Repository: python-attrs/attrs (python)

Issue:
Creating a Subclassed instance now errors with `got an unexpected keyword argument`
We just noticed this in Airflow CI on upgrading to 25.2.0.

The error is

```
ERROR    airflow.models.dagbag.DagBag:dagbag.py:394 Failed to import: /opt/airflow/airflow/example_dags/example_dynamic_task_mapping.py
Traceback (most recent call last):
  File "/opt/airflow/airflow/models/dagbag.py", line 384, in parse
    loader.exec_module(new_module)
  File "<frozen importlib._bootstrap_external>", line 999, in exec_module
  File "<frozen importlib._bootstrap>", line 488, in _call_with_frames_removed
  File "/opt/airflow/airflow/example_dags/example_dynamic_task_mapping.py", line 27, in <module>
    with DAG(dag_id="example_dynamic_task_mapping", schedule=None, start_date=datetime(2022, 3, 4)) as dag:
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: DAG.__init__() got an unexpected keyword argument 'dag_id'
```

(Yes, I know your view on subclassing. I don't disagree either)

Minimal repro case:

```python
import attrs

def _all_after_dag_id_to_kw_only(cls, fields: list[attrs.Attribute]):
    i = iter(fields)
    f = next(i)
    if f.name != "dag_id":
        raise RuntimeError("dag_id was not the first field")
    yield f

    for f in i:
        yield f.evolve(kw_only=True)


@attrs.define(field_transformer=_all_after_dag_id_to_kw_only)
class Base:
    dag_id: str
    has_default: str = "default"


@attrs.define()
class Subclass(Base):
    other: bool


x = Subclass(dag_id="foo", other=False)

print(repr(x))
print(attrs.__version__)
```

Pull request description:
Cope with `field_transformer` being a generator
This worked on 25.1.0 but was inadvertently broken by #1401

Fixes #1416

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- [x] Added **tests** for changed code.
  Our CI fails if coverage is not 100%.
- [ ] New features have been added to our [Hypothesis testing strategy](https://github.com/python-attrs/attrs/blob/main/tests/strategies.py).
- [ ] Changes or additions to public APIs are reflected in our type stubs (files ending in ``.pyi``).
    - [ ] ...and used in the stub test file `tests/typing_example.py`.
    - [ ] If they've been added to `attr/__init__.pyi`, they've *also* been re-imported in `attrs/__init__.pyi`.
- [ ] Updated **documentation** for changed code.
    - [ ] New functions/classes have to be added to `docs/api.rst` by hand.
    - [ ] Changes to the signatures of `@attr.s()` and `@attrs.define()` have to be added by hand too.
    - [ ] Changed/added classes/methods/functions have appropriate `versionadded`, `versionchanged`, or `deprecated` [directives](http://www.sphinx-doc.org/en/stable/markup/para.html#directive-versionadded).
          The next version is the second number in the current release + 1.
          The first number represents the current year.
          So if the current version on PyPI is 22.2.0, the next version is gonna be 22.3.0.
          If the next version is the first in the new year, it'll be 23.1.0.
      - [ ] If something changed that affects both `attrs.define()` and `attr.s()`, you have to add version directives to both.
- [ ] Documentation in `.rst` and `.md` files is written using [semantic newlines](https://rhodesmill.org/brandon/2012/one-sentence-per-line/).
- [ ] Changes (and possible deprecations) have news fragments in [`changelog.d`](https://github.com/python-attrs/attrs/blob/main/changelog.d).
- [x] Consider granting [push permissions to the PR branch](https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/allowing-changes-to-a-pull-request-branch-created-from-a-fork), so maintainers can fix minor issues themselves without pestering you.

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Patch:
diff --git a/src/attr/_make.py b/src/attr/_make.py
index 7439dc2..e84d979 100644
--- a/src/attr/_make.py
+++ b/src/attr/_make.py
@@ -448,7 +448,7 @@ def _transform_attrs(
     attrs = base_attrs + own_attrs
 
     if field_transformer is not None:
-        attrs = field_transformer(cls, attrs)
+        attrs = tuple(field_transformer(cls, attrs))
 
     # Check attr order after executing the field_transformer.
     # Mandatory vs non-mandatory attr order only matters when they are part of


Test patch:
diff --git a/tests/test_hooks.py b/tests/test_hooks.py
index 6b34206..4dcd352 100644
--- a/tests/test_hooks.py
+++ b/tests/test_hooks.py
@@ -217,6 +217,22 @@ class TestTransformHook:
         assert "CAttributes" == fields_type.__name__
         assert issubclass(fields_type, tuple)
 
+    def test_hook_generator(self):
+        """
+        Ensure that `attrs.fields` are correctly recorded when field_transformer is a generator
+
+        Regression test for #1417
+        """
+
+        def hook(cls, attribs):
+            yield from attribs
+
+        @attr.s(auto_attribs=True, field_transformer=hook)
+        class Base:
+            x: int
+
+        assert ["x"] == [a.name for a in attr.fields(Base)]
+
 
 class TestAsDictHook:
     def test_asdict(self):