test: properly reset a few globals between every test

tests/conftest.py

@@ -16,6 +16,7 @@
 
 from coverage import env
 from coverage.exceptions import _StopEverything
+from coverage.files import set_relative_directory
 
 
 # Pytest will rewrite assertions in test modules, but not elsewhere.
@@ -72,6 +73,22 @@ def reset_sys_path():
     sys.path[:] = sys_path
 
 
+@pytest.fixture(autouse=True)
+def reset_environment():
+    """Make sure a test setting an envvar doesn't leak into another test."""
+    old_environ = os.environ.copy()
+    yield
+    os.environ.clear()
+    os.environ.update(old_environ)
+
+
+@pytest.fixture(autouse=True)
+def reset_filesdotpy_globals():
+    """coverage/files.py has some unfortunate globals. Reset them every test."""
+    set_relative_directory()
+    yield
+
+
 TRACK_TESTS = False
 TEST_TXT = "/tmp/tests.txt"
 

tests/mixins.py

@@ -66,10 +66,9 @@ def _temp_dir(self, tmpdir_factory):
                 # nose's caching importer can get confused.  The full path prevents
                 # problems.
                 sys.path.insert(0, os.getcwd())
-
-                yield None
+                yield
         else:
-            yield None
+            yield
 
     def make_file(self, filename, text="", bytes=b"", newline=None):
         """Make a file. See `tests.helpers.make_file`"""