Updating with an empty data is ok

With no arc or line data, it used to choose lines arbitrarily, which
would fail if an empty data was updating an arc data.

1 files changed, 16 insertions, 0 deletions

diff --git a/tests/test_data.py b/tests/test_data.py
index e09aaf44..eb389e35 100644
--- a/tests/test_data.py
+++ b/tests/test_data.py
@@ -428,6 +428,22 @@ class CoverageDataTest(DataTestHelpers, CoverageTest):
         with self.assertRaisesRegex(CoverageException, msg):
             covdata2.update(covdata1)
 
+    def test_update_lines_empty(self):
+        covdata1 = CoverageData(suffix='1')
+        covdata1.add_lines(LINES_1)
+
+        covdata2 = CoverageData(suffix='2')
+        covdata1.update(covdata2)
+        self.assert_line_counts(covdata1, SUMMARY_1)
+
+    def test_update_arcs_empty(self):
+        covdata1 = CoverageData(suffix='1')
+        covdata1.add_arcs(ARCS_3)
+
+        covdata2 = CoverageData(suffix='2')
+        covdata1.update(covdata2)
+        self.assert_line_counts(covdata1, SUMMARY_3)
+
     def test_asking_isnt_measuring(self):
         # Asking about an unmeasured file shouldn't make it seem measured.
         covdata = CoverageData()