Use monkeypatch

Tests/test_file_png.py

@@ -772,22 +772,18 @@ def test_seek(self) -> None:
                 im.seek(1)
 
     @pytest.mark.parametrize("buffer", (True, False))
-    def test_save_stdout(self, buffer: bool) -> None:
-        old_stdout = sys.stdout
+    def test_save_stdout(self, buffer: bool, monkeypatch: pytest.MonkeyPatch) -> None:
 
         class MyStdOut:
             buffer = BytesIO()
 
         mystdout: MyStdOut | BytesIO = MyStdOut() if buffer else BytesIO()
 
-        sys.stdout = mystdout
+        monkeypatch.setattr(sys, "stdout", mystdout)
 
         with Image.open(TEST_PNG_FILE) as im:
             im.save(sys.stdout, "PNG")
 
-        # Reset stdout
-        sys.stdout = old_stdout
-
         if isinstance(mystdout, MyStdOut):
             mystdout = mystdout.buffer
         with Image.open(mystdout) as reloaded:

Tests/test_file_ppm.py

@@ -367,22 +367,18 @@ def test_mimetypes(tmp_path: Path) -> None:
 
 
 @pytest.mark.parametrize("buffer", (True, False))
-def test_save_stdout(buffer: bool) -> None:
-    old_stdout = sys.stdout
+def test_save_stdout(buffer: bool, monkeypatch: pytest.MonkeyPatch) -> None:
 
     class MyStdOut:
         buffer = BytesIO()
 
     mystdout: MyStdOut | BytesIO = MyStdOut() if buffer else BytesIO()
 
-    sys.stdout = mystdout
+    monkeypatch.setattr(sys, "stdout", mystdout)
 
     with Image.open(TEST_FILE) as im:
         im.save(sys.stdout, "PPM")
 
-    # Reset stdout
-    sys.stdout = old_stdout
-
     if isinstance(mystdout, MyStdOut):
         mystdout = mystdout.buffer
     with Image.open(mystdout) as reloaded: