Add support for tagged output type hints.

sdks/python/apache_beam/pvalue.py

@@ -246,6 +246,8 @@ def __init__(
     self._tags = tags
     self._main_tag = main_tag
     self._transform = transform
+    self._tagged_output_types = (
+        transform.get_type_hints().tagged_output_types() if transform else {})
     self._allow_unknown_tags = (
         not tags if allow_unknown_tags is None else allow_unknown_tags)
     # The ApplyPTransform instance for the application of the multi FlatMap
@@ -303,7 +305,7 @@ def __getitem__(self, tag: Union[int, str, None]) -> PCollection:
       pcoll = PCollection(
           self._pipeline,
           tag=tag,
-          element_type=typehints.Any,
+          element_type=self._tagged_output_types.get(tag, typehints.Any),
           is_bounded=is_bounded)
       # Transfer the producer from the DoOutputsTuple to the resulting
       # PCollection.
@@ -323,15 +325,19 @@ def __getitem__(self, tag: Union[int, str, None]) -> PCollection:
     return pcoll
 
 
-class TaggedOutput(object):
+TagType = TypeVar('TagType', bound=str)
+ValueType = TypeVar('ValueType')
+
+
+class TaggedOutput(Generic[TagType, ValueType]):
   """An object representing a tagged value.
 
   ParDo, Map, and FlatMap transforms can emit values on multiple outputs which
   are distinguished by string tags. The DoFn will return plain values
   if it wants to emit on the main output and TaggedOutput objects
   if it wants to emit a value on a specific tagged output.
   """
-  def __init__(self, tag: str, value: Any) -> None:
+  def __init__(self, tag: TagType, value: ValueType) -> None:
     if not isinstance(tag, str):
       raise TypeError(
           'Attempting to create a TaggedOutput with non-string tag %s' %

sdks/python/apache_beam/transforms/core.py

@@ -1834,6 +1834,17 @@ def with_outputs(self, *tags, main=None, allow_unknown_tags=None):
       raise ValueError(
           'Main output tag %r must be different from side output tags %r.' %
           (main, tags))
+    type_hints = self.get_type_hints()
+    declared_tags = set(type_hints.tagged_output_types().keys())
+    requested_tags = set(tags)
+
+    unknown = requested_tags - declared_tags
+    if unknown and declared_tags:  # Only warn if type hints exist
+      logging.warning(
+          "Tags %s requested in with_outputs() but not declared "
+          "in type hints. Declared tags: %s",
+          unknown,
+          declared_tags)
     return _MultiParDo(self, tags, main, allow_unknown_tags)
 
   def _do_fn_info(self):
@@ -2120,8 +2131,10 @@ def Map(fn, *args, **kwargs):  # pylint: disable=invalid-name
             wrapper)
   output_hint = type_hints.simple_output_type(label)
   if output_hint:
+    tagged_output_types = type_hints.tagged_output_types()
     wrapper = with_output_types(
-        typehints.Iterable[_strip_output_annotations(output_hint)])(
+        typehints.Iterable[_strip_output_annotations(output_hint)],
+        **tagged_output_types)(
             wrapper)
   # pylint: disable=protected-access
   wrapper._argspec_fn = fn
@@ -2189,8 +2202,10 @@ def MapTuple(fn, *args, **kwargs):  # pylint: disable=invalid-name
     pass
   output_hint = type_hints.simple_output_type(label)
   if output_hint:
+    tagged_output_types = type_hints.tagged_output_types()
     wrapper = with_output_types(
-        typehints.Iterable[_strip_output_annotations(output_hint)])(
+        typehints.Iterable[_strip_output_annotations(output_hint)],
+        **tagged_output_types)(
             wrapper)
 
   # Replace the first (args) component.
@@ -2261,7 +2276,10 @@ def FlatMapTuple(fn, *args, **kwargs):  # pylint: disable=invalid-name
     pass
   output_hint = type_hints.simple_output_type(label)
   if output_hint:
-    wrapper = with_output_types(_strip_output_annotations(output_hint))(wrapper)
+    tagged_output_types = type_hints.tagged_output_types()
+    wrapper = with_output_types(
+        _strip_output_annotations(output_hint), **tagged_output_types)(
+            wrapper)
 
   # Replace the first (args) component.
   modified_arg_names = ['tuple_element'] + arg_names[-num_defaults:]

sdks/python/apache_beam/transforms/ptransform.py

@@ -414,12 +414,15 @@ def with_input_types(self, input_type_hint):
         input_type_hint, 'Type hints for a PTransform')
     return super().with_input_types(input_type_hint)
 
-  def with_output_types(self, type_hint):
+  def with_output_types(self, type_hint, **tagged_type_hints):
     """Annotates the output type of a :class:`PTransform` with a type-hint.
 
     Args:
       type_hint (type): An instance of an allowed built-in type, a custom class,
-        or a :class:`~apache_beam.typehints.typehints.TypeConstraint`.
+        or a :class:`~apache_beam.typehints.typehints.TypeConstraint`. This is
+        the type hint for the main output.
+      **tagged_type_hints: Type hints for tagged outputs. Each keyword argument
+        specifies the type for a tagged output e.g., ``errors=str``.
 
     Raises:
       TypeError: If **type_hint** is not a valid type-hint. See
@@ -430,10 +433,22 @@ def with_output_types(self, type_hint):
       PTransform: A reference to the instance of this particular
       :class:`PTransform` object. This allows chaining type-hinting related
       methods.
+
+    Example::
+      result = pcoll | beam.ParDo(MyDoFn()).with_output_types(
+          int,  # main output type
+          errors=str,  # 'errors' tagged output type
+          warnings=str  # 'warnings' tagged output type
+      ).with_outputs('errors', 'warnings', main='main')
     """
     type_hint = native_type_compatibility.convert_to_beam_type(type_hint)
     validate_composite_type_param(type_hint, 'Type hints for a PTransform')
-    return super().with_output_types(type_hint)
+    for tag, hint in tagged_type_hints.items():
+      tagged_type_hints[tag] = native_type_compatibility.convert_to_beam_type(
+          hint)
+      validate_composite_type_param(
+          tagged_type_hints[tag], f'Tagged output type hint for {tag!r}')
+    return super().with_output_types(type_hint, **tagged_type_hints)
 
   def with_resource_hints(self, **kwargs):  # type: (...) -> PTransform
     """Adds resource hints to the :class:`PTransform`.
@@ -479,10 +494,11 @@ def type_check_inputs_or_outputs(self, pvalueish, input_or_output):
     if hints is None or not any(hints):
       return
     arg_hints, kwarg_hints = hints
-    if arg_hints and kwarg_hints:
+    # Output types can have kwargs for tagged output types.
+    if arg_hints and kwarg_hints and input_or_output != 'output':
       raise TypeCheckError(
-          'PTransform cannot have both positional and keyword type hints '
-          'without overriding %s._type_check_%s()' %
+          'PTransform cannot have both positional and keyword input type hints'
+          ' without overriding %s._type_check_%s()' %
           (self.__class__, input_or_output))
     root_hint = (
         arg_hints[0] if len(arg_hints) == 1 else arg_hints or kwarg_hints)