De-dup logic for converting None policy tags

Author: plamut

google/cloud/bigquery/schema.py

@@ -15,6 +15,7 @@
 """Schemas for BigQuery tables / queries."""
 
 import collections
+from typing import Optional
 
 from google.cloud.bigquery_v2 import types
 
@@ -106,11 +107,25 @@ def __init__(
             self._properties["maxLength"] = max_length
         self._fields = tuple(fields)
 
-        if policy_tags is None:
-            is_record = field_type is not None and field_type.upper() in _STRUCT_TYPES
-            self._policy_tags = None if is_record else PolicyTagList()
-        else:
-            self._policy_tags = policy_tags
+        self._policy_tags = self._determine_policy_tags(field_type, policy_tags)
+
+    @staticmethod
+    def _determine_policy_tags(
+        field_type: str, given_policy_tags: Optional["PolicyTagList"]
+    ) -> Optional["PolicyTagList"]:
+        """Return the given policy tags, or their suitable representation if `None`.
+
+        Args:
+            field_type: The type of the schema field.
+            given_policy_tags: The policy tags to maybe ajdust.
+        """
+        if given_policy_tags is not None:
+            return given_policy_tags
+
+        if field_type is not None and field_type.upper() in _STRUCT_TYPES:
+            return None
+
+        return PolicyTagList()
 
     @staticmethod
     def __get_int(api_repr, name):
@@ -138,9 +153,9 @@ def from_api_repr(cls, api_repr: dict) -> "SchemaField":
         description = api_repr.get("description", _DEFAULT_VALUE)
         fields = api_repr.get("fields", ())
 
-        policy_tags = PolicyTagList.from_api_repr(api_repr.get("policyTags"))
-        if policy_tags is None and field_type not in _STRUCT_TYPES:
-            policy_tags = PolicyTagList()
+        policy_tags = cls._determine_policy_tags(
+            field_type, PolicyTagList.from_api_repr(api_repr.get("policyTags"))
+        )
 
         return cls(
             field_type=field_type,