Adjust is_animated behavior

- Make is_animated APNG behavior consistent with other Pillow formats
- is_animated will be true when n_frames is greater than 1 (for APNG
  this depends on animation frame count + presence or absence of a
  default image)

Tests/test_file_apng.py

@@ -12,7 +12,8 @@ class TestFilePng(PillowTestCase):
     # (referenced from https://wiki.mozilla.org/APNG_Specification)
     def test_apng_basic(self):
         with Image.open("Tests/images/apng/single_frame.png") as im:
-            self.assertTrue(im.is_animated)
+            self.assertFalse(im.is_animated)
+            self.assertEqual(im.n_frames, 1)
             self.assertEqual(im.get_format_mimetype(), "image/apng")
             self.assertIsNone(im.info.get("default_image"))
             self.assertEqual(im.getpixel((0, 0)), (0, 255, 0, 255))
@@ -20,6 +21,7 @@ class TestFilePng(PillowTestCase):
 
         with Image.open("Tests/images/apng/single_frame_default.png") as im:
             self.assertTrue(im.is_animated)
+            self.assertEqual(im.n_frames, 2)
             self.assertEqual(im.get_format_mimetype(), "image/apng")
             self.assertTrue(im.info.get("default_image"))
             self.assertEqual(im.getpixel((0, 0)), (255, 0, 0, 255))
@@ -311,7 +313,8 @@ class TestFilePng(PillowTestCase):
 
         with Image.open(test_file) as im:
             im.load()
-            self.assertTrue(im.is_animated)
+            self.assertFalse(im.is_animated)
+            self.assertEqual(im.n_frames, 1)
             self.assertEqual(im.get_format_mimetype(), "image/apng")
             self.assertIsNone(im.info.get("default_image"))
             self.assertEqual(im.getpixel((0, 0)), (0, 255, 0, 255))
@@ -328,6 +331,7 @@ class TestFilePng(PillowTestCase):
         with Image.open(test_file) as im:
             im.load()
             self.assertTrue(im.is_animated)
+            self.assertEqual(im.n_frames, 2)
             self.assertEqual(im.get_format_mimetype(), "image/apng")
             self.assertTrue(im.info.get("default_image"))
             im.seek(1)

docs/handbook/image-file-formats.rst

@@ -559,8 +559,11 @@ APNG sequences
 The PNG loader includes limited support for reading and writing Animated Portable
 Network Graphics (APNG) files.
 When an APNG file is loaded, :py:meth:`~PIL.ImageFile.ImageFile.get_format_mimetype`
-will return ``"image/apng"``, and the :py:attr:`~PIL.Image.Image.is_animated` property
-will be ``True`` (even for single frame APNG files).
+will return ``"image/apng"``. The value of the :py:attr:`~PIL.Image.Image.is_animated`
+property will be ``True`` when the :py:attr:`~PIL.Image.Image.n_frames` property is
+greater than 1. For APNG files, the ``n_frames`` property depends on both the animation
+frame count as well as the presence or absence of a default image. See the
+``default_image`` property documentation below for more details.
 The :py:meth:`~PIL.Image.Image.seek` and :py:meth:`~PIL.Image.Image.tell` methods
 are supported.
 

src/PIL/PngImagePlugin.py

@@ -717,7 +717,7 @@ class PngImageFile(ImageFile.ImageFile):
 
     @property
     def is_animated(self):
-        return self._n_frames is not None
+        return self._n_frames is not None and self._n_frames > 1
 
     def verify(self):
         """Verify PNG file"""