conv: Use NotImplementedError when no backends are available

to_bgr888() raised ValueError when no requested backend was installed
and NotImplementedError when an installed backend could not handle the
format. Both mean "we can't convert this", so collapse them into one
exception type. The tests that probe backends or skip on missing
backends can now catch a single exception type instead of matching on
the ValueError message string.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>

Author: tomba

pixutils/conv/conv.py

@@ -100,7 +100,7 @@ def to_bgr888(
     # Get list of backends to try
     backends = get_backends(options.get('backends') if options else None)
     if not backends:
-        raise ValueError('No backends available')
+        raise NotImplementedError('No backends available')
 
     size = 0
 

tests/test_conv.py

@@ -230,10 +230,8 @@ def test_function(self):
             rgb_buf = buffer_to_bgr888(
                 test_case.pixel_format, WIDTH, HEIGHT, 0, src_buf, test_case.options
             )
-        except ValueError as e:
-            if str(e) == 'No backends available':
-                self.skipTest('No backend available')
-            raise
+        except NotImplementedError as e:
+            raise unittest.SkipTest('No backend available') from e
 
         src_sha = hashlib.sha256(src_buf.tobytes()).hexdigest()
         rgb_sha = hashlib.sha256(rgb_buf.tobytes()).hexdigest()
@@ -283,10 +281,8 @@ def test_function(self):
 
         try:
             rgb_buf = buffer_to_bgr888(fmt, WIDTH, HEIGHT, bpl, test_buf, test_case.options)
-        except ValueError as e:
-            if str(e) == 'No backends available':
-                self.skipTest('No backend available')
-            raise
+        except NotImplementedError as e:
+            raise unittest.SkipTest('No backend available') from e
 
         rgb_sha = hashlib.sha256(rgb_buf.tobytes()).hexdigest()
         self.assertEqual(
@@ -345,10 +341,8 @@ def test_function(self):
 
         try:
             rgb_buf = buffer_to_bgr888(fmt, WIDTH, HEIGHT, bpl, test_buf, test_case.options)
-        except ValueError as e:
-            if str(e) == 'No backends available':
-                self.skipTest('No backend available')
-            raise
+        except NotImplementedError as e:
+            raise unittest.SkipTest('No backend available') from e
 
         rgb_sha = hashlib.sha256(rgb_buf.tobytes()).hexdigest()
         self.assertEqual(