hjtran opened a new issue, #34732:
URL: https://github.com/apache/beam/issues/34732
### What happened?
If you use a TypeVar for type hints in the callable passed to FlatMap, it
sometimes will not infer the correct output type hint. Example:
```python
@typehints.with_input_types(str)
class FooTransform(beam.PTransform):
def expand(self, pcoll):
return pcoll
T = typing.TypeVar('T')
def list_str_identity(s: T) -> T:
return s
with beam.Pipeline() as p:
(p | beam.Create([['hello world']])
| beam.FlatMap(list_str_identity)
| FooTransform()
| beam.LogElements())
```
This should log "hello world", but instead results in the following error:
```
utput: Traceback (most recent call last):
File
"/opt/playground/backend/executable_files/99dcf1c5-5131-46fe-b66a-f572a0d45ded/99dcf1c5-5131-46fe-b66a-f572a0d45ded.py",
line 44, in <module>
(p | beam.Create([['hello world']])
File "/usr/local/lib/python3.10/site-packages/apache_beam/pvalue.py", line
138, in __or__
return self.pipeline.apply(ptransform, self)
File "/usr/local/lib/python3.10/site-packages/apache_beam/pipeline.py",
line 781, in apply
transform.type_check_inputs(pvalueish)
File
"/usr/local/lib/python3.10/site-packages/apache_beam/transforms/ptransform.py",
line 465, in type_check_inputs
self.type_check_inputs_or_outputs(pvalueish, 'input')
File
"/usr/local/lib/python3.10/site-packages/apache_beam/transforms/ptransform.py",
line 494, in type_check_inputs_or_outputs
raise TypeCheckError(
apache_beam.typehints.decorators.TypeCheckError: Input type hint violation
at FooTransform: expected <class 'str'>, got List[<class 'str'>]
```
The default callable for `FlatMap` is an identity function typed with `T =
TypeVar("T")`, which means this incorrect type hint comes up a lot
### Issue Priority
Priority: 2 (default / most bugs should be filed as P2)
### Issue Components
- [ ] Component: Python SDK
- [ ] Component: Java SDK
- [ ] Component: Go SDK
- [ ] Component: Typescript SDK
- [ ] Component: IO connector
- [ ] Component: Beam YAML
- [ ] Component: Beam examples
- [ ] Component: Beam playground
- [ ] Component: Beam katas
- [ ] Component: Website
- [ ] Component: Infrastructure
- [ ] Component: Spark Runner
- [ ] Component: Flink Runner
- [ ] Component: Samza Runner
- [ ] Component: Twister2 Runner
- [ ] Component: Hazelcast Jet Runner
- [ ] Component: Google Cloud Dataflow Runner
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