Better Entity Memory code documentation (#6318)

Just adds some comments and docstring improvements.

There was some behaviour that was quite unclear to me at first like:
- "when do things get updated?"
- "why are there only entity names and no summaries?"
- "why do the entity names disappear?" 

Now it can be much more obvious to many.

I am lukestanley on Twitter.

langchain/memory/entity.py

@@ -241,20 +241,35 @@ class SQLiteEntityStore(BaseEntityStore):
 
 
 class ConversationEntityMemory(BaseChatMemory):
-    """Entity extractor & summarizer to memory."""
+    """Entity extractor & summarizer memory.
+
+    Extracts named entities from the recent chat history and generates summaries.
+    With a swapable entity store, persisting entities across conversations.
+    Defaults to an in-memory entity store, and can be swapped out for a Redis,
+    SQLite, or other entity store.
+    """
 
     human_prefix: str = "Human"
     ai_prefix: str = "AI"
     llm: BaseLanguageModel
     entity_extraction_prompt: BasePromptTemplate = ENTITY_EXTRACTION_PROMPT
     entity_summarization_prompt: BasePromptTemplate = ENTITY_SUMMARIZATION_PROMPT
+
+    # Cache of recently detected entity names, if any
+    # It is updated when load_memory_variables is called:
     entity_cache: List[str] = []
+
+    # Number of recent message pairs to consider when updating entities:
     k: int = 3
+
     chat_history_key: str = "history"
+
+    # Store to manage entity-related data:
     entity_store: BaseEntityStore = Field(default_factory=InMemoryEntityStore)
 
     @property
     def buffer(self) -> List[BaseMessage]:
+        """Access chat memory messages."""
         return self.chat_memory.messages
 
     @property
@@ -266,40 +281,78 @@ class ConversationEntityMemory(BaseChatMemory):
         return ["entities", self.chat_history_key]
 
     def load_memory_variables(self, inputs: Dict[str, Any]) -> Dict[str, Any]:
-        """Return history buffer."""
+        """
+        Returns chat history and all generated entities with summaries if available,
+        and updates or clears the recent entity cache.
+
+        New entity name can be found when calling this method, before the entity
+        summaries are generated, so the entity cache values may be empty if no entity
+        descriptions are generated yet.
+        """
+
+        # Create an LLMChain for predicting entity names from the recent chat history:
         chain = LLMChain(llm=self.llm, prompt=self.entity_extraction_prompt)
+
         if self.input_key is None:
             prompt_input_key = get_prompt_input_key(inputs, self.memory_variables)
         else:
             prompt_input_key = self.input_key
+
+        # Extract an arbitrary window of the last message pairs from
+        # the chat history, where the hyperparameter k is the
+        # number of message pairs:
         buffer_string = get_buffer_string(
             self.buffer[-self.k * 2 :],
             human_prefix=self.human_prefix,
             ai_prefix=self.ai_prefix,
         )
+
+        # Generates a comma-separated list of named entities,
+        # e.g. "Jane, White House, UFO"
+        # or "NONE" if no named entities are extracted:
         output = chain.predict(
             history=buffer_string,
             input=inputs[prompt_input_key],
         )
+
+        # If no named entities are extracted, assigns an empty list.
         if output.strip() == "NONE":
             entities = []
         else:
+            # Make a list of the extracted entities:
             entities = [w.strip() for w in output.split(",")]
+
+        # Make a dictionary of entities with summary if exists:
         entity_summaries = {}
+
         for entity in entities:
             entity_summaries[entity] = self.entity_store.get(entity, "")
+
+        # Replaces the entity name cache with the most recently discussed entities,
+        # or if no entities were extracted, clears the cache:
         self.entity_cache = entities
+
+        # Should we return as message objects or as a string?
         if self.return_messages:
+            # Get last `k` pair of chat messages:
             buffer: Any = self.buffer[-self.k * 2 :]
         else:
+            # Reuse the string we made earlier:
             buffer = buffer_string
+
         return {
             self.chat_history_key: buffer,
             "entities": entity_summaries,
         }
 
     def save_context(self, inputs: Dict[str, Any], outputs: Dict[str, str]) -> None:
-        """Save context from this conversation to buffer."""
+        """
+        Save context from this conversation history to the entity store.
+
+        Generates a summary for each entity in the entity cache by prompting
+        the model, and saves these summaries to the entity store.
+        """
+
         super().save_context(inputs, outputs)
 
         if self.input_key is None:
@@ -307,15 +360,23 @@ class ConversationEntityMemory(BaseChatMemory):
         else:
             prompt_input_key = self.input_key
 
+        # Extract an arbitrary window of the last message pairs from
+        # the chat history, where the hyperparameter k is the
+        # number of message pairs:
         buffer_string = get_buffer_string(
             self.buffer[-self.k * 2 :],
             human_prefix=self.human_prefix,
             ai_prefix=self.ai_prefix,
         )
+
         input_data = inputs[prompt_input_key]
+
+        # Create an LLMChain for predicting entity summarization from the context
         chain = LLMChain(llm=self.llm, prompt=self.entity_summarization_prompt)
 
+        # Generate new summaries for entities and save them in the entity store
         for entity in self.entity_cache:
+            # Get existing summary if it exists
             existing_summary = self.entity_store.get(entity, "")
             output = chain.predict(
                 summary=existing_summary,
@@ -323,6 +384,7 @@ class ConversationEntityMemory(BaseChatMemory):
                 history=buffer_string,
                 input=input_data,
             )
+            # Save the updated summary to the entity store
             self.entity_store.set(entity, output.strip())
 
     def clear(self) -> None: