Add additional test coverage

Tests/test_file_avif.py

@@ -110,18 +110,21 @@ def has_alpha_premultiplied(im_bytes):
 
 
 class TestUnsupportedAvif:
-    def test_unsupported(self):
-        if features.check("avif"):
-            AvifImagePlugin.SUPPORTED = False
+    def test_unsupported(self, monkeypatch):
+        monkeypatch.setattr(AvifImagePlugin, "SUPPORTED", False)
 
-        try:
-            file_path = "Tests/images/avif/hopper.avif"
-            pytest.warns(
-                UserWarning,
-                lambda: pytest.raises(UnidentifiedImageError, Image.open, file_path),
-            )
-        finally:
-            AvifImagePlugin.SUPPORTED = features.check("avif")
+        file_path = "Tests/images/avif/hopper.avif"
+        pytest.warns(
+            UserWarning,
+            lambda: pytest.raises(UnidentifiedImageError, Image.open, file_path),
+        )
+
+    def test_unsupported_open(self, monkeypatch):
+        monkeypatch.setattr(AvifImagePlugin, "SUPPORTED", False)
+
+        file_path = "Tests/images/avif/hopper.avif"
+        with pytest.raises(SyntaxError):
+            AvifImagePlugin.AvifImageFile(file_path)
 
 
 @skip_unless_feature("avif")
@@ -198,6 +201,27 @@ def test_AvifDecoder_with_invalid_args(self):
         with pytest.raises(TypeError):
             _avif.AvifDecoder()
 
+    def test_encoder_finish_none_error(self, monkeypatch, tmp_path):
+        """Save should raise an OSError if AvifEncoder.finish returns None"""
+
+        class _mock_avif:
+            class AvifEncoder:
+                def __init__(self, *args, **kwargs):
+                    pass
+
+                def add(self, *args, **kwargs):
+                    pass
+
+                def finish(self):
+                    return None
+
+        monkeypatch.setattr(AvifImagePlugin, "_avif", _mock_avif)
+
+        im = Image.new("RGB", (150, 150))
+        test_file = str(tmp_path / "temp.avif")
+        with pytest.raises(OSError):
+            im.save(test_file)
+
     def test_no_resource_warning(self, tmp_path):
         with Image.open(TEST_AVIF_FILE) as image:
             temp_file = str(tmp_path / "temp.avif")