fix(core): accept non-dict Mapping values in mustache templates (#39680)

libs/core/langchain_core/utils/mustache.py

@@ -394,7 +394,7 @@ def _get_key(
                 if resolved_scope in (0, False):
                     return resolved_scope
                 # Move into the scope
-                if isinstance(resolved_scope, dict):
+                if isinstance(resolved_scope, Mapping):
                     try:
                         resolved_scope = resolved_scope[child]
                     except (KeyError, TypeError):

libs/core/tests/unit_tests/prompts/test_prompt.py

@@ -1,7 +1,9 @@
 """Test functionality related to prompts."""
 
 import re
+from collections import ChainMap, UserDict
 from tempfile import NamedTemporaryFile
+from types import MappingProxyType
 from typing import Any, Literal
 from unittest import mock
 
@@ -251,6 +253,16 @@ def test_mustache_prompt_from_template(snapshot: SnapshotAssertion) -> None:
     }
 
 
+def test_mustache_prompt_with_non_dict_mapping() -> None:
+    """Test mustache templates accept non-`dict` `Mapping` values."""
+    template = "Hello {{user.name}}"
+    prompt = PromptTemplate.from_template(template, template_format="mustache")
+
+    assert prompt.format(user=ChainMap({"name": "Alice"})) == "Hello Alice"
+    assert prompt.format(user=UserDict({"name": "Alice"})) == "Hello Alice"
+    assert prompt.format(user=MappingProxyType({"name": "Alice"})) == "Hello Alice"
+
+
 def test_prompt_from_template_with_partial_variables() -> None:
     """Test prompts can be constructed from a template with partial variables."""
     # given