Correct the fallback handling when an icon cannot be loaded.

changes/2558.bugfix.rst

@@ -0,0 +1 @@
+If an icon file exists but cannot be loaded, or an application icon cannot be found, Toga will fall back to a default icon.

core/src/toga/icons.py

@@ -95,7 +95,6 @@ def __init__(
         :param system: **For internal use only**
         """
         self.factory = get_platform_factory()
-
         try:
             # Try to load the icon with the given path snippet. If the request is for the
             # app icon, use ``resources/<app name>`` as the path.
@@ -130,7 +129,7 @@ def __init__(
                 )
 
             self._impl = self.factory.Icon(interface=self, path=full_path)
-        except FileNotFoundError:
+        except (FileNotFoundError, ValueError):
             # Icon path couldn't be loaded. If the path is the sentinel for the app
             # icon, and this isn't running as a script, fall back to the application
             # binary
@@ -143,7 +142,7 @@ def __init__(
                     try:
                         # Use the application binary's icon
                         self._impl = self.factory.Icon(interface=self, path=None)
-                    except FileNotFoundError:
+                    except (FileNotFoundError, ValueError):
                         # Can't find the application binary's icon.
                         print(
                             "WARNING: Can't find app icon; falling back to default icon"

core/tests/test_icons.py

@@ -113,8 +113,15 @@ def test_create(monkeypatch, app, path, system, sizes, extensions, final_paths):
     assert icon._impl.path == final_paths
 
 
-def test_create_fallback(app, capsys):
+def test_create_fallback_missing(monkeypatch, app, capsys):
     """If a resource doesn't exist, a fallback icon is used."""
+    # Prime the dummy so the app icon cannot be loaded
+    monkeypatch.setattr(
+        DummyIcon,
+        "ICON_FAILURE",
+        FileNotFoundError(),
+    )
+
     icon = toga.Icon("resources/missing")
 
     assert icon._impl is not None
@@ -127,6 +134,27 @@ def test_create_fallback(app, capsys):
     )
 
 
+def test_create_fallback_unloadable(monkeypatch, app, capsys):
+    """If a resource exists, but can't be loaded, a fallback icon is used."""
+    # Prime the dummy so the app icon cannot be loaded
+    monkeypatch.setattr(
+        DummyIcon,
+        "ICON_FAILURE",
+        ValueError("Icon could not be loaded"),
+    )
+
+    icon = toga.Icon("resources/sample")
+
+    assert icon._impl is not None
+    assert icon._impl.interface == toga.Icon.DEFAULT_ICON
+
+    # A warning was printed; allow for windows separators
+    assert (
+        "WARNING: Can't find icon resources/sample"
+        in capsys.readouterr().out.replace("\\", "/")
+    )
+
+
 def test_create_fallback_variants(monkeypatch, app, capsys):
     """If a resource with size variants doesn't exist, a fallback icon is used."""
     monkeypatch.setattr(DummyIcon, "SIZES", [32, 72])
@@ -164,7 +192,7 @@ def test_create_app_icon(monkeypatch, app, capsys):
 
 
 def test_create_app_icon_missing(monkeypatch, app, capsys):
-    """The app icon can be constructed"""
+    """If the app icon is missing, a fallback is used"""
     # When running under pytest, code will identify as running as a script
 
     # Load the app default icon.
@@ -195,10 +223,38 @@ def test_create_app_icon_non_script(monkeypatch, app, capsys):
     assert capsys.readouterr().out == ""
 
 
-def test_create_app_icon_missing_non_script(monkeypatch, app, capsys):
-    """The binary executableThe app icon can be reset to the default"""
+def test_create_app_icon_unloadable_non_script(monkeypatch, app, capsys):
+    """If the icon from binary executable cannot be loaded, the app icon is reset to the default"""
     # Prime the dummy so the app icon cannot be loaded
-    monkeypatch.setattr(DummyIcon, "ICON_EXISTS", False)
+    monkeypatch.setattr(
+        DummyIcon,
+        "ICON_FAILURE",
+        ValueError("Icon could not be loaded"),
+    )
+
+    # Patch sys.executable so the test looks like it's running as a packaged binary
+    monkeypatch.setattr(sys, "executable", "/path/to/App")
+
+    # Load the app default icon
+    icon = toga.Icon(_APP_ICON)
+
+    assert isinstance(icon, toga.Icon)
+    # App icon path reports as `resources/<app_name>`; impl is the default toga icon
+    assert icon.path == Path("resources/icons")
+    assert icon._impl.path == Path(TOGA_RESOURCES / "toga.png")
+
+    # A warning was printed; allow for windows separators
+    assert "WARNING: Can't find app icon" in capsys.readouterr().out.replace("\\", "/")
+
+
+def test_create_app_icon_missing_non_script(monkeypatch, app, capsys):
+    """If the icon from binary executable cannot be found, the app icon is reset to the default"""
+    # Prime the dummy so the app icon cannot be found
+    monkeypatch.setattr(
+        DummyIcon,
+        "ICON_FAILURE",
+        FileNotFoundError(),
+    )
 
     # Patch sys.executable so the test looks like it's running as a packaged binary
     monkeypatch.setattr(sys, "executable", "/path/to/App")

dummy/src/toga_dummy/icons.py

@@ -2,18 +2,19 @@
 
 
 class Icon(LoggedObject):
-    ICON_EXISTS = True
+    ICON_FAILURE = None
     EXTENSIONS = [".png", ".ico"]
     SIZES = None
 
     def __init__(self, interface, path):
         super().__init__()
         self.interface = interface
-        if not self.ICON_EXISTS:
-            raise FileNotFoundError("Couldn't find icon")
-        elif path is None:
-            self.path = "<APP ICON>"
-        elif path == {}:
-            raise FileNotFoundError("No image variants found")
+        if self.ICON_FAILURE:
+            raise self.ICON_FAILURE
         else:
-            self.path = path
+            if path is None:
+                self.path = "<APP ICON>"
+            elif path == {}:
+                raise FileNotFoundError("No image variants found")
+            else:
+                self.path = path