add numbered list parser (#9837)

libs/langchain/langchain/output_parsers/list.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import re
 from abc import abstractmethod
 from typing import List
 
@@ -34,3 +35,29 @@ class CommaSeparatedListOutputParser(ListOutputParser):
     def parse(self, text: str) -> List[str]:
         """Parse the output of an LLM call."""
         return text.strip().split(", ")
+
+    @property
+    def _type(self) -> str:
+        return "comma-separated-list"
+
+
+class NumberedListOutputParser(ListOutputParser):
+    """Parse a numbered list."""
+
+    def get_format_instructions(self) -> str:
+        return (
+            "Your response should be a numbered list with each item on a new line. "
+            "For example: \n\n1. foo\n\n2. bar\n\n3. baz"
+        )
+
+    def parse(self, text: str) -> List[str]:
+        """Parse the output of an LLM call."""
+        pattern = r"\d+\.\s([^\n]+)"
+
+        # Extract the text of each item
+        matches = re.findall(pattern, text)
+        return matches
+
+    @property
+    def _type(self) -> str:
+        return "numbered-list"

libs/langchain/tests/unit_tests/output_parsers/test_list_parser.py

@@ -1,4 +1,7 @@
-from langchain.output_parsers.list import CommaSeparatedListOutputParser
+from langchain.output_parsers.list import (
+    CommaSeparatedListOutputParser,
+    NumberedListOutputParser,
+)
 
 
 def test_single_item() -> None:
@@ -11,3 +14,19 @@ def test_multiple_items() -> None:
     """Test that a string with multiple comma-separated items is parsed to a list."""
     parser = CommaSeparatedListOutputParser()
     assert parser.parse("foo, bar, baz") == ["foo", "bar", "baz"]
+
+
+def test_numbered_list() -> None:
+    parser = NumberedListOutputParser()
+    text1 = (
+        "Your response should be a numbered list with each item on a new line. "
+        "For example: \n\n1. foo\n\n2. bar\n\n3. baz"
+    )
+
+    text2 = "Items:\n\n1. apple\n\n2. banana\n\n3. cherry"
+
+    text3 = "No items in the list."
+
+    assert parser.parse(text1) == ["foo", "bar", "baz"]
+    assert parser.parse(text2) == ["apple", "banana", "cherry"]
+    assert parser.parse(text3) == []