hjtran opened a new pull request, #35164:
URL: https://github.com/apache/beam/pull/35164
The following pipeline currently throws an error:
```python
with beam.Pipeline() as pipeline:
letters = (
pipeline
| beam.Create([['abc']], reshuffle=False)
| beam.FlatMap()
| beam.Map(lambda s: s.upper()).with_input_types(str))
```
with error: (roughly)
```
TypeCheckError: The transform '[4]: Map(<lambda at
<ipython-input-4-04b6d203d131>:6>)' requires PCollections of type '<class
'str'>' but was applied to a PCollection of type 'List[<class 'str'>]'
(produced by the transform '[4]: FlatMap(identity)').
```
This can be fixed by just updating the `FlatMap` identity function to just
have no type hints, which allows trivial inference to correctly determine the
right pcollection types.
Before realizing that solution, I ended up fixing a few type hinting
functions that strip iterables so that they handle generic `TypeVariable`s
better (the yielded type of `TypeVar('T')` should not be `TypeVar('T')` for
example).
Those changes aren't actually necessary for this fix, but they seem like a
bug in the typehinting logic anyways so I've left them in.
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