Add test of the debug output when reading/writing data

2 files changed, 44 insertions, 2 deletions

diff --git a/tests/coveragetest.py b/tests/coveragetest.py
index 2f488616..a8535418 100644
--- a/tests/coveragetest.py
+++ b/tests/coveragetest.py
@@ -12,7 +12,7 @@ import sys
 import coverage
 from coverage.backunittest import TestCase
 from coverage.backward import StringIO, import_local_file, string_class
-from coverage.debug import _TEST_NAME_FILE
+from coverage.debug import _TEST_NAME_FILE, DebugControl
 from coverage.test_helpers import (
     EnvironmentAwareMixin, StdStreamCapturingMixin, TempDirMixin,
 )
@@ -373,3 +373,13 @@ class CoverageTest(
     def last_line_squeezed(self, report):
         """Return the last line of `report` with the spaces squeezed down."""
         return self.squeezed_lines(report)[-1]
+
+
+class DebugControlString(DebugControl):
+    """A `DebugControl` that writes to a StringIO, for testing."""
+    def __init__(self, options):
+        super(DebugControlString, self).__init__(options, StringIO())
+
+    def get_output(self):
+        """Get the output text from the `DebugControl`."""
+        return self.output.getvalue()
diff --git a/tests/test_data.py b/tests/test_data.py
index 4acfd63e..9d909705 100644
--- a/tests/test_data.py
+++ b/tests/test_data.py
@@ -7,7 +7,7 @@ from coverage.backward import pickle
 from coverage.data import CoverageData, CoverageDataFiles
 from coverage.files import PathAliases, canonical_filename
 
-from tests.coveragetest import CoverageTest
+from tests.coveragetest import CoverageTest, DebugControlString
 
 
 DATA_1 = {
@@ -128,6 +128,38 @@ class DataFilesTest(DataTestHelpers, CoverageTest):
         self.data_files.read(covdata2)
         self.assert_line_counts(covdata2, SUMMARY_1)
 
+    def test_debug_output_with_debug_option(self):
+        # With debug option dataio, we get debug output about reading and
+        # writing files.
+        debug = DebugControlString(options=["dataio"])
+        covdata1 = CoverageData(debug=debug)
+        covdata1.add_lines(DATA_1)
+        self.data_files.write(covdata1)
+
+        covdata2 = CoverageData(debug=debug)
+        self.data_files.read(covdata2)
+        self.assert_line_counts(covdata2, SUMMARY_1)
+
+        self.assertRegex(
+            debug.get_output(),
+            r"^Writing data to '.*\.coverage'\n"
+            r"Reading data from '.*\.coverage'\n$"
+        )
+
+    def test_debug_output_without_debug_option(self):
+        # With a debug object, but not the dataio option, we don't get debug
+        # output.
+        debug = DebugControlString(options=[])
+        covdata1 = CoverageData(debug=debug)
+        covdata1.add_lines(DATA_1)
+        self.data_files.write(covdata1)
+
+        covdata2 = CoverageData(debug=debug)
+        self.data_files.read(covdata2)
+        self.assert_line_counts(covdata2, SUMMARY_1)
+
+        self.assertEqual(debug.get_output(), "")
+
     def test_combining(self):
         covdata1 = CoverageData()
         covdata1.add_lines(DATA_1)