From 31660aa14c04ca149339c5ff52c338a921b2a122 Mon Sep 17 00:00:00 2001
From: vishkaty <147529812+vishkaty@users.noreply.github.com>
Date: Wed, 29 Jul 2026 11:43:20 -0400
Subject: [PATCH] fix(rest/python): seed valid totals when creating a checkout

The SDK now enforces the totals contains cardinality (exactly one subtotal
and one total) on the response models. create_checkout constructed the
checkout with an empty totals list and filled it in during recalculation,
so construction validated the empty list and rejected it, returning 500 on
every create.

Seed a valid placeholder (one subtotal + one total) at construction so the
in-progress model satisfies the constraint; _recalculate_totals overwrites
it with the server-authoritative amounts immediately after. Construction
stays fully validated, and the emitted wire document is unchanged.

Verified against the pinned SDK build with the totals validators active:
full conformance suite 69 passing / 0 failures (was 67 failures). Also
green without the validators: server integration tests (16) and the
happy-path client, so it is safe to land independently.

Relates to Universal-Commerce-Protocol/python-sdk#57
---
 rest/python/server/services/checkout_service.py | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/rest/python/server/services/checkout_service.py b/rest/python/server/services/checkout_service.py
index c3f445e..a209f23 100644
--- a/rest/python/server/services/checkout_service.py
+++ b/rest/python/server/services/checkout_service.py
@@ -286,6 +286,12 @@ async def create_checkout(
 
       fulfillment_resp = FulfillmentResponseClass(methods=resp_methods)
 
+    # The SDK enforces the totals contains cardinality (exactly one subtotal
+    # and one total) on the response model. The server is the authority for
+    # totals and computes them in _recalculate_totals below, but that runs
+    # after construction. Seed a valid placeholder so the in-progress model
+    # satisfies the constraint at construction; it is overwritten with the
+    # authoritative amounts immediately. See python-sdk#57.
     checkout = Checkout(
       ucp=ResponseCheckout(
         version=config.get_server_version(),
@@ -303,7 +309,10 @@ async def create_checkout(
       status=CheckoutStatus.IN_PROGRESS,
       currency=checkout_req.currency,
       line_items=line_items,
-      totals=[],
+      totals=[
+        {"type": "subtotal", "amount": 0},
+        {"type": "total", "amount": 0},
+      ],
       links=[],
       payment=PaymentResponse(
         instruments=checkout_req.payment.instruments