Strip code block fences and extra test from xml when doing streaming … (#15293)

…parse

<!-- Thank you for contributing to LangChain!

Please title your PR "<package>: <description>", where <package> is
whichever of langchain, community, core, experimental, etc. is being
modified.

Replace this entire comment with:
  - **Description:** a description of the change, 
  - **Issue:** the issue # it fixes if applicable,
  - **Dependencies:** any dependencies required for this change,
- **Twitter handle:** we announce bigger features on Twitter. If your PR
gets announced, and you'd like a mention, we'll gladly shout you out!

Please make sure your PR is passing linting and testing before
submitting. Run `make format`, `make lint` and `make test` from the root
of the package you've modified to check this locally.

See contribution guidelines for more information on how to write/run
tests, lint, etc: https://python.langchain.com/docs/contributing/

If you're adding a new integration, please include:
1. a test for the integration, preferably unit tests that do not rely on
network access,
2. an example notebook showing its use. It lives in
`docs/docs/integrations` directory.

If no one reviews your PR within a few days, please @-mention one of
@baskaryan, @eyurtsev, @hwchase17.
 -->

libs/core/langchain_core/output_parsers/xml.py

@@ -55,9 +55,12 @@ class XMLOutputParser(BaseTransformOutputParser):
     def _transform(
         self, input: Iterator[Union[str, BaseMessage]]
     ) -> Iterator[AddableDict]:
+        xml_start_re = re.compile(r"<[a-zA-Z:_]")
         parser = ET.XMLPullParser(["start", "end"])
+        xml_started = False
         current_path: List[str] = []
         current_path_has_children = False
+        buffer = ""
         for chunk in input:
             if isinstance(chunk, BaseMessage):
                 # extract text
@@ -65,8 +68,19 @@ class XMLOutputParser(BaseTransformOutputParser):
                 if not isinstance(chunk_content, str):
                     continue
                 chunk = chunk_content
-            # pass chunk to parser
+            # add chunk to buffer of unprocessed text
-            parser.feed(chunk)
+            buffer += chunk
+            # if xml string hasn't started yet, continue to next chunk
+            if not xml_started:
+                if match := xml_start_re.search(buffer):
+                    # if xml string has started, remove all text before it
+                    buffer = buffer[match.start() :]
+                    xml_started = True
+                else:
+                    continue
+            # feed buffer to parser
+            parser.feed(buffer)
+            buffer = ""
             # yield all events
             for event, elem in parser.read_events():
                 if event == "start":
@@ -80,7 +94,10 @@ class XMLOutputParser(BaseTransformOutputParser):
                     if not current_path_has_children:
                         yield nested_element(current_path, elem)
                     # prevent yielding of parent element
-                    current_path_has_children = True
+                    if current_path:
+                        current_path_has_children = True
+                    else:
+                        xml_started = False
         # close parser
         parser.close()
 

libs/core/tests/unit_tests/output_parsers/test_xml_parser.py

@@ -22,7 +22,22 @@ DEF_RESULT_EXPECTED = {
 
 @pytest.mark.parametrize(
     "result",
-    [DEF_RESULT_ENCODING, DEF_RESULT_ENCODING[DEF_RESULT_ENCODING.find("\n") :]],
+    [
+        DEF_RESULT_ENCODING,
+        DEF_RESULT_ENCODING[DEF_RESULT_ENCODING.find("\n") :],
+        f"""
+```xml
+{DEF_RESULT_ENCODING}
+```
+""",
+        f"""
+Some random text
+```xml
+{DEF_RESULT_ENCODING}
+```
+More random text
+""",
+    ],
 )
 def test_xml_output_parser(result: str) -> None:
     """Test XMLOutputParser."""