feat: add markdown list parser (#11411)

**Description:** add `MarkdownListOutputParser` as a new
`ListOutputParser`
 **Issue:** #11410

libs/langchain/langchain/output_parsers/__init__.py

@@ -20,6 +20,7 @@ from langchain.output_parsers.fix import OutputFixingParser
 from langchain.output_parsers.list import (
     CommaSeparatedListOutputParser,
     ListOutputParser,
+    MarkdownListOutputParser,
     NumberedListOutputParser,
 )
 from langchain.output_parsers.pydantic import PydanticOutputParser
@@ -38,6 +39,7 @@ __all__ = [
     "EnumOutputParser",
     "GuardrailsOutputParser",
     "ListOutputParser",
+    "MarkdownListOutputParser",
     "NumberedListOutputParser",
     "OutputFixingParser",
     "PydanticOutputParser",

libs/langchain/langchain/output_parsers/list.py

@@ -61,3 +61,19 @@ class NumberedListOutputParser(ListOutputParser):
     @property
     def _type(self) -> str:
         return "numbered-list"
+
+
+class MarkdownListOutputParser(ListOutputParser):
+    """Parse a markdown list."""
+
+    def get_format_instructions(self) -> str:
+        return "Your response should be a markdown list, " "eg: `- foo\n- bar\n- baz`"
+
+    def parse(self, text: str) -> List[str]:
+        """Parse the output of an LLM call."""
+        pattern = r"-\s([^\n]+)"
+        return re.findall(pattern, text)
+
+    @property
+    def _type(self) -> str:
+        return "markdown-list"

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

@@ -1,5 +1,6 @@
 from langchain.output_parsers.list import (
     CommaSeparatedListOutputParser,
+    MarkdownListOutputParser,
     NumberedListOutputParser,
 )
 
@@ -30,3 +31,19 @@ def test_numbered_list() -> None:
     assert parser.parse(text1) == ["foo", "bar", "baz"]
     assert parser.parse(text2) == ["apple", "banana", "cherry"]
     assert parser.parse(text3) == []
+
+
+def test_markdown_list() -> None:
+    parser = MarkdownListOutputParser()
+    text1 = (
+        "Your response should be a numbered list with each item on a new line."
+        "For example: \n- foo\n- bar\n- baz"
+    )
+
+    text2 = "Items:\n- apple\n- banana\n- 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) == []