Improve error message for saving attempt when no layer is selected

[melissawm]

🐛 Bug Report

When using Ctrl+S (on Linux) to try to save a layer with no layer selected through the GUI, an OSError appears which is not very friendly to users.

💡 Steps to Reproduce

1. Create any layer on the gui
2. Click Ctrl+S (Linux/Windows) or Command+S (Mac)
3. Observe the OSError

💡 Expected Behavior

Maybe a dialog with a friendlier "No layer selected. Please select a layer to save" message.

🌎 Environment

napari dev Manjaro Linux + KDE Python 3.11

💡 Additional Context

Full traceback

``` --------------------------------------------------------------------------- OSError Traceback (most recent call last) File ~/micromamba/envs/napari-dev/lib/python3.11/site-packages/app_model/backends/qt/_qaction.py:57, in QCommandAction._on_triggered(self=QMenuItemAction(MenuItem(when=None, group='4_sav...tle=None, toggled=None), alt=None), app='napari'), checked=False) 53 def _on_triggered(self, checked: bool) -> None: 54 # execute_command returns a Future, for the sake of eventually being 55 # asynchronous without breaking the API. For now, we call result() 56 # to raise any exceptions. ---> 57 self._app.commands.execute_command(self._command_id).result() self._command_id = 'napari.window.file.save_layers_dialog.selected' self = QMenuItemAction(MenuItem(when=None, group='4_save', order=None, command=CommandRule(id='napari.window.file.save_layers_dialog.selected', title='Save Selected Layers...', category=None, tooltip=None, status_tip=None, icon=None, icon_visible_in_menu=True, enablement=Expr.parse('num_selected_layers > 0'), short_title=None, toggled=None), alt=None), app='napari') self._app = Application('napari') File ~/micromamba/envs/napari-dev/lib/python3.11/site-packages/app_model/registries/_commands_reg.py:245, in CommandsRegistry.execute_command(self=, id='napari.window.file.save_layers_dialog.selected', execute_asynchronously=False, *args=(), **kwargs={}) 241 except Exception as e: 242 if self._raise_synchronous_exceptions: 243 # note, the caller of this function can also achieve this by 244 # calling `future.result()` on the returned future object. --> 245 raise e 246 future.set_exception(e) 248 return future File ~/micromamba/envs/napari-dev/lib/python3.11/site-packages/app_model/registries/_commands_reg.py:240, in CommandsRegistry.execute_command(self=, id='napari.window.file.save_layers_dialog.selected', execute_asynchronously=False, *args=(), **kwargs={}) 238 future: Future = Future() 239 try: --> 240 future.set_result(cmd(*args, **kwargs)) future = cmd = args = () kwargs = {} 241 except Exception as e: 242 if self._raise_synchronous_exceptions: 243 # note, the caller of this function can also achieve this by 244 # calling `future.result()` on the returned future object. File ~/micromamba/envs/napari-dev/lib/python3.11/site-packages/in_n_out/_store.py:934, in Store.inject_processors.._deco.._exec(*args=(), **kwargs={}) 932 @wraps(func) 933 def _exec(*args: P.args, **kwargs: P.kwargs) -> R: --> 934 result = func(*args, **kwargs) func = args = () kwargs = {} 935 if result is not None: 936 self.process( 937 result, 938 type_hint=type_hint, (...) 941 _funcname=getattr(func, "__qualname__", str(func)), 942 ) File ~/micromamba/envs/napari-dev/lib/python3.11/site-packages/in_n_out/_store.py:804, in Store.inject.._inner.._exec(*args=(), **kwargs={}) 797 logger.debug( 798 " Calling %s with %r (injected %r)", 799 _fname, 800 bound.arguments, 801 _injected_names, 802 ) 803 try: --> 804 result = func(**bound.arguments) bound = )> func = bound.arguments = {'qt_viewer': } 805 except TypeError as e: 806 if "missing" not in e.args[0]: File ~/projects/napari/napari/_qt/_qapp_model/qactions/_file.py:105, in _save_selected_layers(qt_viewer=) 104 def _save_selected_layers(qt_viewer: QtViewer): --> 105 qt_viewer._save_layers_dialog(selected=True) qt_viewer = File ~/projects/napari/napari/_qt/qt_viewer.py:756, in QtViewer._save_layers_dialog(self=, selected=True) 751 msg = trans._( 752 'Please select one or more layers to save,' 753 '\nor use "Save all layers..."' 754 ) 755 if msg: --> 756 raise OSError(trans._('Nothing to save')) trans = 758 # prepare list of extensions for drop down menu. 759 ext_str, writers = _extension_string_for_layers( 760 list(self.viewer.layers.selection) 761 if selected 762 else self.viewer.layers 763 ) OSError: Nothing to save ```

[willingc]

Great catch @melissawm and good suggestion.

[willingc]

Looking at the traceback it seems as if there is a message created but it's not displayed to the user and sent to error handling.

File ~/Code/repos-napari/napari/napari/_qt/qt_viewer.py:756, in QtViewer._save_layers_dialog(self=<napari._qt.qt_viewer.QtViewer object>, selected=True)
    751     msg = trans._(
    752         'Please select one or more layers to save,'
    753         '\nor use "Save all layers..."'
    754     )
    755 if msg:
--> 756     raise OSError(trans._('Nothing to save'))

[psobolewskiPhD]

Interesting, on macOS if I have no layer selected, the menu item is greyed-out and Command-S does nothing: no error, no warning. Odd that it behaves differently with different OS. @lucyleeow any ideas?

[willingc]

I was able to reproduce on macOS sequoia when using the keyboard shortcut (on main not released version)