add output parser

langchain/prompts/base.py

@@ -1,8 +1,8 @@
 """BasePrompt schema definition."""
 from abc import ABC, abstractmethod
-from typing import Any, Dict, List
+from typing import Any, Dict, List, Union
 
-from pydantic import BaseModel, root_validator
+from pydantic import BaseModel, Field, root_validator
 
 from langchain.formatting import formatter
 
@@ -29,11 +29,29 @@ def check_valid_template(
         raise ValueError("Invalid prompt schema.")
 
 
+class OutputParser(ABC):
+    """Class to parse the output of an LLM call."""
+
+    @abstractmethod
+    def parse(self, text: str) -> Union[str, List[str], Dict[str, str]]:
+        """Parse the output of an LLM call."""
+
+
+class DefaultParser(OutputParser):
+    """Just return the text."""
+
+    def parse(self, text: str) -> Union[str, List[str], Dict[str, str]]:
+        """Parse the output of an LLM call."""
+        return text
+
+
 class BasePromptTemplate(BaseModel, ABC):
     """Base prompt should expose the format method, returning a prompt."""
 
     input_variables: List[str]
     """A list of the names of the variables the prompt template expects."""
+    output_parser: OutputParser = Field(default_factory=DefaultParser)
+    """How to parse the output of calling an LLM on this formatted prompt."""
 
     @root_validator()
     def validate_variable_names(cls, values: Dict) -> Dict: