Viewer class properties: src/PIL/ImageShow.py, Tests/test_imageshow.py files changed:

- Viewer class format property changed to 'PNG', format property removed from child classes.
- Viewer class options property changed to {"compress_level": 1}, format property removed from child classes.
- IPythonViewer format option set to None, IPythonViewer options set to {}.
- Corresponding test Tests/test_imageshow.py::test_viewer changed to meet the changes mentioned above.

Tests/test_imageshow.py

@@ -54,7 +54,7 @@ def test_show():
 def test_viewer():
     viewer = ImageShow.Viewer()
 
-    assert viewer.get_format(None) is None
+    assert viewer.get_format(None) == "PNG"
 
     with pytest.raises(NotImplementedError):
         viewer.get_command(None)

src/PIL/ImageShow.py

@@ -61,6 +61,11 @@ def show(image, title=None, **options):
 class Viewer:
     """Base class for viewers."""
 
+    format = "PNG"
+    """The format to convert the image into."""
+    options = {"compress_level": 1}
+    """Additional options used to convert the image."""
+
     # main api
 
     def show(self, image, **options):
@@ -81,11 +86,6 @@ class Viewer:
 
     # hook methods
 
-    format = None
-    """The format to convert the image into."""
-    options = {}
-    """Additional options used to convert the image."""
-
     def get_format(self, image):
         """Return format name, or ``None`` to save as PGM/PPM."""
         return self.format
@@ -143,9 +143,6 @@ class Viewer:
 class WindowsViewer(Viewer):
     """The default viewer on Windows is the default system application for PNG files."""
 
-    format = "PNG"
-    options = {"compress_level": 1}
-
     def get_command(self, file, **options):
         return (
             f'start "Pillow" /WAIT "{file}" '
@@ -161,9 +158,6 @@ if sys.platform == "win32":
 class MacViewer(Viewer):
     """The default viewer on macOS using ``Preview.app``."""
 
-    format = "PNG"
-    options = {"compress_level": 1}
-
     def get_command(self, file, **options):
         # on darwin open returns immediately resulting in the temp
         # file removal while app is opening
@@ -199,9 +193,6 @@ if sys.platform == "darwin":
 
 
 class UnixViewer(Viewer):
-    format = "PNG"
-    options = {"compress_level": 1}
-
     def get_command(self, file, **options):
         command = self.get_command_ex(file, **options)[0]
         return f"({command} {quote(file)}; rm -f {quote(file)})&"
@@ -395,6 +386,9 @@ if sys.platform not in ("win32", "darwin"):  # unixoids
 class IPythonViewer(Viewer):
     """The viewer for IPython frontends."""
 
+    format = None
+    options = {}
+
     def show_image(self, image, **options):
         ipython_display(image)
         return 1