core[patch]: Add encoding options when create prompt template from a file (#24054)

- Uses default utf-8 encoding for loading prompt templates from file

---------

Co-authored-by: Eugene Yurtsev <eyurtsev@gmail.com>

libs/core/langchain_core/prompts/prompt.py

@@ -214,6 +214,7 @@ class PromptTemplate(StringPromptTemplate):
         cls,
         template_file: Union[str, Path],
         input_variables: Optional[List[str]] = None,
+        encoding: Optional[str] = None,
         **kwargs: Any,
     ) -> PromptTemplate:
         """Load a prompt from a file.
@@ -222,13 +223,15 @@ class PromptTemplate(StringPromptTemplate):
             template_file: The path to the file containing the prompt template.
             input_variables: [DEPRECATED] A list of variable names the final prompt
                 template will expect. Defaults to None.
+            encoding: The encoding system for opening the template file.
+                If not provided, will use the OS default.
 
         input_variables is ignored as from_file now delegates to from_template().
 
         Returns:
             The prompt loaded from the file.
         """
-        with open(str(template_file), "r") as f:
+        with open(str(template_file), "r", encoding=encoding) as f:
             template = f.read()
         if input_variables:
             warnings.warn(

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

@@ -18,6 +18,29 @@ def test_prompt_valid() -> None:
     assert prompt.input_variables == input_variables
 
 
+def test_from_file_encoding() -> None:
+    """Test that we can load a template from a file with a non utf-8 encoding."""
+    template = "This is a {foo} test with special character €."
+    input_variables = ["foo"]
+
+    # First write to a file using CP-1252 encoding.
+    from tempfile import NamedTemporaryFile
+
+    with NamedTemporaryFile(delete=True, mode="w", encoding="cp1252") as f:
+        f.write(template)
+        f.flush()
+        file_name = f.name
+
+        # Now read from the file using CP-1252 encoding and test
+        prompt = PromptTemplate.from_file(file_name, encoding="cp1252")
+        assert prompt.template == template
+        assert prompt.input_variables == input_variables
+
+        # Now read from the file using UTF-8 encoding and test
+        with pytest.raises(UnicodeDecodeError):
+            PromptTemplate.from_file(file_name, encoding="utf-8")
+
+
 def test_prompt_from_template() -> None:
     """Test prompts can be constructed from a template."""
     # Single input variable.