add mako template

langchain/formatting.py

@@ -1,6 +1,7 @@
 """Utilities for formatting strings."""
 from string import Formatter
 from typing import Any, Mapping, Sequence, Union
+from mako.template import Template
 
 
 class StrictFormatter(Formatter):
@@ -28,5 +29,10 @@ class StrictFormatter(Formatter):
             )
         return super().vformat(format_string, args, kwargs)
 
+    def mako_format(self, format_string: str, **kwargs: Any) -> str:
+        """Format a string using mako."""
+        template = Template(format_string)
+        return template.render(**kwargs)
+
 
 formatter = StrictFormatter()

langchain/prompts/base.py

@@ -6,6 +6,7 @@ from langchain.formatting import formatter
 
 DEFAULT_FORMATTER_MAPPING = {
     "f-string": formatter.format,
+    "mako": formatter.mako_format,
 }
 
 

langchain/prompts/prompt.py

@@ -3,6 +3,7 @@ from typing import Any, Dict, List
 
 from pydantic import BaseModel, Extra, root_validator
 
+
 from langchain.prompts.base import (
     DEFAULT_FORMATTER_MAPPING,
     BasePromptTemplate,
@@ -106,6 +107,27 @@ class PromptTemplate(BaseModel, BasePromptTemplate):
             template = f.read()
         return cls(input_variables=input_variables, template=template)
 
+    @classmethod
+    def from_mako_template(
+        cls, template_file: str, input_variables: List[str]
+    ) -> "PromptTemplate":
+        """Load a prompt from a mako template file.
+
+        Args:
+            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 mako template file.
+        """
+        with open(template_file, "r") as f:
+            template = f.read()
+        return cls(
+            input_variables=input_variables,
+            template=template,
+            template_format="mako",
+        )
+
 
 # For backwards compatibility.
 Prompt = PromptTemplate

tests/unit_tests/data/mako_prompt.txt

@@ -0,0 +1 @@
+This is a ${foo} test.

tests/unit_tests/prompts/test_prompt.py

@@ -87,3 +87,12 @@ def test_prompt_from_file() -> None:
     input_variables = ["question"]
     prompt = PromptTemplate.from_file(template_file, input_variables)
     assert prompt.template == "Question: {question}\nAnswer:"
+
+
+def test_mako_template() -> None:
+    """Test mako template can be used."""
+    template_file = "tests/unit_tests/data/mako_prompt.txt"
+    input_variables = ["foo"]
+    prompt = PromptTemplate.from_mako_template(template_file, input_variables)
+    assert prompt.template == "This is a ${foo} test."
+    assert prompt.format(foo="bar") == "This is a bar test."