core: update json output parser (#15079)

- **Description:** changed json.py to handle additional cases of partial
json string to be parsed, basically by dropping the last character in
the string until a valid json string is found or the string is empty.
Also added additional test cases.
  
- **Issue:** function parse_partial_json could not parse cases where the
key is present but the value is not.

---------

Co-authored-by: Nuno Campos <nuno@langchain.dev>

libs/core/langchain_core/output_parsers/json.py

@@ -102,12 +102,27 @@ def parse_partial_json(s: str, *, strict: bool = False) -> Any:
     if is_inside_string:
         new_s += '"'
 
-    # Close any remaining open structures in the reverse order that they were opened.
-    for closing_char in reversed(stack):
-        new_s += closing_char
+    # Try to parse mods of string until we succeed or run out of characters.
+    while new_s:
+        final_s = new_s
 
-    # Attempt to parse the modified string as JSON.
-    return json.loads(new_s, strict=strict)
+        # Close any remaining open structures in the reverse
+        # order that they were opened.
+        for closing_char in reversed(stack):
+            final_s += closing_char
+
+        # Attempt to parse the modified string as JSON.
+        try:
+            return json.loads(final_s, strict=strict)
+        except json.JSONDecodeError:
+            # If we still can't parse the string as JSON,
+            # try removing the last character
+            new_s = new_s[:-1]
+
+    # If we got here, we ran out of characters to remove
+    # and still couldn't parse the string as JSON, so return the parse error
+    # for the original string.
+    return json.loads(s, strict=strict)
 
 
 def parse_json_markdown(

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

@@ -199,6 +199,9 @@ TEST_CASES_PARTIAL = [
     ('{"foo": "bar", "bar": "foo}', '{"foo": "bar", "bar": "foo}"}'),
     ('{"foo": "bar", "bar": "foo[', '{"foo": "bar", "bar": "foo["}'),
     ('{"foo": "bar", "bar": "foo\\"', '{"foo": "bar", "bar": "foo\\""}'),
+    ('{"foo": "bar", "bar":', '{"foo": "bar"}'),
+    ('{"foo": "bar", "bar"', '{"foo": "bar"}'),
+    ('{"foo": "bar", ', '{"foo": "bar"}'),
 ]