Be sure not to write lines if we are measuring arcs

2 files changed, 3 insertions, 3 deletions

diff --git a/coverage/data.py b/coverage/data.py
index adacaecc..be34da1b 100644
--- a/coverage/data.py
+++ b/coverage/data.py
@@ -147,10 +147,10 @@ class CoverageData(object):
         # Create the file data.
         file_data = {}
 
-        file_data['lines'] = dict((f, list(lmap.keys())) for f, lmap in iitems(self._lines))
-
         if self._arcs:
             file_data['arcs'] = dict((f, list(amap.keys())) for f, amap in iitems(self._arcs))
+        else:
+            file_data['lines'] = dict((f, list(lmap.keys())) for f, lmap in iitems(self._lines))
 
         if self._collector:
             file_data['collector'] = self._collector
diff --git a/tests/test_data.py b/tests/test_data.py
index 734c3423..6278e36a 100644
--- a/tests/test_data.py
+++ b/tests/test_data.py
@@ -174,7 +174,7 @@ class DataFilesTest(DataTestHelpers, CoverageTest):
         with open(".coverage", 'rb') as fdata:
             data = pickle.load(fdata)
 
-        self.assertCountEqual(data['lines'].keys(), [])
+        self.assertNotIn('lines', data)
         arcs = data['arcs']
         self.assertCountEqual(arcs['x.py'], X_PY_ARCS_3)
         self.assertCountEqual(arcs['y.py'], Y_PY_ARCS_3)