[Python] Fix AttributeError in ExternalTransform.expand by using get_type_hints() (#37299)

* [Python] Fix AttributeError in ExternalTransform.expand by using get_type_hints() (Fixes #37289)

* Fix lint: wrap long comment at line 808

* Fix yapf formatting and ignore local venv

Author: Eliaaazzz

.gitignore

@@ -154,3 +154,4 @@ playground/cloudfunction.zip
 # Ignore .test-infra/metrics/github_runs_prefetcher/code.zip
 # as its generated with terraform
 .test-infra/metrics/sync/github/github_runs_prefetcher/code.zip
+.venv/

sdks/python/apache_beam/transforms/external.py

@@ -804,16 +804,19 @@ def expand(self, pvalueish: pvalue.PCollection) -> pvalue.PCollection:
               spec=beam_runner_api_pb2.FunctionSpec(
                   urn=common_urns.primitives.IMPULSE.urn),
               outputs={'out': transform_proto.inputs[tag]}))
+
+    # Retrieve type hints and store them in variables
+    # to avoid duplicate calls and AttributeError
+    hints = self.get_type_hints()
     output_coders = None
-    if self._type_hints.output_types:
-      if self._type_hints.output_types[0]:
-        output_coders = dict(
-            (str(k), context.coder_id_from_element_type(v))
-            for (k, v) in enumerate(self._type_hints.output_types[0]))
-      elif self._type_hints.output_types[1]:
+    if hints.output_types:
+      if hints.output_types[0]:
+        output_coders = dict((str(k), context.coder_id_from_element_type(v))
+                             for (k, v) in enumerate(hints.output_types[0]))
+      elif hints.output_types[1]:
         output_coders = {
             k: context.coder_id_from_element_type(v)
-            for (k, v) in self._type_hints.output_types[1].items()
+            for (k, v) in hints.output_types[1].items()
         }
     components = context.to_runner_api()
     request = beam_expansion_api_pb2.ExpansionRequest(