feat (core): allow nested prompt templates

libs/core/langchain_core/prompts/base.py

@@ -7,15 +7,7 @@
 from collections.abc import Mapping
 from functools import cached_property
 from pathlib import Path
-from typing import (
-    TYPE_CHECKING,
-    Any,
-    Callable,
-    Generic,
-    Optional,
-    TypeVar,
-    Union,
-)
+from typing import TYPE_CHECKING, Any, Callable, Generic, Optional, TypeVar, Union
 
 import yaml
 from pydantic import BaseModel, ConfigDict, Field, model_validator
@@ -36,7 +28,6 @@
 if TYPE_CHECKING:
     from langchain_core.documents import Document
 
-
 FormatOutputType = TypeVar("FormatOutputType")
 
 
@@ -260,27 +251,47 @@ async def aformat_prompt(self, **kwargs: Any) -> PromptValue:
         """
         return self.format_prompt(**kwargs)
 
-    def partial(self, **kwargs: Union[str, Callable[[], str]]) -> BasePromptTemplate:
+    def partial(
+        self, **kwargs: Union[str, Callable[[], str], BasePromptTemplate]
+    ) -> BasePromptTemplate:
         """Return a partial of the prompt template.
 
         Args:
-            kwargs: Union[str, Callable[[], str], partial variables to set.
+            kwargs: Union[str, Callable[[], str], BasePromptTemplate],
+            partial variables to set.
 
         Returns:
             BasePromptTemplate: A partial of the prompt template.
         """
         prompt_dict = self.__dict__.copy()
-        prompt_dict["input_variables"] = list(
-            set(self.input_variables).difference(kwargs)
+        input_vars = set(self.input_variables).difference(kwargs)
+        partial_vars = {}
+        for key, partial_var in kwargs.items():
+            if isinstance(partial_var, BasePromptTemplate):
+                # Prepare partial arguments, excluding the current key
+                new_kwargs = kwargs.copy()
+                new_kwargs.pop(key)
+                partial_var = partial_var.partial(**new_kwargs)
+            partial_vars[key] = partial_var
+        prompt_dict.update(
+            {
+                "input_variables": list(input_vars),
+                "partial_variables": {**kwargs, **partial_vars},
+            }
         )
-        prompt_dict["partial_variables"] = {**self.partial_variables, **kwargs}
         return type(self)(**prompt_dict)
 
     def _merge_partial_and_user_variables(self, **kwargs: Any) -> dict[str, Any]:
         # Get partial params:
-        partial_kwargs = {
-            k: v if not callable(v) else v() for k, v in self.partial_variables.items()
-        }
+        partial_kwargs = {}
+        for k, v in self.partial_variables.items():
+            if isinstance(v, BasePromptTemplate):
+                # Propagate partial variables and kwargs to nested prompt templates
+                partial_kwargs[k] = v.format(**kwargs)
+            elif callable(v):
+                partial_kwargs[k] = v()
+            else:
+                partial_kwargs[k] = v
         return {**partial_kwargs, **kwargs}
 
     @abstractmethod

libs/core/langchain_core/prompts/prompt.py

@@ -8,6 +8,7 @@
 
 from pydantic import BaseModel, model_validator
 
+from langchain_core.prompts import BasePromptTemplate
 from langchain_core.prompts.string import (
     DEFAULT_FORMATTER_MAPPING,
     PromptTemplateFormat,
@@ -104,12 +105,24 @@ def pre_init_validation(cls, values: dict) -> Any:
             )
 
         if values["template_format"]:
+            # Collect nested partial variables from
+            # BasePromptTemplate instances in partial_variables
+            nested_partial_vars = {
+                key
+                for partial_var in values["partial_variables"].values()
+                if isinstance(partial_var, BasePromptTemplate)
+                for key in partial_var.partial_variables
+            }
+
+            # Filter template variables based on
+            # partial_variables and nested_partial_vars
             values["input_variables"] = [
                 var
                 for var in get_template_variables(
                     values["template"], values["template_format"]
                 )
                 if var not in values["partial_variables"]
+                and var not in nested_partial_vars
             ]
 
         return values

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

@@ -403,6 +403,58 @@ def test_partial() -> None:
     assert result == "This is a foo test."
 
 
+def test_nested_prompt_template_as_partial() -> None:
+    """Test prompt with PromptTemplate as partial variable."""
+    template_nested = "{bar}"
+    prompt_nested = PromptTemplate(input_variables=["bar"], template=template_nested)
+
+    template = "This is a {foo} test."
+    prompt = PromptTemplate(input_variables=["foo"], template=template)
+    assert prompt.template == template
+    assert prompt.input_variables == ["foo"]
+
+    new_prompt = prompt.partial(foo=prompt_nested)
+    assert new_prompt.input_variables == []
+    assert new_prompt.partial_variables["foo"].input_variables == ["bar"]
+    assert new_prompt.partial_variables["foo"].partial_variables == {}
+    result = new_prompt.format(bar="bar")
+    assert result == "This is a bar test."
+
+    new_prompt = prompt.partial(foo=prompt_nested, bar="bar")
+    assert new_prompt.input_variables == []
+    assert new_prompt.partial_variables["foo"].input_variables == []
+    assert new_prompt.partial_variables["foo"].partial_variables == {"bar": "bar"}
+    result = new_prompt.format()
+    assert result == "This is a bar test."
+
+
+def test_nested_prompt_template_with_shared_variable() -> None:
+    """Test prompt with PromptTemplate as partial variable, sharing another variable."""
+    template_nested = "{bar}"
+    prompt_nested = PromptTemplate(
+        input_variables=["bar", "foo"], template=template_nested
+    )
+
+    template = "This is a {foo} {bar} test."
+    prompt = PromptTemplate(input_variables=["foo", "bar"], template=template)
+    assert prompt.template == template
+    assert prompt.input_variables == ["bar", "foo"]
+
+    new_prompt = prompt.partial(foo=prompt_nested)
+    assert new_prompt.input_variables == ["bar"]
+    assert new_prompt.partial_variables["foo"].input_variables == ["bar"]
+    assert new_prompt.partial_variables["foo"].partial_variables == {}
+    result = new_prompt.format(bar="bar")
+    assert result == "This is a bar bar test."
+
+    new_prompt = prompt.partial(foo=prompt_nested, bar="bar")
+    assert new_prompt.input_variables == []
+    assert new_prompt.partial_variables["foo"].input_variables == []
+    assert new_prompt.partial_variables["foo"].partial_variables == {"bar": "bar"}
+    result = new_prompt.format()
+    assert result == "This is a bar bar test."
+
+
 @pytest.mark.requires("jinja2")
 def test_prompt_from_jinja2_template() -> None:
     """Test prompts can be constructed from a jinja2 template."""
@@ -508,7 +560,7 @@ def test_prompt_jinja2_missing_input_variables() -> None:
 
 @pytest.mark.requires("jinja2")
 def test_prompt_jinja2_extra_input_variables() -> None:
-    """Test error is raised when there are too many input variables."""
+    """Test warning is raised when there are too many input variables."""
     template = "This is a {{ foo }} test."
     input_variables = ["foo", "bar"]
     with pytest.warns(UserWarning):
@@ -525,7 +577,7 @@ def test_prompt_jinja2_extra_input_variables() -> None:
 
 @pytest.mark.requires("jinja2")
 def test_prompt_jinja2_wrong_input_variables() -> None:
-    """Test error is raised when name of input variable is wrong."""
+    """Test warning is raised when name of input variable is wrong."""
     template = "This is a {{ foo }} test."
     input_variables = ["bar"]
     with pytest.warns(UserWarning):