fix: Simplify loop stopping condition to prevent off-by-one errors (#6696)

* 🐛 (loop.py): Fix logic in LoopComponent to correctly evaluate if loop should stop based on current index and data length
📝 (LoopTemplate.json): Update output_types to be arrays for consistency and readability in LoopTemplate JSON file

* 🐛 (loop.py): fix logic in stop condition to correctly evaluate if loop should stop or continue based on current index and data length

* [autofix.ci] apply automated fixes

* ✅ (loop-component.spec.ts): update test expectation to match the correct number of occurrences of "modified_value" in the output

---------

Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>
Co-authored-by: Gabriel Luiz Freitas Almeida <[email protected]>

Author: 3 people

src/backend/base/langflow/components/logic/loop.py

@@ -54,7 +54,7 @@ def evaluate_stop_loop(self) -> bool:
         """Evaluate whether to stop item or done output."""
         current_index = self.ctx.get(f"{self._id}_index", 0)
         data_length = len(self.ctx.get(f"{self._id}_data", []))
-        return current_index > data_length
+        return current_index > max(data_length - 1, 0)
 
     def item_output(self) -> Data:
         """Output the next item in the list or stop if done."""

src/backend/base/langflow/initial_setup/starter_projects/LoopTemplate.json

@@ -443,7 +443,7 @@
                 "show": true,
                 "title_case": false,
                 "type": "code",
-                "value": "from langflow.custom import Component\nfrom langflow.io import DataInput, Output\nfrom langflow.schema import Data\n\n\nclass LoopComponent(Component):\n    display_name = \"Loop\"\n    description = (\n        \"Iterates over a list of Data objects, outputting one item at a time and aggregating results from loop inputs.\"\n    )\n    icon = \"infinity\"\n\n    inputs = [\n        DataInput(\n            name=\"data\",\n            display_name=\"Data\",\n            info=\"The initial list of Data objects to iterate over.\",\n        ),\n    ]\n\n    outputs = [\n        Output(display_name=\"Item\", name=\"item\", method=\"item_output\", allows_loop=True),\n        Output(display_name=\"Done\", name=\"done\", method=\"done_output\"),\n    ]\n\n    def initialize_data(self) -> None:\n        \"\"\"Initialize the data list, context index, and aggregated list.\"\"\"\n        if self.ctx.get(f\"{self._id}_initialized\", False):\n            return\n\n        # Ensure data is a list of Data objects\n        data_list = self._validate_data(self.data)\n\n        # Store the initial data and context variables\n        self.update_ctx(\n            {\n                f\"{self._id}_data\": data_list,\n                f\"{self._id}_index\": 0,\n                f\"{self._id}_aggregated\": [],\n                f\"{self._id}_initialized\": True,\n            }\n        )\n\n    def _validate_data(self, data):\n        \"\"\"Validate and return a list of Data objects.\"\"\"\n        if isinstance(data, Data):\n            return [data]\n        if isinstance(data, list) and all(isinstance(item, Data) for item in data):\n            return data\n        msg = \"The 'data' input must be a list of Data objects or a single Data object.\"\n        raise TypeError(msg)\n\n    def evaluate_stop_loop(self) -> bool:\n        \"\"\"Evaluate whether to stop item or done output.\"\"\"\n        current_index = self.ctx.get(f\"{self._id}_index\", 0)\n        data_length = len(self.ctx.get(f\"{self._id}_data\", []))\n        return current_index > data_length\n\n    def item_output(self) -> Data:\n        \"\"\"Output the next item in the list or stop if done.\"\"\"\n        self.initialize_data()\n        current_item = Data(text=\"\")\n\n        if self.evaluate_stop_loop():\n            self.stop(\"item\")\n            return Data(text=\"\")\n\n        # Get data list and current index\n        data_list, current_index = self.loop_variables()\n        if current_index < len(data_list):\n            # Output current item and increment index\n            try:\n                current_item = data_list[current_index]\n            except IndexError:\n                current_item = Data(text=\"\")\n        self.aggregated_output()\n        self.update_ctx({f\"{self._id}_index\": current_index + 1})\n        return current_item\n\n    def done_output(self) -> Data:\n        \"\"\"Trigger the done output when iteration is complete.\"\"\"\n        self.initialize_data()\n\n        if self.evaluate_stop_loop():\n            self.stop(\"item\")\n            self.start(\"done\")\n\n            return self.ctx.get(f\"{self._id}_aggregated\", [])\n        self.stop(\"done\")\n        return Data(text=\"\")\n\n    def loop_variables(self):\n        \"\"\"Retrieve loop variables from context.\"\"\"\n        return (\n            self.ctx.get(f\"{self._id}_data\", []),\n            self.ctx.get(f\"{self._id}_index\", 0),\n        )\n\n    def aggregated_output(self) -> Data:\n        \"\"\"Return the aggregated list once all items are processed.\"\"\"\n        self.initialize_data()\n\n        # Get data list and aggregated list\n        data_list = self.ctx.get(f\"{self._id}_data\", [])\n        aggregated = self.ctx.get(f\"{self._id}_aggregated\", [])\n\n        # Check if loop input is provided and append to aggregated list\n        if self.item is not None and not isinstance(self.item, str) and len(aggregated) <= len(data_list):\n            aggregated.append(self.item)\n            self.update_ctx({f\"{self._id}_aggregated\": aggregated})\n        return aggregated\n"
+                "value": "from langflow.custom import Component\nfrom langflow.io import DataInput, Output\nfrom langflow.schema import Data\n\n\nclass LoopComponent(Component):\n    display_name = \"Loop\"\n    description = (\n        \"Iterates over a list of Data objects, outputting one item at a time and aggregating results from loop inputs.\"\n    )\n    icon = \"infinity\"\n\n    inputs = [\n        DataInput(\n            name=\"data\",\n            display_name=\"Data\",\n            info=\"The initial list of Data objects to iterate over.\",\n        ),\n    ]\n\n    outputs = [\n        Output(display_name=\"Item\", name=\"item\", method=\"item_output\", allows_loop=True),\n        Output(display_name=\"Done\", name=\"done\", method=\"done_output\"),\n    ]\n\n    def initialize_data(self) -> None:\n        \"\"\"Initialize the data list, context index, and aggregated list.\"\"\"\n        if self.ctx.get(f\"{self._id}_initialized\", False):\n            return\n\n        # Ensure data is a list of Data objects\n        data_list = self._validate_data(self.data)\n\n        # Store the initial data and context variables\n        self.update_ctx(\n            {\n                f\"{self._id}_data\": data_list,\n                f\"{self._id}_index\": 0,\n                f\"{self._id}_aggregated\": [],\n                f\"{self._id}_initialized\": True,\n            }\n        )\n\n    def _validate_data(self, data):\n        \"\"\"Validate and return a list of Data objects.\"\"\"\n        if isinstance(data, Data):\n            return [data]\n        if isinstance(data, list) and all(isinstance(item, Data) for item in data):\n            return data\n        msg = \"The 'data' input must be a list of Data objects or a single Data object.\"\n        raise TypeError(msg)\n\n    def evaluate_stop_loop(self) -> bool:\n        \"\"\"Evaluate whether to stop item or done output.\"\"\"\n        current_index = self.ctx.get(f\"{self._id}_index\", 0)\n        data_length = len(self.ctx.get(f\"{self._id}_data\", []))\n        return current_index > max(data_length - 1, 0)\n\n    def item_output(self) -> Data:\n        \"\"\"Output the next item in the list or stop if done.\"\"\"\n        self.initialize_data()\n        current_item = Data(text=\"\")\n\n        if self.evaluate_stop_loop():\n            self.stop(\"item\")\n            return Data(text=\"\")\n\n        # Get data list and current index\n        data_list, current_index = self.loop_variables()\n        if current_index < len(data_list):\n            # Output current item and increment index\n            try:\n                current_item = data_list[current_index]\n            except IndexError:\n                current_item = Data(text=\"\")\n        self.aggregated_output()\n        self.update_ctx({f\"{self._id}_index\": current_index + 1})\n        return current_item\n\n    def done_output(self) -> Data:\n        \"\"\"Trigger the done output when iteration is complete.\"\"\"\n        self.initialize_data()\n\n        if self.evaluate_stop_loop():\n            self.stop(\"item\")\n            self.start(\"done\")\n\n            return self.ctx.get(f\"{self._id}_aggregated\", [])\n        self.stop(\"done\")\n        return Data(text=\"\")\n\n    def loop_variables(self):\n        \"\"\"Retrieve loop variables from context.\"\"\"\n        return (\n            self.ctx.get(f\"{self._id}_data\", []),\n            self.ctx.get(f\"{self._id}_index\", 0),\n        )\n\n    def aggregated_output(self) -> Data:\n        \"\"\"Return the aggregated list once all items are processed.\"\"\"\n        self.initialize_data()\n\n        # Get data list and aggregated list\n        data_list = self.ctx.get(f\"{self._id}_data\", [])\n        aggregated = self.ctx.get(f\"{self._id}_aggregated\", [])\n\n        # Check if loop input is provided and append to aggregated list\n        if self.item is not None and not isinstance(self.item, str) and len(aggregated) <= len(data_list):\n            aggregated.append(self.item)\n            self.update_ctx({f\"{self._id}_aggregated\": aggregated})\n        return aggregated\n"
               },
               "data": {
                 "_input_type": "DataInput",

src/frontend/tests/extended/features/loop-component.spec.ts

@@ -237,6 +237,6 @@ test(
 
     // Count occurrences of modified_value in output
     const matches = output?.match(/modified_value/g) || [];
-    expect(matches).toHaveLength(2);
+    expect(matches).toHaveLength(1);
   },
 );