add inline comment for python tests

python/pyspark/sql/tests.py

@@ -710,6 +710,7 @@ class SQLTests(ReusedPySparkTestCase):
         from pyspark.sql.tests import UTC
         utc = UTC()
         utcnow = datetime.datetime.utcfromtimestamp(ts)  # without microseconds
+        # add microseconds to utcnow (keeping year,month,day,hour,minute,second)
         utcnow = datetime.datetime(*(utcnow.timetuple()[:6] + (now.microsecond, utc)))
         df = self.sqlCtx.createDataFrame([(day, now, utcnow)])
         day1, now1, utcnow1 = df.first()