Sandy Chapman created BEAM-13439:
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Summary: Correct type signature on ptransform_fn to better support
typecheckers
Key: BEAM-13439
URL: https://issues.apache.org/jira/browse/BEAM-13439
Project: Beam
Issue Type: Improvement
Components: sdk-py-core
Affects Versions: 2.34.0
Reporter: Sandy Chapman
Because the return type of the ptransform_fn is not specified, [certain
typecheckers|https://github.com/microsoft/pyright/issues/2688#issuecomment-990238720]
will assume the type signature remains unchanged. However, ptransform_fn
specifically adjusts the type signature such that the function no longer takes
a pcoll.
This can be corrected by changing the the definition of ptransform_fn from:
{code:java}
def ptransform_fn(fn):
{code}
to:
{code:java}
def ptransform_fn(fn) -> Callable[..., _PTransformFnPTransform]: {code}
This allows the typechecker to understand the return type of a function
decorated with ptransform_fn will instead be a _PTransformFnPTransform type.
Additionally, for Python 3.10, ParamSpec could be used to preserve the
signature of the internal function as well.
{code:java}
from typing import ParamSpec, TypeVar, Concatenate
P = ParamSpec("P")
T = TypeVar("T")
R = TypeVar("R")
def ptransform_fn(fn: Callable[Concatenate[PCollection[T], P], R]) ->
Callable[[P], _PTransformFnPTransform]: ...{code}
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