Raise ValueError instead of asserting number of FTEX texture formats (#9805)

Author: ritsth

Tests/test_file_ftex.py

@@ -38,3 +38,13 @@ def test_invalid_texture() -> None:
     with pytest.raises(ValueError, match="Invalid texture compression format: 2"):
         with Image.open(io.BytesIO(data)):
             pass
+
+
+def test_unsupported_texture_format_count() -> None:
+    with open("Tests/images/ftex_dxt1.ftc", "rb") as fp:
+        # Change the format count to an unsupported value
+        data = fp.read(20) + struct.pack("<i", 2)
+
+    with pytest.raises(ValueError, match="Unsupported number of texture formats: 2"):
+        with Image.open(io.BytesIO(data)):
+            pass

src/PIL/FtexImagePlugin.py

@@ -82,7 +82,9 @@ def _open(self) -> None:
 
         # Only support single-format files.
         # I don't know of any multi-format file.
-        assert format_count == 1
+        if format_count != 1:
+            msg = f"Unsupported number of texture formats: {format_count}"
+            raise ValueError(msg)
 
         format, where = struct.unpack("<2i", self.fp.read(8))
         self.fp.seek(where)