docs(api): update documentation for `SegmentUnion` and `VariableUnion`

api/core/app/apps/advanced_chat/app_runner.py

@@ -153,7 +153,8 @@ class AdvancedChatAppRunner(WorkflowBasedAppRunner):
                 system_variables=system_inputs,
                 user_inputs=inputs,
                 environment_variables=workflow.environment_variables,
-                # TODO(QuantumGhost): find a better way to resolve typing issue.
+                # Based on the definition of `VariableUnion`,
+                # `list[Variable]` can be safely used as `list[VariableUnion]` since they are compatible.
                 conversation_variables=cast(list[VariableUnion], conversation_variables),
             )
 

api/core/variables/segments.py

@@ -1,7 +1,7 @@
 import json
 import sys
 from collections.abc import Mapping, Sequence
-from typing import Annotated, Any
+from typing import Annotated, Any, TypeAlias
 
 from pydantic import BaseModel, ConfigDict, Discriminator, Tag, field_validator
 
@@ -205,7 +205,15 @@ def get_segment_discriminator(v: Any) -> SegmentType | None:
         return None
 
 
-SegmentUnion = Annotated[
+# The `SegmentUnion`` type is used to enable serialization and deserialization with Pydantic.
+# Use `Segment` for type hinting when serialization is not required.
+#
+# Note:
+# - All variants in `SegmentUnion` must inherit from the `Segment` class.
+# - The union must include all non-abstract subclasses of `Segment`, except:
+#   - `SegmentGroup`, which is not added to the variable pool.
+#   - `Variable` and its subclasses, which are handled by `VariableUnion`.
+SegmentUnion: TypeAlias = Annotated[
     (
         Annotated[NoneSegment, Tag(SegmentType.NONE)]
         | Annotated[StringSegment, Tag(SegmentType.STRING)]

api/core/variables/types.py

@@ -43,8 +43,11 @@ class SegmentType(StrEnum):
 
     @classmethod
     def infer_segment_type(cls, value: Any) -> Optional["SegmentType"]:
-        """Try to infer the SegmentType from the Python type of
+        """
-        the `value` parameter.
+        Attempt to infer the `SegmentType` based on the Python type of the `value` parameter.
+
+        Returns `None` if no appropriate `SegmentType` can be determined for the given `value`.
+        For example, this may occur if the input is a generic Python object of type `object`.
         """
 
         if isinstance(value, list):

api/core/variables/variables.py

@@ -100,6 +100,12 @@ class ArrayFileVariable(ArrayFileSegment, ArrayVariable):
     pass
 
 
+# The `VariableUnion`` type is used to enable serialization and deserialization with Pydantic.
+# Use `Variable` for type hinting when serialization is not required.
+#
+# Note:
+# - All variants in `VariableUnion` must inherit from the `Variable` class.
+# - The union must include all non-abstract subclasses of `Segment`, except:
 VariableUnion: TypeAlias = Annotated[
     (
         Annotated[NoneVariable, Tag(SegmentType.NONE)]

api/core/workflow/entities/variable_pool.py

@@ -34,10 +34,6 @@ class VariablePool(BaseModel):
         description="User inputs",
         default_factory=dict,
     )
-    # system_variables: Mapping[SystemVariableKey, Any] = Field(
-    #     description="System variables",
-    #     default_factory=dict,
-    # )
     system_variables: SystemVariable = Field(
         description="System variables",
     )
@@ -92,6 +88,8 @@ class VariablePool(BaseModel):
         # Ensure the first-level key exists in the dictionary
         if key not in self.variable_dictionary:
             self.variable_dictionary[key] = {}
+        # Based on the definition of `VariableUnion`,
+        # `list[Variable]` can be safely used as `list[VariableUnion]` since they are compatible.
         self.variable_dictionary[key][hash_key] = cast(VariableUnion, variable)
 
     @classmethod
@@ -199,7 +197,3 @@ class VariablePool(BaseModel):
     @classmethod
     def loads(cls, json_data: str) -> "VariablePool":
         return VariablePool.model_validate_json(json_data)
-
-    def reload_storage_keys_for_file_types(self):
-        # TODO
-        pass

api/services/workflow_service.py

@@ -693,7 +693,9 @@ def _setup_variable_pool(
         system_variables=system_variable,
         user_inputs=user_inputs,
         environment_variables=workflow.environment_variables,
-        conversation_variables=cast(list[VariableUnion], conversation_variables),
+        # Based on the definition of `VariableUnion`,
+        # `list[Variable]` can be safely used as `list[VariableUnion]` since they are compatible.
+        conversation_variables=cast(list[VariableUnion], conversation_variables),  #
     )
 
     return variable_pool

api/tests/unit_tests/core/workflow/nodes/test_continue_on_error.py

@@ -5,7 +5,6 @@ from core.app.entities.app_invoke_entities import InvokeFrom
 from core.workflow.entities.node_entities import NodeRunResult, WorkflowNodeExecutionMetadataKey
 from core.workflow.entities.variable_pool import VariablePool
 from core.workflow.entities.workflow_node_execution import WorkflowNodeExecutionStatus
-from core.workflow.enums import SystemVariableKey
 from core.workflow.graph_engine.entities.event import (
     GraphRunPartialSucceededEvent,
     NodeRunExceptionEvent,