Clear spans list after finish (#64)

Author: liyanhui1228

trace/opencensus/trace/span.py

@@ -174,8 +174,13 @@ def format_span_json(span):
         'endTime': span.end_time,
     }
 
+    parent_span_id = None
+
     if span.parent_span is not None:
-        span_json['parentSpanId'] = span.parent_span.span_id
+        parent_span_id = span.parent_span.span_id
+
+    if parent_span_id is not None:
+        span_json['parentSpanId'] = parent_span_id
 
     if span.labels is not None:
         span_json['labels'] = span.labels

trace/opencensus/trace/tracer/context_tracer.py

@@ -45,6 +45,7 @@ def finish(self):
         :returns: JSON format trace.
         """
         trace = self._get_trace_json()
+        self._spans_list = []
 
         return trace
 

trace/tests/unit/tracer/test_context_tracer.py

@@ -50,6 +50,7 @@ def test_finish_without_spans(self):
         trace = tracer.finish()
 
         self.assertIsNone(trace)
+        self.assertEqual(tracer._spans_list, [])
 
     def test_finish_with_spans(self):
         from opencensus.trace.enums import Enum
@@ -125,6 +126,7 @@ def test_finish_with_spans(self):
         trace_json = tracer.finish()
 
         self.assertEqual(trace_json, trace)
+        self.assertEqual(tracer._spans_list, [])
 
     @mock.patch.object(context_tracer.ContextTracer, 'current_span')
     def test_span(self, current_span_mock):