Print available Proton versions on missing value

Print available Proton versions when $PROTON_VERSION has been set but
the installation in question cannot be found, as the list of allowed
values is otherwise hard to determine.

Also refactor the Proton app discovery in CLI entrypoint, as it was
already duplicated and needed to be updated to print the new error
message.

src/protontricks/cli/main.py

@@ -32,11 +32,45 @@ def cli(args=None):
     main(args)
 
 
+
 @cli_error_handler
 def main(args=None, steam_path=None, steam_root=None):
     """
     'protontricks' script entrypoint
     """
+    def _find_proton_app_or_exit(steam_path, steam_apps, appid):
+        """
+        Attempt to find a Proton app. Fail with an appropriate CLI error
+        message if one cannot be found.
+        """
+        proton_app = find_proton_app(
+            steam_path=steam_path, steam_apps=steam_apps, appid=appid
+        )
+
+        if not proton_app:
+            if os.environ.get("PROTON_VERSION"):
+                # Print an error listing accepted values if PROTON_VERSION was
+                # set, as the user is trying to use a certain Proton version
+                proton_names = sorted(set([
+                    app.name for app in steam_apps if app.is_proton
+                ]))
+                exit_(
+                    "Protontricks installation could not be found with given "
+                    "$PROTON_VERSION!\n\n"
+                    f"Valid values include: {", ".join(proton_names)}"
+                )
+            else:
+                exit_("Proton installation could not be found!")
+
+        if not proton_app.is_proton_ready:
+            exit_(
+                "Proton installation is incomplete. Have you launched a Steam "
+                "app using this Proton version at least once to finish the "
+                "installation?"
+            )
+
+        return proton_app
+
     if args is None:
         args = sys.argv[1:]
 
@@ -297,19 +331,9 @@ def exit_(error):
         cwd = str(steam_app.install_path) if args.cwd_app else None
 
         # 6. Find Proton version of selected app
-        proton_app = find_proton_app(
+        proton_app = _find_proton_app_or_exit(
             steam_path=steam_path, steam_apps=steam_apps, appid=steam_app.appid
         )
-        if not proton_app:
-            exit_("Proton installation could not be found!")
-
-        if not proton_app.is_proton_ready:
-            exit_(
-                "Proton installation is incomplete. Have you launched a Steam "
-                "app using this Proton version at least once to finish the "
-                "installation?"
-            )
-
 
         run_command(
             winetricks_path=winetricks_path,
@@ -361,19 +385,9 @@ def exit_(error):
         return
 
     # 6. Find globally active Proton version now
-    proton_app = find_proton_app(
+    proton_app = _find_proton_app_or_exit(
         steam_path=steam_path, steam_apps=steam_apps, appid=args.appid)
 
-    if not proton_app:
-        exit_("Proton installation could not be found!")
-
-    if not proton_app.is_proton_ready:
-        exit_(
-            "Proton installation is incomplete. Have you launched a Steam app "
-            "using this Proton version at least once to finish the "
-            "installation?"
-        )
-
     # If neither search or GUI are set, do a normal Winetricks command
     # Find game by appid
     steam_appid = int(args.appid)

src/protontricks/steam.py

@@ -925,6 +925,7 @@ def find_proton_app(steam_path, steam_apps, appid=None):
                 "$PROTON_VERSION was set but matching Proton installation "
                 "could not be found."
             )
+
             return None
 
     tool_app = find_steam_compat_tool_app(

tests/cli/test_main.py

@@ -69,6 +69,27 @@ def test_run_winetricks_select_proton(
         assert command_mock.commands[-1].env["PROTON_PATH"] \
             == str(custom_proton.install_path)
 
+    def test_run_winetricks_select_proton_accepted_values(
+            self, cli, steam_app_factory, custom_proton_factory, command_mock):
+        """
+        Perform a Protonrticks command while selecting a non-existent Proton
+        version using PROTON_VERSION env var. Ensure list of allowed values
+        is printed in the error message
+        """
+        steam_app_factory(name="Fake game", appid=10)
+        custom_proton_factory(name="Custom Proton C")
+        custom_proton_factory(name="Custom Proton A")
+
+        result = cli(
+            ["10", "winecfg"],
+            env={"PROTON_VERSION": "Nonexistent Proton"},
+            expect_returncode=1
+        )
+
+        assert "Protontricks installation could not be found" in result
+        assert \
+            "Valid values include: Custom Proton A, Custom Proton C" in result
+
     def test_run_winetricks_select_steam(
             self, cli, steam_app_factory, default_proton, command_mock,
             home_dir):