Idempotent approval: no double charges on resume

"""Side effects must not repeat when a node re-executes on resume after interrupt().
Shows: (1) NAIVE: side effect before interrupt() in the same node -> runs twice.
       (2) FIXED: split nodes + idempotency key (derived from thread_id) checked against a ledger,
           so even a retried/re-run execute node charges once. No LLM; all LangGraph code real.
The 'payment API' is a fake in-process ledger (stands in for Stripe etc.)."""
from typing import TypedDict
from langgraph.graph import StateGraph, START, END
from langgraph.checkpoint.memory import InMemorySaver
from langgraph.types import interrupt, Command

class FakePaymentAPI:
    def __init__(self): self.charges = []; self.by_key = {}
    def charge(self, amount, idempotency_key=None):
        if idempotency_key and idempotency_key in self.by_key:   # server-side dedupe, like Stripe
            return self.by_key[idempotency_key]
        cid = f"ch_{len(self.charges)+1}"; self.charges.append((cid, amount))
        if idempotency_key: self.by_key[idempotency_key] = cid
        return cid

class S(TypedDict, total=False):
    amount: int
    charge_id: str
    approved: bool

def naive_graph(api):
    def node(s):
        api.charge(s["amount"])                       # BAD: before interrupt -> re-runs on resume
        ok = interrupt({"approve_charge": s["amount"]})
        return {"approved": ok}
    b = StateGraph(S); b.add_node("node", node); b.add_edge(START, "node"); b.add_edge("node", END)

# ... PREVIEW ENDS HERE: first 30 of 57 lines (setup + the NAIVE version).
# The FIXED version and the runnable scenarios are in the Signal Pro pack.

langgraph==1.2.15
langgraph-checkpoint==4.3.0

== NAIVE: after pause, charges=[('ch_1', 1999)]
   after resume, charges=[('ch_1', 1999), ('ch_2', 1999)] state={'amount': 1999, 'approved': True}
== FIXED: after pause, charges=[]
   after resume, charges=[('ch_1', 1999)] state={'amount': 1999, 'charge_id': 'ch_1', 'approved': True}
   after replaying execute from checkpoint, charges=[('ch_1', 1999)]  (still 1 -> idempotent)

MIT License

Copyright (c) 2026 Agent Signal

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.