Ensure file touching happens if nothing was measured. #884

CHANGES.rst

@@ -35,6 +35,9 @@ Unreleased
   unwanted warnings ("Already imported a file that will be measured") and a
   reduction in coverage totals (`issue 909`_).  This is now fixed.
 
+- If no data was collected, an exception about "No data to report" could happen
+  instead of a 0% report being created (`issue 884`_).  This is now fixed.
+
 - The handling of source files with non-encodable file names has changed.
   Previously, if a file name could not be encoded as UTF-8, an error occurred,
   as described in `issue 891`_.  Now, those files will not be measured, since
@@ -46,6 +49,7 @@ Unreleased
 - ``coverage run --debug=sys`` would fail with an AttributeError. This is now
   fixed (`issue 907`_).
 
+.. _issue 884: https://github.com/nedbat/coveragepy/issues/884
 .. _issue 890: https://github.com/nedbat/coveragepy/issues/890
 .. _issue 891: https://github.com/nedbat/coveragepy/issues/891
 .. _issue 901: https://github.com/nedbat/coveragepy/issues/901

coverage/control.py

@@ -716,7 +716,7 @@ def _post_save_work(self):
 
         # Touch all the files that could have executed, so that we can
         # mark completely unexecuted files as 0% covered.
-        if self._data:
+        if self._data is not None:
             for file_path, plugin_name in self._inorout.find_possibly_unexecuted_files():
                 file_path = self._file_mapper(file_path)
                 self._data.touch_file(file_path, plugin_name)

tests/test_api.py

@@ -295,6 +295,21 @@ def test_empty_reporting(self):
         with self.assertRaisesRegex(CoverageException, "No data to report."):
             cov.report()
 
+    def test_completely_zero_reporting(self):
+        # https://github.com/nedbat/coveragepy/issues/884
+        # If nothing was measured, the file-touching didn't happen properly.
+        self.make_file("foo/bar.py", "print('Never run')")
+        self.make_file("test.py", "assert True")
+        cov = coverage.Coverage(source=["foo"])
+        self.start_import_stop(cov, "test")
+        cov.report()
+        # Name         Stmts   Miss  Cover
+        # --------------------------------
+        # foo/bar.py       1      1     0%
+
+        last = self.last_line_squeezed(self.stdout()).replace("\\", "/")
+        self.assertEqual("foo/bar.py 1 1 0%", last)
+
     def test_cov4_data_file(self):
         cov4_data = (
             "!coverage.py: This is a private format, don't read it directly!"