[Feature Request]: Support generic PTransforms in Python SDK

[mattalbr]

What would you like to happen?

It would be awesome for the following code to work (written in 3.12 syntax, but would be great in <=3.11 syntax too)

T = TypeVar("T")

@dataclass.dataclass()
class NeatOutputClass[T]:
  data: T

class ReusableTransform[T](beam.PTransform):
  def __init__(self, value_extractor: Callable[[Any], T):
    # This is a bit contrived, but not unreasonable for a transform acting
    # on slightly different data. Could be done as a generic DoFn too.
    self.value_extractor = value_extractor

  def expand(self, pcoll: beam.PCollection) -> NeatOutputClass[T]:
    # Overly simplified for simple reproducibility.
    return pcoll | beam.Map(lambda x: self.value_extractor(x))

But unfortunately this yields TypeError: Subscripted generics cannot be used with class and instance checks

Full Stack Trace

/home/vscode/.cache/pants/named_caches/pex_root/venvs/s/192fb705/venv/lib/python3.11/site-packages/apache_beam/transforms/ptransform.py:622: in __ror__
    result = p.apply(self, pvalueish, label)
/home/vscode/.cache/pants/named_caches/pex_root/venvs/s/192fb705/venv/lib/python3.11/site-packages/apache_beam/pipeline.py:754: in apply
    pvalueish_result = self.runner.apply(transform, pvalueish, self._options)
/home/vscode/.cache/pants/named_caches/pex_root/venvs/s/192fb705/venv/lib/python3.11/site-packages/apache_beam/runners/runner.py:191: in apply
    return self.apply_PTransform(transform, input, options)
/home/vscode/.cache/pants/named_caches/pex_root/venvs/s/192fb705/venv/lib/python3.11/site-packages/apache_beam/runners/runner.py:195: in apply_PTransform
    return transform.expand(input)
my_repo/transformation.py:263: in expand
    pcolls
/home/vscode/.cache/pants/named_caches/pex_root/venvs/s/192fb705/venv/lib/python3.11/site-packages/apache_beam/transforms/ptransform.py:622: in __ror__
    result = p.apply(self, pvalueish, label)
/home/vscode/.cache/pants/named_caches/pex_root/venvs/s/192fb705/venv/lib/python3.11/site-packages/apache_beam/pipeline.py:757: in apply
    transform.type_check_outputs(pvalueish_result)
/home/vscode/.cache/pants/named_caches/pex_root/venvs/s/192fb705/venv/lib/python3.11/site-packages/apache_beam/transforms/ptransform.py:475: in type_check_outputs
    self.type_check_inputs_or_outputs(pvalueish, 'output')
/home/vscode/.cache/pants/named_caches/pex_root/venvs/s/192fb705/venv/lib/python3.11/site-packages/apache_beam/transforms/ptransform.py:496: in type_check_inputs_or_outputs
    if hint and not typehints.is_consistent_with(pvalue_.element_type, hint):
/home/vscode/.cache/pants/named_caches/pex_root/venvs/s/192fb705/venv/lib/python3.11/site-packages/apache_beam/typehints/typehints.py:1323: in is_consistent_with
    return base._consistent_with_check_(sub)
/home/vscode/.cache/pants/named_caches/pex_root/venvs/s/192fb705/venv/lib/python3.11/site-packages/apache_beam/typehints/typehints.py:711: in _consistent_with_check_
    len(sub.tuple_types) == len(self.tuple_types) and all(
/home/vscode/.cache/pants/named_caches/pex_root/venvs/s/192fb705/venv/lib/python3.11/site-packages/apache_beam/typehints/typehints.py:712: in <genexpr>
    is_consistent_with(sub_elem, elem) for sub_elem,
/home/vscode/.cache/pants/named_caches/pex_root/venvs/s/192fb705/venv/lib/python3.11/site-packages/apache_beam/typehints/typehints.py:1323: in is_consistent_with
    return base._consistent_with_check_(sub)
/home/vscode/.cache/pants/named_caches/pex_root/venvs/s/192fb705/venv/lib/python3.11/site-packages/apache_beam/typehints/typehints.py:271: in _consistent_with_check_
    is_consistent_with(sub.inner_type, self.inner_type))
/home/vscode/.cache/pants/named_caches/pex_root/venvs/s/192fb705/venv/lib/python3.11/site-packages/apache_beam/typehints/typehints.py:1329: in is_consistent_with
    return issubclass(sub, base)

Issue Priority

Priority: 2 (default / most feature requests should be filed as P2)

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[jrmccluskey]

I believe this runs into a similar problem as #33189, the output of NeatOutputClass[T] doesn't play nice with our type hinting infrastructure. I'm not sure I've ever seen someone try to do a fully generic PTransform definition before, you may be able to get away with a similar solution as outlined in that issue - #33189 (comment)

The .with_output_types() call to parameterize NeatOutputClass in some way might work, although I'm not 100% sure on if it would be happy.

[mattalbr]

From my experimentation, it seems like the workaround in #33189 requires the DoFn or PTransform to only operate on a concrete instance of a generic type, but doesn't work for a fully generic DoFn or PTransform (e.g. if a DoFn needs to accept a fully generic type).

It would be cool to support fully generic PTransforms. FWIW, here's my use case:

I'm building out a pipeline that processes data from various historical tables and basically groups historical values by key and computes intervals of values across time for each key.

The logic there to handle the intervalization is complex enough that I don't want to repeat it, so I have my PTransform that computes the intervals accept Callables to go from each history row to a key+value pair. Then the output is:

tuple[K, list[Interval[V]]]

Without support for generics, I need to throw a ton of Anys in here and slowly but surely chip away at the value of the typechecking. With generics I can say:

K = TypeVar("K")
V = TypeVar("V")
R = TypeVar("R")

@dataclasses.dataclass
class Interval[V]:
  start: datetime.datetime
  end: datetime.datetime
  value: V | None

class Intervalizer[K, V, R](beam.PTransform):
  def __init__(self, key_fn: Callable[[R], K], value_fn: Callable[[R], V]):
    self._key_fn = key_fn
    self._value_fn = value_fn

  def process(self, pcoll) -> list[tuple[K, Interval[V]]]:
    # Fancy implementation.
    return pcoll

class FooIntervalizer(beam.PTransform):
  def process(self, history_rows_pcoll) -> list[tuple[int, Interval[str]]]:
    return history_rows_pcoll | Intervalizer[int, str](key_fn=lambda x: x.id, value_fn=lambda x: x.val)
    

[github-actions]

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[mattalbr]

Would love to keep this open for consideration in 3.0

[Abacn]

This Issue is marked as Beam 3 - Milestone 1: No Code and ML focus. Please help us burning down the open blocker tasks.

• If this issue is solved, close it. If it is obsolete, close as not planned. If it is no longer necessary for Beam 3, move it out of the Milestone.

• If the work is pending, please be reminded that Milestone 1 is targeted for completion by Dec 30, 2025

• If this issue involves multiple milestones (e.g. remove a public API should first mark it as deprecated, scoped in Milestone 1; then finally remove it in Beam 3 release candidate, scoped in Milestone 3) and Milestone 1 portion has completed, consider move the Issue to Milestone 3.

cc: @jrmccluskey @mattalbr any plan to support this?