core: mustache prompt templates (#19980)

Co-authored-by: Erick Friis <erick@langchain.dev>

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

@@ -191,6 +191,34 @@ async def test_chat_prompt_template_from_messages_using_role_strings() -> None:
     assert messages == expected
 
 
+def test_chat_prompt_template_from_messages_mustache() -> None:
+    """Test creating a chat prompt template from role string messages."""
+    template = ChatPromptTemplate.from_messages(
+        [
+            ("system", "You are a helpful AI bot. Your name is {{name}}."),
+            ("human", "Hello, how are you doing?"),
+            ("ai", "I'm doing well, thanks!"),
+            ("human", "{{user_input}}"),
+        ],
+        "mustache",
+    )
+
+    messages = template.format_messages(name="Bob", user_input="What is your name?")
+
+    assert messages == [
+        SystemMessage(
+            content="You are a helpful AI bot. Your name is Bob.", additional_kwargs={}
+        ),
+        HumanMessage(
+            content="Hello, how are you doing?", additional_kwargs={}, example=False
+        ),
+        AIMessage(
+            content="I'm doing well, thanks!", additional_kwargs={}, example=False
+        ),
+        HumanMessage(content="What is your name?", additional_kwargs={}, example=False),
+    ]
+
+
 def test_chat_prompt_template_with_messages(
     messages: List[BaseMessagePromptTemplate],
 ) -> None:

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

@@ -38,6 +38,135 @@ def test_prompt_from_template() -> None:
     assert prompt == expected_prompt
 
 
+def test_mustache_prompt_from_template() -> None:
+    """Test prompts can be constructed from a template."""
+    # Single input variable.
+    template = "This is a {{foo}} test."
+    prompt = PromptTemplate.from_template(template, template_format="mustache")
+    assert prompt.format(foo="bar") == "This is a bar test."
+    assert prompt.input_variables == ["foo"]
+    assert prompt.input_schema.schema() == {
+        "title": "PromptInput",
+        "type": "object",
+        "properties": {"foo": {"title": "Foo", "type": "string"}},
+    }
+
+    # Multiple input variables.
+    template = "This {{bar}} is a {{foo}} test."
+    prompt = PromptTemplate.from_template(template, template_format="mustache")
+    assert prompt.format(bar="baz", foo="bar") == "This baz is a bar test."
+    assert prompt.input_variables == ["bar", "foo"]
+    assert prompt.input_schema.schema() == {
+        "title": "PromptInput",
+        "type": "object",
+        "properties": {
+            "bar": {"title": "Bar", "type": "string"},
+            "foo": {"title": "Foo", "type": "string"},
+        },
+    }
+
+    # Multiple input variables with repeats.
+    template = "This {{bar}} is a {{foo}} test {{foo}}."
+    prompt = PromptTemplate.from_template(template, template_format="mustache")
+    assert prompt.format(bar="baz", foo="bar") == "This baz is a bar test bar."
+    assert prompt.input_variables == ["bar", "foo"]
+    assert prompt.input_schema.schema() == {
+        "title": "PromptInput",
+        "type": "object",
+        "properties": {
+            "bar": {"title": "Bar", "type": "string"},
+            "foo": {"title": "Foo", "type": "string"},
+        },
+    }
+
+    # Nested variables.
+    template = "This {{obj.bar}} is a {{obj.foo}} test {{foo}}."
+    prompt = PromptTemplate.from_template(template, template_format="mustache")
+    assert prompt.format(obj={"bar": "foo", "foo": "bar"}, foo="baz") == (
+        "This foo is a bar test baz."
+    )
+    assert prompt.input_variables == ["foo", "obj"]
+    assert prompt.input_schema.schema() == {
+        "title": "PromptInput",
+        "type": "object",
+        "properties": {
+            "foo": {"title": "Foo", "type": "string"},
+            "obj": {"$ref": "#/definitions/obj"},
+        },
+        "definitions": {
+            "obj": {
+                "title": "obj",
+                "type": "object",
+                "properties": {
+                    "foo": {"title": "Foo", "type": "string"},
+                    "bar": {"title": "Bar", "type": "string"},
+                },
+            }
+        },
+    }
+
+    # . variables
+    template = "This {{.}} is a test."
+    prompt = PromptTemplate.from_template(template, template_format="mustache")
+    assert prompt.format(foo="baz") == ("This {'foo': 'baz'} is a test.")
+    assert prompt.input_variables == []
+    assert prompt.input_schema.schema() == {
+        "title": "PromptInput",
+        "type": "object",
+        "properties": {},
+    }
+
+    # section/context variables
+    template = """This{{#foo}}
+        {{bar}}
+    {{/foo}}is a test."""
+    prompt = PromptTemplate.from_template(template, template_format="mustache")
+    assert prompt.format(foo={"bar": "yo"}) == (
+        """This
+        yo
+    is a test."""
+    )
+    assert prompt.input_variables == ["foo"]
+    assert prompt.input_schema.schema() == {
+        "title": "PromptInput",
+        "type": "object",
+        "properties": {"foo": {"$ref": "#/definitions/foo"}},
+        "definitions": {
+            "foo": {
+                "title": "foo",
+                "type": "object",
+                "properties": {"bar": {"title": "Bar", "type": "string"}},
+            }
+        },
+    }
+
+    # section/context variables with repeats
+    template = """This{{#foo}}
+        {{bar}}
+    {{/foo}}is a test."""
+    prompt = PromptTemplate.from_template(template, template_format="mustache")
+    assert prompt.format(foo=[{"bar": "yo"}, {"bar": "hello"}]) == (
+        """This
+        yo
+    
+        hello
+    is a test."""
+    )
+    assert prompt.input_variables == ["foo"]
+    assert prompt.input_schema.schema() == {
+        "title": "PromptInput",
+        "type": "object",
+        "properties": {"foo": {"$ref": "#/definitions/foo"}},
+        "definitions": {
+            "foo": {
+                "title": "foo",
+                "type": "object",
+                "properties": {"bar": {"title": "Bar", "type": "string"}},
+            }
+        },
+    }
+
+
 def test_prompt_from_template_with_partial_variables() -> None:
     """Test prompts can be constructed from a template with partial variables."""
     # given