Fixes error loading Obsidian templates (#13888)

- **Description:** Obsidian templates can include
[variables](https://help.obsidian.md/Plugins/Templates#Template+variables)
using double curly braces. `ObsidianLoader` uses PyYaml to parse the
frontmatter of documents. This parsing throws an error when encountering
variables' curly braces. This is avoided by temporarily substituting
safe strings before parsing.
  - **Issue:** #13887
  - **Tag maintainer:** @hwchase17

libs/langchain/langchain/document_loaders/obsidian.py

@@ -1,7 +1,8 @@
+import functools
 import logging
 import re
 from pathlib import Path
-from typing import List
+from typing import Any, Dict, List
 
 import yaml
 from langchain_core.documents import Document
@@ -15,6 +16,7 @@ class ObsidianLoader(BaseLoader):
     """Load `Obsidian` files from directory."""
 
     FRONT_MATTER_REGEX = re.compile(r"^---\n(.*?)\n---\n", re.DOTALL)
+    TEMPLATE_VARIABLE_REGEX = re.compile(r"{{(.*?)}}", re.DOTALL)
     TAG_REGEX = re.compile(r"[^\S\/]#([a-zA-Z_]+[-_/\w]*)")
     DATAVIEW_LINE_REGEX = re.compile(r"^\s*(\w+)::\s*(.*)$", re.MULTILINE)
     DATAVIEW_INLINE_BRACKET_REGEX = re.compile(r"\[(\w+)::\s*(.*)\]", re.MULTILINE)
@@ -35,6 +37,27 @@ def __init__(
         self.encoding = encoding
         self.collect_metadata = collect_metadata
 
+    def _replace_template_var(
+        self, placeholders: Dict[str, str], match: re.Match
+    ) -> str:
+        """Replace a template variable with a placeholder."""
+        placeholder = f"__TEMPLATE_VAR_{len(placeholders)}__"
+        placeholders[placeholder] = match.group(1)
+        return placeholder
+
+    def _restore_template_vars(self, obj: Any, placeholders: Dict[str, str]) -> Any:
+        """Restore template variables replaced with placeholders to original values."""
+        if isinstance(obj, str):
+            for placeholder, value in placeholders.items():
+                obj = obj.replace(placeholder, f"{{{{{value}}}}}")
+        elif isinstance(obj, dict):
+            for key, value in obj.items():
+                obj[key] = self._restore_template_vars(value, placeholders)
+        elif isinstance(obj, list):
+            for i, item in enumerate(obj):
+                obj[i] = self._restore_template_vars(item, placeholders)
+        return obj
+
     def _parse_front_matter(self, content: str) -> dict:
         """Parse front matter metadata from the content and return it as a dict."""
         if not self.collect_metadata:
@@ -44,8 +67,17 @@ def _parse_front_matter(self, content: str) -> dict:
         if not match:
             return {}
 
+        placeholders: Dict[str, str] = {}
+        replace_template_var = functools.partial(
+            self._replace_template_var, placeholders
+        )
+        front_matter_text = self.TEMPLATE_VARIABLE_REGEX.sub(
+            replace_template_var, match.group(1)
+        )
+
         try:
-            front_matter = yaml.safe_load(match.group(1))
+            front_matter = yaml.safe_load(front_matter_text)
+            front_matter = self._restore_template_vars(front_matter, placeholders)
 
             # If tags are a string, split them into a list
             if "tags" in front_matter and isinstance(front_matter["tags"], str):

libs/langchain/tests/unit_tests/document_loaders/sample_documents/obsidian/template_var_frontmatter.md

@@ -0,0 +1,12 @@
+---
+aString: {{var}}
+anArray:
+- element
+- {{varElement}}
+aDict:
+  dictId1: 'val'
+  dictId2: '{{varVal}}'
+tags: [ 'tag', '{{varTag}}' ]
+---
+
+Frontmatter contains template variables.

libs/langchain/tests/unit_tests/document_loaders/test_obsidian.py

@@ -17,7 +17,7 @@
 
 def test_page_content_loaded() -> None:
     """Verify that all docs have page_content"""
-    assert len(docs) == 5
+    assert len(docs) == 6
     assert all(doc.page_content for doc in docs)
 
 
@@ -27,7 +27,7 @@ def test_disable_collect_metadata() -> None:
         str(OBSIDIAN_EXAMPLE_PATH), collect_metadata=False
     )
     docs_wo = loader_without_metadata.load()
-    assert len(docs_wo) == 5
+    assert len(docs_wo) == 6
     assert all(doc.page_content for doc in docs_wo)
     assert all(set(doc.metadata) == STANDARD_METADATA_FIELDS for doc in docs_wo)
 
@@ -45,6 +45,24 @@ def test_metadata_with_frontmatter() -> None:
     assert set(doc.metadata["tags"].split(",")) == {"journal/entry", "obsidian"}
 
 
+def test_metadata_with_template_vars_in_frontmatter() -> None:
+    """Verify frontmatter fields with template variables are loaded."""
+    doc = next(
+        doc for doc in docs if doc.metadata["source"] == "template_var_frontmatter.md"
+    )
+    FRONTMATTER_FIELDS = {
+        "aString",
+        "anArray",
+        "aDict",
+        "tags",
+    }
+    assert set(doc.metadata) == FRONTMATTER_FIELDS | STANDARD_METADATA_FIELDS
+    assert doc.metadata["aString"] == "{{var}}"
+    assert doc.metadata["anArray"] == "['element', '{{varElement}}']"
+    assert doc.metadata["aDict"] == "{'dictId1': 'val', 'dictId2': '{{varVal}}'}"
+    assert set(doc.metadata["tags"].split(",")) == {"tag", "{{varTag}}"}
+
+
 def test_metadata_with_bad_frontmatter() -> None:
     """Verify a doc with non-yaml frontmatter."""
     doc = next(doc for doc in docs if doc.metadata["source"] == "bad_frontmatter.md")