -> None:
        super().__init__()
        self._greeting = "Hello"
        self._subject = subject
        self._started = time.monotonic()

    @AttrRW.declare
    async def greeting(self) -> str:
        """The word to greet with."""
        return self._greeting

    @greeting.setter
    async def set_greeting(self, value: str) -> None:
        self._greeting = value

    @AttrR.declare(Polled(period=0.2))
    async def message(self) -> str:
        """The greeting as it currently reads."""
        return f"{self._greeting}, {self._subject}!"

    @AttrR.declare(Polled(period=0.2), units="s", precision=1)
    async def uptime(self) -> float:
        """Seconds since the controller was constructed."""
        return time.monotonic() - self._started

Scope

• New src/fastcs/demo/hello_world.py, covering the four things the decorator is: a bare decorated getter as an AttrR, an AttrRW.declare plus @x.setter pair, the docstring becoming the description, and the decorator's positional schedule plus keyword metadata (Polled(period=0.2), units="s", precision=1).
• message recomputes from greeting, so setting one attribute visibly moves another. That is the one thing a soft example can otherwise not show — a value changing for a reason other than you writing it — and it earns the Polled schedule honestly rather than polling a constant.
• tests/demo/test_hello_world.py: access modes, inferred datatypes, docstring descriptions, the ONCE-vs-polled schedules, the metadata, the static types, and that the setter is still callable as an ordinary method.
• The demo README's ladder already had the hello_world.py row; its "baselines vs framework PRs" paragraph said this module was still waiting on the decorator (@attr decorator sugar over getter/setter constructors #397), which merged as attributes: @attr decorator sugar over the getter/setter constructors #423, so that sentence is updated.

Also in this PR: the decorator names the class it builds

Added on review (thread r3944915082 and its follow-up), so that this example is the one we actually want. It is a change to the decorator itself (#397's code, merged as #423) rather than to the demo, and it is what makes the module above read as it does. It landed in two steps, and the second supersedes the first:

1. The setter got a name of its own, as PyTango's write_voltage does for a voltage attribute, rather than redeclaring the getter's name the way @property does. That removed the # pyright: ignore[reportRedeclaration] attributes: @attr decorator sugar over the getter/setter constructors #423 documented as its one wart, but left a second cost: a type checker binds the name at the decorator line and nothing later in the class body can change it, so self.greeting read as AttrR[str] even though it was an AttrRW[str] at runtime, and .set() needed assert isinstance(...) narrowing.

2. @attr is replaced by AttrR.declare/AttrRW.declare — an alternate constructor on the class the decorator builds, which is where @coretl and @shihab-dls landed on the thread. The type is now in the declaration rather than inferred from what follows it, so both costs are gone:

   • AttrR.declare is read-only and carries no setter at all — @serial.setter on one is an AttributeError, not a quiet promotion to read-write.
   • AttrRW.declare expects a setter, and a declaration that never gets one raises when the controller is constructed, naming the attribute.
   • self.greeting is an AttrRW[str] to pyright as well as at runtime. tests/demo/test_hello_world.py pins that with assert_type, so the type-checking job fails if it drifts.
   • set_greeting stays an ordinary bound method, so await controller.set_greeting("Goodbye") writes to the device directly while await controller.greeting.set("Goodbye") writes through the attribute and updates what clients see.
   • There is no # pyright: ignore left in the repo for either half of a read-write attribute.

   @x.setter still replaces the getter's declaration in the declaring class's own namespace, so @Base.voltage.setter in a subclass leaves Base without one (unchanged behaviour, same test).

   ADR 0018 gains an amendment recording the spelling and why it changed, since its resolved question 1 writes @attr + @x.setter and a reader would otherwise be given a spelling that no longer exists. ADRs 0013 and 0014 mention @attr in passing and are left alone as records of their own moment.

docs/how-to/fastcs-for-pytango-users.md is updated throughout: the PyTango pairing reads as voltage/write_voltage, and the note about narrowing is gone rather than reworded, because there is nothing left to narrow.

Instructions to reviewer on how to test:

1. uv run pytest tests/demo/test_hello_world.py tests/test_attr_decorator.py -v — nothing external needed.
2. uv run --locked tox -e type-checking — the assert_type calls in the demo tests are what prove the static type.
3. uv run python -c "import asyncio; from fastcs.demo.hello_world import HelloWorldController as C; c = C(); print(asyncio.run(c.message.poll()))"

Checks for reviewer

• Would the PR title make sense to a user on a set of release notes
• attr is removed rather than kept as an alias. Two spellings for one thing is what the review exchange was trying to avoid, and it is pre-1.0 with the decorator only days old (attributes: @attr decorator sugar over the getter/setter constructors #423), so everything in the repo is migrated instead. Say if you would rather attr stayed as a deprecated alias for a release.
• A read-only declaration can no longer be promoted by a subclass. AttrR.declare has no setter, which is what "@AttrR does not expect a setter" asks for — but it means a base that is read-only and instantiable, with a subclass adding writing, is no longer expressible. The subclass-namespace test now declares AttrRW.declare on the base and supplies the setter in the child, which still proves the thing it is for. Say if you want the promotion back.
• AttrRW.declare with no setter raises rather than binding a soft attribute. The procedural AttrRW(str) with no setter is legitimate — set() pushes straight to the readback — so this could have been allowed. A decorated getter is device IO by construction, though, so a silent soft round-trip is almost certainly a forgotten setter. Say if you would rather it were permitted.
• Three attributes, where the issue says "one or two". greeting alone would not show the @AttrR.declare(Polled(...), units=...) form, which is half the decorator's surface and the half a reader will reach for first. message and uptime are two lines each.
• The class is HelloWorldController, not HelloWorld. It matches TemperatureController/EigerDetector in reading as a controller, but the issue text says "hello world"; trivial to rename.
• uptime is a soft value that moves on its own (time.monotonic()). It is the only thing here with no user-visible cause, which is what makes it a fair demonstration of polling — but it is also the one attribute whose value is not reproducible, so its test only asserts it does not go backwards.

Notes

• The two per-instance tests (test_each_instance_gets_its_own_attributes, test_setting_one_instance_leaves_the_other_alone) are removed on review, as controllers: ControllerFiller, and one declarative mechanism for attributes #426 covers that generally.
• Overlaps controllers: ControllerFiller, and one declarative mechanism for attributes #426, which uses attr in tests/test_controller_filler.py and reads UnboundAttr in base_controller._bind_attrs. Both are independent branches off refactor; whichever merges second needs those decorators renamed, which is mechanical.
• Not added to src/fastcs/demo/fastcs.yaml. That file launches the temperature controllers over GraphQL and EPICS CA on fixed ports, and a soft example does not need a transport to be the thing it demonstrates; adding it would also churn the checked-in schema.json. Say if you would like it launchable.
• No literalinclude region markers yet — per the demo README those are added with each module's tutorial in the docs pass (DOCS: Rewrite tutorials around the examples/ controllers (one per example, single-sourced) #408).
• declare is a staticmethod with its own Declared_T TypeVar, which sits in datatypes/types.py next to Inferred_T: a class-scoped TypeVar leaves pyright resolving the decorator to AttrR[Unknown].
• Verified locally with uv run --locked tox -e pre-commit,type-checking, both green in full. As on demo: use ControllerVector for temperature ramp sub-controllers #409/demo: cut-down Eiger REST sim + introspectable controller example #410/demo: convert temperature controller to getter/setter style #411/attributes: getter/setter IO rework, remove AttributeIORef/AttributeIO #412/attributes: replace the DataType family with python types and *Meta typed dicts #418/methods: typed commands — positional arguments and a return value #419/controllers: ControllerRunner, plus native timestamps and severity on attributes #420/attributes: @attr decorator sugar over the getter/setter constructors #423, this sandbox cannot run docs (needs outbound network) or the PVA/p4p-backed tests (RuntimeError: Address family not supported by protocol). Excluding those, pytest src tests --ignore=tests/benchmarking passes 451, with only the same 10 pre-existing p4p/socket-family failures. Real CI covers docs and PVA. mypy is not in this repo's dev dependencies or tox envs, so only pyright has been run.

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Summary by CodeRabbit

• New Features

  • Added declarative AttrR.declare and AttrRW.declare APIs for read-only and read/write attributes.
  • Added validation for missing setters and support for schedules and metadata.
  • Added a Hello World controller demo showcasing declarative attributes and polling.
  • Exposed the Declared_T datatype for public use.

• Documentation

  • Updated guides, examples, and design documentation to use the new declaration API.
  • Clarified read-only, read/write, write-only, and setter requirements.

• Breaking Changes

  • Removed the previous @attr decorator API.

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📝 Walkthrough

Walkthrough

The pull request replaces @attr with AttrR.declare and AttrRW.declare, adds class-level setter binding, updates exports and typing, and introduces a tested soft-value HelloWorldController example with corresponding documentation.

Changes

Declarative attribute API

Layer / File(s)	Summary
Declaration contracts and typing src/fastcs/attributes/attr_r.py, src/fastcs/attributes/attr_rw.py, src/fastcs/attributes/attr_w.py, src/fastcs/datatypes/*, src/fastcs/attributes/__init__.py	Adds typed AttrR.declare and AttrRW.declare overloads, getter/setter aliases, Declared_T, and updated package exports.
Declaration binding and setter installation src/fastcs/attributes/attr_decorator.py, src/fastcs/controllers/base_controller.py	Replaces the attr() factory with declaration helpers. Adds AttrSetter, class-level setter installation, declaration names, and errors for missing or invalid setters.
Soft controller example and validation src/fastcs/demo/hello_world.py, tests/demo/test_hello_world.py, tests/test_attr_decorator.py	Adds HelloWorldController and validates declaration types, metadata, polling, setter behavior, type inference, and declaration errors.
API documentation and examples docs/explanations/decisions/0018-attr-decorator-sugar.md, docs/how-to/fastcs-for-pytango-users.md, src/fastcs/demo/README.md, docs/conf.py	Updates examples and design documentation for the declarative API and configures Sphinx to ignore nested UnboundGetter references.

Estimated code review effort: 4 (Complex) | ~45 minutes

Merge Risk: 🟡 Moderate · up to f46ef

Documentation can direct users to an unavailable decorator, and valid read-write array declarations may fail during controller class creation. Resolve these before merge.

Sequence Diagram(s)

sequenceDiagram
  participant ControllerClass
  participant AttrRWDeclare
  participant AttrSetter
  participant BaseController
  participant HelloWorldController
  ControllerClass->>AttrRWDeclare: declare greeting getter
  AttrRWDeclare->>AttrSetter: attach set_greeting
  AttrSetter->>ControllerClass: install read-write declaration
  BaseController->>HelloWorldController: bind declared attributes
  HelloWorldController-->>BaseController: provide greeting, message, and uptime

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Suggested reviewers: shihab-dls

🚥 Pre-merge checks | ✅ 3 | ❌ 2

❌ Failed checks (2 warnings)

Check name	Status	Explanation	Resolution
Title check	⚠️ Warning	The title correctly identifies the pure-soft hello-world demo, but it says the demo uses the removed @attr decorator. The implementation uses AttrR.declare and AttrRW.declare instead.	Update the title to describe the actual API, for example: "demo: pure-soft hello-world example using declarative attributes".
Docstring Coverage	⚠️ Warning	Docstring coverage is 15.65% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 115 functions across 12 files. (3 skipped…	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Linked Issues check	✅ Passed	The pull request adds src/fastcs/demo/hello_world.py with pure-soft declarative attributes and adds unit tests, satisfying the coding objectives for issue #398. The implementation uses the updated Att…
Out of Scope Changes check	✅ Passed	The API migration, documentation updates, typing changes, and related tests support the stated objective of replacing @attr with AttrR.declare and AttrRW.declare. No unrelated code changes are identif…

Full details: Docstring Coverage

Explanation

Docstring coverage is 15.65% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 115 functions across 12 files. (3 skipped: 3 unsupported.)

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[codecov]

Codecov Report

❌ Patch coverage is 96.29630% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.64%. Comparing base (e73453b) to head (02665b7).
⚠️ Report is 6 commits behind head on refactor.

Files with missing lines	Patch %	Lines
src/fastcs/attributes/attr_decorator.py	94.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           refactor     #425      +/-   ##
============================================
+ Coverage     91.25%   92.64%   +1.38%     
============================================
  Files            72       70       -2     
  Lines          2892     3343     +451     
============================================
+ Hits           2639     3097     +458     
+ Misses          253      246       -7     

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[shihab-dls]

The example has brought up a design choice we must change regarding @attr setters, so this must be changed in this PR, such that the example is correct.

[shihab-dls]

Some changes required on tests

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Actionable comments posted: 2

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Inline comments:
In `@docs/explanations/decisions/0018-attr-decorator-sugar.md`:
- Around line 160-163: Update the ADR’s earlier `@attr` decision and examples to
explicitly mark that spelling as superseded, while preserving the historical
text and documenting the replacement spelling so the ADR no longer presents
`@attr` as an available decorator.

In `@src/fastcs/attributes/attr_decorator.py`:
- Line 300: Update UnboundAttrRW.setter() to compare the datatype returned by
_datatype_for_annotation(value.annotation) with self._datatype using equality
rather than identity, and add a regression test covering matching
Array1D[np.int32] read-write annotations during class definition.

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📥 Commits

Reviewing files that changed from the base of the PR and between fc74689 and f46effb.

📒 Files selected for processing (15)

• docs/conf.py
• docs/explanations/decisions/0018-attr-decorator-sugar.md
• docs/how-to/fastcs-for-pytango-users.md
• src/fastcs/attributes/__init__.py
• src/fastcs/attributes/attr_decorator.py
• src/fastcs/attributes/attr_r.py
• src/fastcs/attributes/attr_rw.py
• src/fastcs/attributes/attr_w.py
• src/fastcs/controllers/base_controller.py
• src/fastcs/datatypes/__init__.py
• src/fastcs/datatypes/types.py
• src/fastcs/demo/README.md
• src/fastcs/demo/hello_world.py
• tests/demo/test_hello_world.py
• tests/test_attr_decorator.py

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