fix: don't create a data file when just trying to read one. #1328

Author: nedbat

CHANGES.rst

@@ -41,13 +41,18 @@ Version 6.4 — 2022-05-22
 - A new debug option ``debug=sqldata`` adds more detail to ``debug=sql``,
   logging all the data being written to the database.
 
+- Previously, running ``coverage report`` (or any of the reporting commands) in
+  an empty directory would create a .coverage data file.  Now they do not,
+  fixing `issue 1328`_.
+
 - On Python 3.11, the ``[toml]`` extra no longer installs tomli, instead using
   tomllib from the standard library.  Thanks `Shantanu <pull 1359_>`_.
 
 - In-memory CoverageData objects now properly update(), closing `issue 1323`_.
 
 .. _issue 1310: https://github.com/nedbat/coveragepy/issues/1310
 .. _issue 1323: https://github.com/nedbat/coveragepy/issues/1323
+.. _issue 1328: https://github.com/nedbat/coveragepy/issues/1328
 .. _issue 1351: https://github.com/nedbat/coveragepy/issues/1351
 .. _pull 1354: https://github.com/nedbat/coveragepy/pull/1354
 .. _pull 1359: https://github.com/nedbat/coveragepy/pull/1359

coverage/sqldata.py

@@ -783,8 +783,9 @@ def erase(self, parallel=False):
 
     def read(self):
         """Start using an existing data file."""
-        with self._connect():       # TODO: doesn't look right
-            self._have_used = True
+        if os.path.exists(self._filename):
+            with self._connect():
+                self._have_used = True
 
     def write(self):
         """Ensure the data is written to the data file."""

tests/test_data.py

@@ -118,6 +118,13 @@ class CoverageDataTest(CoverageTest):
     def test_empty_data_is_false(self):
         covdata = DebugCoverageData()
         assert not covdata
+        self.assert_doesnt_exist(".coverage")
+
+    def test_empty_data_is_false_when_read(self):
+        covdata = DebugCoverageData()
+        covdata.read()
+        assert not covdata
+        self.assert_doesnt_exist(".coverage")
 
     def test_line_data_is_true(self):
         covdata = DebugCoverageData()

tests/test_summary.py

@@ -450,6 +450,7 @@ def test_report_skip_covered_no_data(self):
         cov.load()
         with pytest.raises(NoDataError, match="No data to report."):
             self.get_report(cov, skip_covered=True)
+        self.assert_doesnt_exist(".coverage")
 
     def test_report_skip_empty(self):
         self.make_file("main.py", """

tests/test_xml.py

@@ -139,6 +139,7 @@ def test_no_data(self):
         with pytest.raises(NoDataError, match="No data to report."):
             self.run_xml_report()
         self.assert_doesnt_exist("coverage.xml")
+        self.assert_doesnt_exist(".coverage")
 
     def test_no_source(self):
         # Written while investigating a bug, might as well keep it.