Support confirmation of SetupIntents with a payment method ID

Previously localstripe only supported confirmation of SetupIntents with
payment_method_data, which is a dictionary containing the information needed
to create a PaymentMethod. The Stripe SetupIntents API also allows the caller
to pass in an already-created PaymentMethod, by ID. This change adds that
capability to localstripe.

localstripe/resources.py

@@ -2195,6 +2195,10 @@ def _api_detach(cls, id, **kwargs):
 
     @classmethod
     def _api_retrieve(cls, id):
+        obj = cls._try_get_canonical_test_article(id)
+        if obj:
+            return obj
+
         # https://stripe.com/docs/payments/payment-methods#transitioning
         # You can retrieve all saved compatible payment instruments through the
         # Payment Methods API.
@@ -2205,6 +2209,41 @@ def _api_retrieve(cls, id):
 
         return super()._api_retrieve(id)
 
+    @classmethod
+    def _try_get_canonical_test_article(cls, id):
+        """Convert special payment method IDs into payment method objects.
+
+        See https://docs.stripe.com/testing?testing-method=payment-methods.
+
+        Oddly, as we do here, Stripe will convert these special test IDs into
+        actual objects and store them on a GET request, meaning the GET has
+        side effects and is not idempotent."""
+
+        if id == 'pm_card_visa':
+            return PaymentMethod(
+                type='card',
+                card=dict(
+                    number='4242424242424242',
+                    exp_month='12',
+                    exp_year='2030',
+                    cvc='123'))
+        if id == 'pm_card_visa_chargeDeclined':
+            return PaymentMethod(
+                type='card',
+                card=dict(
+                    number='4000000000000002',
+                    exp_month='12',
+                    exp_year='2030',
+                    cvc='123'))
+        if id == 'pm_card_chargeCustomerFail':
+            return PaymentMethod(
+                type='card',
+                card=dict(
+                    number='4000000000000341',
+                    exp_month='12',
+                    exp_year='2030',
+                    cvc='123'))
+
     @classmethod
     def _api_list_all(cls, url, customer=None, type=None, limit=None,
                       starting_after=None):
@@ -2696,7 +2735,7 @@ def __init__(self, customer=None, usage=None, payment_method_types=None,
 
     @classmethod
     def _api_confirm(cls, id, use_stripe_sdk=None, client_secret=None,
-                     payment_method_data=None, **kwargs):
+                     payment_method=None, payment_method_data=None, **kwargs):
         if kwargs:
             raise UserError(400, 'Unexpected ' + ', '.join(kwargs.keys()))
 
@@ -2714,27 +2753,13 @@ def _api_confirm(cls, id, use_stripe_sdk=None, client_secret=None,
         if client_secret and client_secret != obj.client_secret:
             raise UserError(401, 'Unauthorized')
 
-        if payment_method_data:
-            if obj.payment_method is not None:
-                raise UserError(400, 'Bad request')
-
+        if payment_method is not None:
+            assert isinstance(payment_method, str)
+            pm = PaymentMethod._api_retrieve(payment_method)
+            obj._attach_pm(pm)
+        elif payment_method_data is not None:
             pm = PaymentMethod(**payment_method_data)
-            obj.payment_method = pm.id
-
-            if pm._attaching_is_declined():
-                obj.status = 'canceled'
-                obj.next_action = None
-                raise UserError(402, 'Your card was declined.',
-                                {'code': 'card_declined'})
-            elif pm._requires_authentication():
-                obj.status = 'requires_action'
-                obj.next_action = {'type': 'use_stripe_sdk',
-                                   'use_stripe_sdk': {
-                                       'type': 'three_d_secure_redirect',
-                                       'stripe_js': ''}}
-            else:
-                obj.status = 'succeeded'
-                obj.next_action = None
+            obj._attach_pm(pm)
         elif obj.payment_method is None:
             obj.status = 'requires_payment_method'
             obj.next_action = None
@@ -2743,6 +2768,25 @@ def _api_confirm(cls, id, use_stripe_sdk=None, client_secret=None,
             obj.next_action = None
         return obj
 
+    def _attach_pm(self, pm):
+        self.payment_method = pm.id
+        self.payment_method_types = [pm.type]
+
+        if pm._attaching_is_declined():
+            self.status = 'canceled'
+            self.next_action = None
+            raise UserError(402, 'Your card was declined.',
+                            {'code': 'card_declined'})
+        elif pm._requires_authentication():
+            self.status = 'requires_action'
+            self.next_action = {'type': 'use_stripe_sdk',
+                                'use_stripe_sdk': {
+                                    'type': 'three_d_secure_redirect',
+                                    'stripe_js': ''}}
+        else:
+            self.status = 'succeeded'
+            self.next_action = None
+
     @classmethod
     def _api_cancel(cls, id, use_stripe_sdk=None, client_secret=None,
                     **kwargs):

test.sh

@@ -560,35 +560,35 @@ cus=$(curl -sSfg -u $SK: $HOST/v1/customers \
            -d [email protected] \
       | grep -oE 'cus_\w+' | head -n 1)
 
-pm=$(curl -sSfg -u $SK: $HOST/v1/payment_methods \
+pm_card_okay=$(curl -sSfg -u $SK: $HOST/v1/payment_methods \
           -d type=card \
           -d card[number]=4242424242424242 \
           -d card[exp_month]=12 \
           -d card[exp_year]=2020 \
           -d card[cvc]=123 \
      | grep -oE 'pm_\w+' | head -n 1)
 
-curl -sSfg -u $SK: $HOST/v1/payment_methods/$pm/attach \
+curl -sSfg -u $SK: $HOST/v1/payment_methods/$pm_card_okay/attach \
      -d customer=$cus
 
 curl -sSfg -u $SK: $HOST/v1/customers/$cus \
-     -d invoice_settings[default_payment_method]=$pm
+     -d invoice_settings[default_payment_method]=$pm_card_okay
 
 curl -sSfg -u $SK: $HOST/v1/customers/$cus?expand[]=invoice_settings.default_payment_method
 
 curl -sSfg -u $SK: $HOST/v1/payment_methods?customer=$cus\&type=card
 
-curl -sSfg -u $SK: $HOST/v1/payment_methods/$pm/detach -X POST
+curl -sSfg -u $SK: $HOST/v1/payment_methods/$pm_card_okay/detach -X POST
 
-pm=$(curl -sSfg -u $SK: $HOST/v1/payment_methods \
+pm_card_decline_on_attach=$(curl -sSfg -u $SK: $HOST/v1/payment_methods \
           -d type=card \
           -d card[number]=4000000000000002 \
           -d card[exp_month]=4 \
           -d card[exp_year]=2042 \
           -d card[cvc]=123 \
      | grep -oE 'pm_\w+' | head -n 1)
 code=$(curl -sg -o /dev/null -w "%{http_code}" -u $SK: \
-            $HOST/v1/payment_methods/$pm/attach \
+            $HOST/v1/payment_methods/$pm_card_decline_on_attach/attach \
             -d customer=$cus)
 [ "$code" = 402 ]
 
@@ -618,6 +618,53 @@ curl -sSfg $HOST/v1/setup_intents/$seti/confirm \
      -d payment_method_data[card][exp_year]=24 \
      -d payment_method_data[billing_details][address][postal_code]=42424
 
+# We can also pass a payment method ID to setup_intents/*/confirm:
+res=$(curl -sSfg -u $SK: $HOST/v1/setup_intents -X POST)
+seti=$(echo "$res" | grep '"id"' | grep -oE 'seti_\w+' | head -n 1)
+seti_secret=$(echo $res | grep -oE 'seti_\w+_secret_\w+' | head -n 1)
+status=$(
+  curl -sSfg $HOST/v1/setup_intents/$seti/confirm \
+       -d key=pk_test_sldkjflaksdfj \
+       -d client_secret=$seti_secret \
+       -d payment_method=$pm_card_okay \
+    | grep -oE '"status": "succeeded"')
+[ -n "$status" ]
+
+# ... and payment method IDs on bad cards fail on setup_intents/*/confirm:
+res=$(curl -sSfg -u $SK: $HOST/v1/setup_intents -X POST)
+seti=$(echo "$res" | grep '"id"' | grep -oE 'seti_\w+' | head -n 1)
+seti_secret=$(echo $res | grep -oE 'seti_\w+_secret_\w+' | head -n 1)
+code=$(
+  curl -sg -w "%{http_code}" -o /dev/null $HOST/v1/setup_intents/$seti/confirm \
+       -d key=pk_test_sldkjflaksdfj \
+       -d client_secret=$seti_secret \
+       -d payment_method=$pm_card_decline_on_attach)
+[ "$code" = 402 ]
+
+# We can also pass a special well-known payment method ID to
+# setup_intents/*/confirm:
+res=$(curl -sSfg -u $SK: $HOST/v1/setup_intents -X POST)
+seti=$(echo "$res" | grep '"id"' | grep -oE 'seti_\w+' | head -n 1)
+seti_secret=$(echo $res | grep -oE 'seti_\w+_secret_\w+' | head -n 1)
+status=$(
+  curl -sSfg $HOST/v1/setup_intents/$seti/confirm \
+       -d key=pk_test_sldkjflaksdfj \
+       -d client_secret=$seti_secret \
+       -d payment_method=pm_card_visa \
+    | grep -oE '"status": "succeeded"')
+[ -n "$status" ]
+
+# ... including well-known bad payment method IDs:
+res=$(curl -sSfg -u $SK: $HOST/v1/setup_intents -X POST)
+seti=$(echo "$res" | grep '"id"' | grep -oE 'seti_\w+' | head -n 1)
+seti_secret=$(echo $res | grep -oE 'seti_\w+_secret_\w+' | head -n 1)
+code=$(
+  curl -sg -w "%{http_code}" -o /dev/null $HOST/v1/setup_intents/$seti/confirm \
+       -d key=pk_test_sldkjflaksdfj \
+       -d client_secret=$seti_secret \
+       -d payment_method=pm_card_visa_chargeDeclined)
+[ "$code" = 402 ]
+
 # off_session cannot be used when confirm is false
 code=$(
   curl -sg -o /dev/null -w "%{http_code}" \