PromptTemplate update documentation and expand kwarg (#8423)

# PromptTemplate

* Update documentation to highlight the classmethod for instantiating a
prompt template.
* Expand kwargs in the classmethod to make parameters easier to discover

This PR got reverted here:
https://github.com/langchain-ai/langchain/pull/8395/files

libs/langchain/langchain/prompts/prompt.py

@@ -3,7 +3,7 @@ from __future__ import annotations
 
 from pathlib import Path
 from string import Formatter
-from typing import Any, Dict, List, Union
+from typing import Any, Dict, List, Optional, Union
 
 from pydantic import root_validator
 
@@ -16,12 +16,24 @@ from langchain.prompts.base import (
 
 
 class PromptTemplate(StringPromptTemplate):
-    """Schema to represent a prompt for an LLM.
+    """A prompt template for a language model.
+
+    A prompt template consists of a string template. It accepts a set of parameters
+    from the user that can be used to generate a prompt for a language model.
+
+    The template can be formatted using either f-strings (default) or jinja2 syntax.
 
     Example:
+
         .. code-block:: python
 
             from langchain import PromptTemplate
+
+            # Instantiation using from_template (recommended)
+            prompt = PromptTemplate.from_template("Say {foo}")
+            prompt.format(foo="bar")
+
+            # Instantiation using initializer
             prompt = PromptTemplate(input_variables=["foo"], template="Say {foo}")
     """
 
@@ -44,6 +56,7 @@ class PromptTemplate(StringPromptTemplate):
     """Whether or not to try validating the template."""
 
     def __add__(self, other: Any) -> PromptTemplate:
+        """Override the + operator to allow for combining prompt templates."""
         # Allow for easy combining
         if isinstance(other, PromptTemplate):
             if self.template_format != "f-string":
@@ -153,6 +166,7 @@ class PromptTemplate(StringPromptTemplate):
             template_file: The path to the file containing the prompt template.
             input_variables: A list of variable names the final prompt template
                 will expect.
+
         Returns:
             The prompt loaded from the file.
         """
@@ -161,25 +175,52 @@ class PromptTemplate(StringPromptTemplate):
         return cls(input_variables=input_variables, template=template, **kwargs)
 
     @classmethod
-    def from_template(cls, template: str, **kwargs: Any) -> PromptTemplate:
-        """Load a prompt template from a template."""
-        if "template_format" in kwargs and kwargs["template_format"] == "jinja2":
+    def from_template(
+        cls,
+        template: str,
+        *,
+        template_format: str = "f-string",
+        partial_variables: Optional[Dict[str, Any]] = None,
+        **kwargs: Any,
+    ) -> PromptTemplate:
+        """Load a prompt template from a template.
+
+        Args:
+            template: The template to load.
+            template_format: The format of the template. Use `jinja2` for jinja2,
+                             and `f-string` or None for f-strings.
+            partial_variables: A dictionary of variables that can be used to partially
+                               fill in the template. For example, if the template is
+                              `"{variable1} {variable2}"`, and `partial_variables` is
+                              `{"variable1": "foo"}`, then the final prompt will be
+                              `"foo {variable2}"`.
+
+        Returns:
+            The prompt template loaded from the template.
+        """
+        if template_format == "jinja2":
             # Get the variables for the template
             input_variables = _get_jinja2_variables_from_template(template)
-
-        else:
+        elif template_format == "f-string":
             input_variables = {
                 v for _, v, _, _ in Formatter().parse(template) if v is not None
             }
+        else:
+            raise ValueError(f"Unsupported template format: {template_format}")
 
-        if "partial_variables" in kwargs:
-            partial_variables = kwargs["partial_variables"]
+        _partial_variables = partial_variables or {}
+
+        if _partial_variables:
             input_variables = {
-                var for var in input_variables if var not in partial_variables
+                var for var in input_variables if var not in _partial_variables
             }
 
         return cls(
-            input_variables=list(sorted(input_variables)), template=template, **kwargs
+            input_variables=sorted(input_variables),
+            template=template,
+            template_format=template_format,
+            partial_variables=_partial_variables,
+            **kwargs,
         )
 
 

libs/langchain/tests/unit_tests/load/__snapshots__/test_dump.ambr

@@ -98,7 +98,8 @@
             "name"
           ],
           "template": "hello {name}!",
-          "template_format": "f-string"
+          "template_format": "f-string",
+          "partial_variables": {}
         }
       }
     }
@@ -176,7 +177,8 @@
                       "name"
                     ],
                     "template": "hello {name}!",
-                    "template_format": "f-string"
+                    "template_format": "f-string",
+                    "partial_variables": {}
                   }
                 }
               }
@@ -245,7 +247,8 @@
             "name"
           ],
           "template": "hello {name}!",
-          "template_format": "f-string"
+          "template_format": "f-string",
+          "partial_variables": {}
         }
       }
     }

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

@@ -161,6 +161,10 @@ Will it get confused{ }?
     )
     assert prompt == expected_prompt
 
+
+@pytest.mark.requires("jinja2")
+def test_prompt_from_jinja2_template_multiple_inputs() -> None:
+    """Test with multiple input variables."""
     # Multiple input variables.
     template = """\
 Hello world
@@ -186,7 +190,10 @@ You just set bar boolean variable to true
 
     assert prompt == expected_prompt
 
-    # Multiple input variables with repeats.
+
+@pytest.mark.requires("jinja2")
+def test_prompt_from_jinja2_template_multiple_inputs_with_repeats() -> None:
+    """Test with multiple input variables and repeats."""
     template = """\
 Hello world