What does __call__ function do in langgraph

Yes, the __call__ method is indeed invoked when the instance of the ReturnNodeValue class is used in the context of the LangGraph node. Here's an explanation of how it works:

Code Breakdown

Class Definition (ReturnNodeValue):

This class has an __init__ method, which initializes the object with a value called node_secret.

It also defines the __call__ method, which allows an instance of the class to be "called" like a function, passing the state argument.

The __call__ Method:

This method takes in a state (likely a State object in LangGraph), prints a message, and returns a dictionary updating the "aggregate" key with the value stored in self._value.

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def __call__(self, state: State) -> Any:

    print(f"Adding {self._value} to {state['aggregate']}")

    return {"aggregate": [self._value]}

When the __call__ method is invoked, it manipulates the state by adding the value to the "aggregate" key.

Using ReturnNodeValue as a Callable:

The line ReturnNodeValue("I'm A") creates an instance of the ReturnNodeValue class with the string "I'm A" as the node_secret.

In Python, if a class defines the __call__ method, then instances of that class can be called as if they were functions.

Adding the Node to the Graph:

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builder.add_node("a", ReturnNodeValue("I'm A"))

This line adds a node labeled "a" to the LangGraph using the ReturnNodeValue("I'm A") instance as the node's callable value.

When this node is executed, it will trigger the __call__ method, passing in the current state.

Does it call __call__?

Yes, when the graph execution framework (LangGraph in this case) reaches node "a", it will invoke ReturnNodeValue("I'm A") like a function. This automatically calls the __call__ method, updating the state and returning the modified value.

Example Execution:

When the node is executed, you will see:

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Adding I'm A to <current state of 'aggregate'>

This is because __call__ is printing that message when invoked.

Summary:

In this example:

The class ReturnNodeValue is defined with an __call__ method.

When ReturnNodeValue("I'm A") is added to the graph, it is used as a callable object.

The LangGraph framework will invoke the __call__ method of this class instance when the node is processed in the graph execution.