Can a node 'reset' a state property decorated with the operator.add reducer?

[ziqizhang]

I know that if my state has a property like this:

lines: Annotated[List[str], operator.add]

Then it means whenever my node returns:

return {"lines": ["this is me", "my name is john"]}

it will append the values to the 'lines' property of my state. This is important and useful for branching, where more than 1 node may be writing to this property.

But I want to ask if it is possible for a node to 'reset' this 'lines' property so it does not append but clearly rewrites it? We can assume that there can be only one such operation at any time.

So

• a few nodes are 'adding' to 'lines' in parallel
• then at this node, it resets 'lines' completely.

Is this possible?

[hinthornw]

Good q! You can write your own reducer

def add_or_rewrite(existing, new):
    existing = existing or []
    if isinstance(new, dict) and new["kind"] == "rewrite":
        return new["value"]
    return existing + new

class State(TypedDict):
    lines: Annotated[List[str], add_or_rewrite]

...
 def regular_node(state):
     ....
    return {"lines": ["my name is john"]}
    
def rewriting_node(state):
    ....
    return {"lines": {"kind": "rewrite", "value": ["this is the wonderful wizard of oz"]}

operator.add only a function that adds a and b. Your function can either add a and b OR do something different depending on the value of b.

Lots a flexibilities! :)