Remind customers before Xero invoices fall due

from datetime import date, timedelta
from clickruntime import on_trigger, credentials, http, state, notify

API = "https://api.xero.com/api.xro/2.0"

def xero_headers():
    token = credentials.connection("xero", "work")
    tenant = http.get("https://api.xero.com/connections", headers={"Authorization": f"Bearer {token}"}).json()[0]["tenantId"]
    return {"Authorization": f"Bearer {token}", "xero-tenant-id": tenant, "Accept": "application/json"}

DAYS_BEFORE = 3

@on_trigger
def run(event):
    headers = xero_headers()
    due = date.today() + timedelta(days=DAYS_BEFORE)
    where = (f'Type=="ACCREC" AND AmountDue>0 AND DueDate==DateTime({due.year},{due.month},{due.day})')
    invoices = http.get(f"{API}/Invoices", headers=headers, params={"Statuses": "AUTHORISED", "where": where}).json()["Invoices"]
    done = set(state.get("reminded") or [])
    sent = []
    for inv in invoices:
        if inv["InvoiceID"] in done:
            continue
        http.post(f"{API}/Invoices/{inv['InvoiceID']}/Email", headers=headers, json={}).raise_for_status()
        http.put(f"{API}/Invoices/{inv['InvoiceID']}/History", headers=headers,
                 json={"HistoryRecords": [{"Details": f"Pre-due reminder sent ({DAYS_BEFORE} days before due date)"}]})
        done.add(inv["InvoiceID"])
        sent.append(f"{inv['InvoiceNumber']} {inv['Contact']['Name']}: {inv['AmountDue']:,.2f}, due {due:%a %d %b}")
    state.set("reminded", list(done)[-2000:])
    if sent:
        notify.email_me(f"{len(sent)} invoices reminded before due date", "Sent through Xero today:\n" + "\n".join(sent))
    return {"sent": len(sent)}