Fix items default value issue in track_ecommerce_transaction (closes #252)

Author: adatzer

snowplow_tracker/test/unit/test_tracker.py

@@ -745,7 +745,7 @@ def test_track_ecommerce_transaction_no_items(self, mok_complete_payload):
             t = Tracker(e)
             ctx = SelfDescribingJson("test.context.schema", {"user": "tester"})
             evTstamp = 1399021242030
-            t.track_ecommerce_transaction("1234", 10, "transAffiliation", 2.5, 1.5, "transCity", "transState", "transCountry", "transCurrency", items=[], context=[ctx], tstamp=evTstamp)
+            t.track_ecommerce_transaction("1234", 10, "transAffiliation", 2.5, 1.5, "transCity", "transState", "transCountry", "transCurrency", context=[ctx], tstamp=evTstamp)
             self.assertEqual(mok_complete_payload.call_count, 1)
             completeArgsList = mok_complete_payload.call_args_list[0][0]
             self.assertEqual(len(completeArgsList), 4)

snowplow_tracker/tracker.py

@@ -540,7 +540,7 @@ def track_ecommerce_transaction(self, order_id, total_value, affiliation=None,
             :param  currency:       The currency the price is expressed in
             :type   currency:       string_or_none
             :param  items:          The items in the transaction
-            :type   items:          list(dict(str:*))
+            :type   items:          list(dict(str:*)) | None
             :param  context:        Custom context for the event
             :type   context:        context_array | None
             :param  tstamp:         Optional event timestamp in milliseconds
@@ -565,6 +565,8 @@ def track_ecommerce_transaction(self, order_id, total_value, affiliation=None,
 
         self.complete_payload(pb, context, tstamp, event_subject)
 
+        if items is None:
+            items = []
         for item in items:
             item["tstamp"] = tstamp
             item["event_subject"] = event_subject