feat: Persist Variables for Enhanced Debugging Workflow (#20699)

This pull request introduces a feature aimed at improving the debugging experience during workflow editing. With the addition of variable persistence, the system will automatically retain the output variables from previously executed nodes. These persisted variables can then be reused when debugging subsequent nodes, eliminating the need for repetitive manual input.

By streamlining this aspect of the workflow, the feature minimizes user errors and significantly reduces debugging effort, offering a smoother and more efficient experience.

Key highlights of this change:

- Automatic persistence of output variables for executed nodes.
- Reuse of persisted variables to simplify input steps for nodes requiring them (e.g., `code`, `template`, `variable_assigner`).
- Enhanced debugging experience with reduced friction.

Closes #19735.

api/core/workflow/entities/variable_pool.py

@@ -7,12 +7,12 @@ from pydantic import BaseModel, Field
 
 from core.file import File, FileAttribute, file_manager
 from core.variables import Segment, SegmentGroup, Variable
+from core.variables.consts import MIN_SELECTORS_LENGTH
 from core.variables.segments import FileSegment, NoneSegment
+from core.workflow.constants import CONVERSATION_VARIABLE_NODE_ID, ENVIRONMENT_VARIABLE_NODE_ID, SYSTEM_VARIABLE_NODE_ID
+from core.workflow.enums import SystemVariableKey
 from factories import variable_factory
 
-from ..constants import CONVERSATION_VARIABLE_NODE_ID, ENVIRONMENT_VARIABLE_NODE_ID, SYSTEM_VARIABLE_NODE_ID
-from ..enums import SystemVariableKey
-
 VariableValue = Union[str, int, float, dict, list, File]
 
 VARIABLE_PATTERN = re.compile(r"\{\{#([a-zA-Z0-9_]{1,50}(?:\.[a-zA-Z_][a-zA-Z0-9_]{0,29}){1,10})#\}\}")
@@ -30,9 +30,11 @@ class VariablePool(BaseModel):
     # TODO: This user inputs is not used for pool.
     user_inputs: Mapping[str, Any] = Field(
         description="User inputs",
+        default_factory=dict,
     )
     system_variables: Mapping[SystemVariableKey, Any] = Field(
         description="System variables",
+        default_factory=dict,
     )
     environment_variables: Sequence[Variable] = Field(
         description="Environment variables.",
@@ -43,28 +45,7 @@ class VariablePool(BaseModel):
         default_factory=list,
     )
 
-    def __init__(
-        self,
-        *,
-        system_variables: Mapping[SystemVariableKey, Any] | None = None,
-        user_inputs: Mapping[str, Any] | None = None,
-        environment_variables: Sequence[Variable] | None = None,
-        conversation_variables: Sequence[Variable] | None = None,
-        **kwargs,
-    ):
-        environment_variables = environment_variables or []
-        conversation_variables = conversation_variables or []
-        user_inputs = user_inputs or {}
-        system_variables = system_variables or {}
-
-        super().__init__(
-            system_variables=system_variables,
-            user_inputs=user_inputs,
-            environment_variables=environment_variables,
-            conversation_variables=conversation_variables,
-            **kwargs,
-        )
-
+    def model_post_init(self, context: Any, /) -> None:
         for key, value in self.system_variables.items():
             self.add((SYSTEM_VARIABLE_NODE_ID, key.value), value)
         # Add environment variables to the variable pool
@@ -91,12 +72,12 @@ class VariablePool(BaseModel):
         Returns:
             None
         """
-        if len(selector) < 2:
+        if len(selector) < MIN_SELECTORS_LENGTH:
             raise ValueError("Invalid selector")
 
         if isinstance(value, Variable):
             variable = value
-        if isinstance(value, Segment):
+        elif isinstance(value, Segment):
             variable = variable_factory.segment_to_variable(segment=value, selector=selector)
         else:
             segment = variable_factory.build_segment(value)
@@ -118,7 +99,7 @@ class VariablePool(BaseModel):
         Raises:
             ValueError: If the selector is invalid.
         """
-        if len(selector) < 2:
+        if len(selector) < MIN_SELECTORS_LENGTH:
             return None
 
         hash_key = hash(tuple(selector[1:]))