Add map-to-output --list-monitors

GNOME47 no longer lists the connector names though that one is likely to
be most commonly used to map to an output since it's the only constant
one (who knows their monitors product name or serial after all...)

Let's add a --list-monitors option to list the current ones, and list
them on failure too while we're there.

README.md

@@ -75,6 +75,9 @@ $ gsetwacom tablet "056A:0357" set-left-handed true
 $ gsetwacom tablet "056A:0357" set-button-action A keybinding "<Control><Alt>t"
 $ gsetwacom tablet "056A:0357" set-ring-action --direction=cw --mode=2 keybinding "x"
 
+$ gsetwacom tablet "056A:0357" map-to-monitor --list-monitors
+- on DP-1: 'GSM' 'LG HDR 4K' with serial no '12345'
+- on HDMI-2: 'GSM' 'LG HDR 4K' with serial no '67890'
 $ gsetwacom tablet "056A:0357" map-to-monitor --connector DP-1
 ```
 And for stylus configuration:

src/gsetwacom/__init__.py

@@ -279,13 +279,10 @@ def wrapper(*args, **kwargs):
 @click.option("--product", type=str, default=None)
 @click.option("--serial", type=str, default=None)
 @click.option("--connector", type=str, default=None)
+@click.option("--list-monitors", is_flag=True, default=False, help="List all currently connected monitors")
 @click.pass_context
 async def tablet_map_to_monitor(
-    ctx,
-    vendor: str | None,
-    product: str | None,
-    serial: str | None,
-    connector: str | None,
+    ctx, vendor: str | None, product: str | None, serial: str | None, connector: str | None, list_monitors: bool
 ):
     """
     Map the tablet to a given monitor. The monitor may be specified with one or more
@@ -297,7 +294,7 @@ async def tablet_map_to_monitor(
         "product": product,
         "serial": serial,
     }
-    if all(args[key] is None for key in args):
+    if not list_monitors and all(args[key] is None for key in args):
         msg = "One of --vendor, --product, --serial or --connector has to be provided"
         raise click.UsageError(msg)
 
@@ -320,24 +317,32 @@ class Monitor:
         product: str
         serial: str
 
-    monitors = (Monitor(*mdata) for (mdata, *_) in monitors)
+    monitors = [Monitor(*mdata) for (mdata, *_) in monitors]
 
-    for monitor in monitors:
-        logger.info("Monitor on %s vendor '%s' product '%s' serial '%s'", *asdict(monitor).values())
-        if any(args[key] is not None and args[key] != getattr(monitor, key) for key in args):
-            continue
-        settings = ctx.obj
-        settings.set_value(
-            "output",
-            GLib.Variant(
-                "as",
-                [monitor.vendor, monitor.product, monitor.serial, monitor.connector],
-            ),
-        )
-        break
+    def print_list():
+        msg = [f"- on {m.connector}: '{m.vendor}' '{m.product}' with serial no '{m.serial}'" for m in monitors]
+        return "\n".join(msg)
+
+    if list_monitors:
+        click.echo(print_list())
     else:
-        msg = "Unable to find this monitor in the current configuration"
-        raise click.UsageError(msg)
+        for monitor in monitors:
+            logger.info("Monitor on %s vendor '%s' product '%s' serial '%s'", *asdict(monitor).values())
+            if any(args[key] is not None and args[key] != getattr(monitor, key) for key in args):
+                continue
+            settings = ctx.obj
+            settings.set_value(
+                "output",
+                GLib.Variant(
+                    "as",
+                    [monitor.vendor, monitor.product, monitor.serial, monitor.connector],
+                ),
+            )
+            break
+        else:
+            msg = f"Unable to find this monitor in the current configuration. Available monitors:\n{print_list()}"
+
+            raise click.UsageError(msg)
 
 
 def change_action(path: str, action: str, keybinding: str | None):