Fix logical type with same language type gets completely hidden intro…

…duced in #22679

sdks/python/apache_beam/io/external/xlang_jdbcio_it_test.py

@@ -36,6 +36,8 @@
 from apache_beam.testing.test_pipeline import TestPipeline
 from apache_beam.testing.util import assert_that
 from apache_beam.testing.util import equal_to
+from apache_beam.typehints.schemas import LogicalType
+from apache_beam.typehints.schemas import MillisInstant
 from apache_beam.utils.timestamp import Timestamp
 
 # pylint: disable=wrong-import-order, wrong-import-position, ungrouped-imports
@@ -192,6 +194,10 @@ def test_xlang_jdbc_read(self, database):
       self.engine.execute(
           "INSERT INTO {} VALUES({},'{}')".format(table_name, i, strtime))
 
+    # Register MillisInstant logical type to override the mapping from Timestamp
+    # originally handled by MicrosInstant.
+    LogicalType.register_logical_type(MillisInstant)
+
     with TestPipeline() as p:
       p.not_use_test_runner_api = True
       result = (

sdks/python/apache_beam/typehints/schemas.py

@@ -588,6 +588,7 @@ def to_language_type(self, value):
   def register_logical_type(cls, logical_type_cls):
     """Register an implementation of LogicalType."""
     cls._known_logical_types.add(logical_type_cls.urn(), logical_type_cls)
+    return logical_type_cls
 
   @classmethod
   def from_typing(cls, typ):
@@ -661,9 +662,16 @@ class MillisInstant(NoArgumentLogicalType[Timestamp, np.int64]):
   """Millisecond-precision instant logical type handles values consistent with
   that encoded by ``InstantCoder`` in the Java SDK.
 
-  This class handles Timestamp language type as :class:`MicrosInstant`, but it
-  only provides millisecond precision, because it is aimed to handle data
-  encoded by Java sdk's InstantCoder which has same precision level.
+  This class handles :class:`apache_beam.utils.timestamp.Timestamp` language
+  type as :class:`MicrosInstant`, but it only provides millisecond precision,
+  because it is aimed to handle data encoded by Java sdk's InstantCoder which
+  has same precision level.
+
+  Timestamp is handled by `MicrosInstant` by default. In some scenario, such as
+  read from cross-language transform with rows containing InstantCoder encoded
+  timestamps, one may need to override the mapping of Timetamp to MillisInstant.
+  To do this, re-register this class with
+  :func:`~LogicalType.register_logical_type`.
   """
   @classmethod
   def representation_type(cls):
@@ -722,6 +730,7 @@ def to_language_type(self, value):
 
 @LogicalType.register_logical_type
 class PythonCallable(NoArgumentLogicalType[PythonCallableWithSource, str]):
+  """A logical type for PythonCallableSource objects."""
   @classmethod
   def urn(cls):
     return common_urns.python_callable.urn