From 528fc76d6a4bb9c59a12fe0eded4e8d9d55ca544 Mon Sep 17 00:00:00 2001
From: William FH <13333726+hinthornw@users.noreply.github.com>
Date: Fri, 1 Dec 2023 09:06:35 -0800
Subject: [PATCH] Update Prompt Format Error (#14044)

The number of times I try to format a string (especially in lcel) is
embarrassingly high. Think this may be more actionable than the default
error message. Now I get nice helpful errors


```
KeyError: "Input to ChatPromptTemplate is missing variable 'input'.  Expected: ['input'] Received: ['dialogue']"
```
---
 libs/core/langchain_core/prompts/base.py | 20 +++++++++++++++++---
 1 file changed, 17 insertions(+), 3 deletions(-)

diff --git a/libs/core/langchain_core/prompts/base.py b/libs/core/langchain_core/prompts/base.py
index dc5003e552..21b31650ff 100644
--- a/libs/core/langchain_core/prompts/base.py
+++ b/libs/core/langchain_core/prompts/base.py
@@ -67,13 +67,27 @@ class BasePromptTemplate(RunnableSerializable[Dict, PromptValue], ABC):
             **{k: (self.input_types.get(k, str), None) for k in self.input_variables},
         )
 
+    def _format_prompt_with_error_handling(self, inner_input: Dict) -> PromptValue:
+        try:
+            input_dict = {key: inner_input[key] for key in self.input_variables}
+        except TypeError as e:
+            raise TypeError(
+                f"Expected mapping type as input to {self.__class__.__name__}. "
+                f"Received {type(inner_input)}."
+            ) from e
+        except KeyError as e:
+            raise KeyError(
+                f"Input to {self.__class__.__name__} is missing variable {e}. "
+                f" Expected: {self.input_variables}"
+                f" Received: {list(inner_input.keys())}"
+            ) from e
+        return self.format_prompt(**input_dict)
+
     def invoke(
         self, input: Dict, config: Optional[RunnableConfig] = None
     ) -> PromptValue:
         return self._call_with_config(
-            lambda inner_input: self.format_prompt(
-                **{key: inner_input[key] for key in self.input_variables}
-            ),
+            self._format_prompt_with_error_handling,
             input,
             config,
             run_type="prompt",