Explicitly check for closed ImageSlide

When an operation is performed on a closed ImageSlide, we've historically
raised an AttributeError upon dereferencing None.  Add proper checks so
the type system understands what we're doing, raising ValueError as
lowlevel does.  This is not an API change because it only affects invalid
caller behavior.

Signed-off-by: Benjamin Gilbert <[email protected]>

Author: bgilbert

openslide/__init__.py

@@ -366,7 +366,7 @@ def __init__(self, file: str | Path | Image.Image):
         self._file_arg = file
         if isinstance(file, Image.Image):
             self._close = False
-            self._image = file
+            self._image: Image.Image | None = file
         else:
             self._close = True
             self._image = Image.open(file)
@@ -391,10 +391,10 @@ def detect_format(cls, filename: str | Path) -> str | None:
     def close(self) -> None:
         """Close the slide object."""
         if self._close:
+            assert self._image is not None
             self._image.close()
             self._close = False
-        # is it necessary to set to None?
-        self._image = None  # type: ignore
+        self._image = None
 
     @property
     def level_count(self) -> Literal[1]:
@@ -406,6 +406,8 @@ def level_dimensions(self) -> tuple[tuple[int, int], ...]:
         """A list of (width, height) tuples, one for each level of the image.
 
         level_dimensions[n] contains the dimensions of level n."""
+        if self._image is None:
+            raise ValueError('Passing closed slide object')
         return (self._image.size,)
 
     @property
@@ -442,6 +444,8 @@ def read_region(
                   reference frame.
         level:    the level number.
         size:     (width, height) tuple giving the region size."""
+        if self._image is None:
+            raise ValueError('Passing closed slide object')
         if level != 0:
             raise OpenSlideError("Invalid level")
         if ['fail' for s in size if s < 0]:

tests/test_imageslide.py

@@ -49,8 +49,9 @@ def test_operations_on_closed_handle(self):
             osr = ImageSlide(img)
             osr.close()
             self.assertRaises(
-                AttributeError, lambda: osr.read_region((0, 0), 0, (100, 100))
+                ValueError, lambda: osr.read_region((0, 0), 0, (100, 100))
             )
+            self.assertRaises(ValueError, lambda: osr.level_dimensions)
             # If an Image is passed to the constructor, ImageSlide.close()
             # shouldn't close it
             self.assertEqual(img.getpixel((0, 0)), 3)
@@ -59,9 +60,8 @@ def test_context_manager(self):
         osr = ImageSlide(file_path('boxes.png'))
         with osr:
             pass
-        self.assertRaises(
-            AttributeError, lambda: osr.read_region((0, 0), 0, (100, 100))
-        )
+        self.assertRaises(ValueError, lambda: osr.read_region((0, 0), 0, (100, 100)))
+        self.assertRaises(ValueError, lambda: osr.level_dimensions)
 
 
 class _SlideTest: