Default to GUI if no args provided

Default to GUI (i.e. set `--gui` flag) if no command-line arguments
are provided.

Fixes #333

CHANGELOG.md

@@ -11,6 +11,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
 ### Changed
  - `protontricks -c` and `protontricks-launch` now use the current working directory instead of the game's installation directory. `--cwd-app` can be used to restore old behavior. Scripts can also `$STEAM_APP_PATH` environment variable to determine the game's installation directory; this has been supported (albeit undocumented) since 1.8.0.
+ - `protontricks` will now launch GUI if no arguments were provided
 
 ## [1.11.1] - 2024-02-20
 ### Fixed

src/protontricks/cli/main.py

@@ -156,6 +156,10 @@ def main(args=None, steam_path=None, steam_root=None):
 
     args = parser.parse_args(args)
 
+    if len(sys.argv) < 2:
+        # No arguments were provided, default to GUI
+        args.gui = True
+
     # 'cli_error_handler' relies on this to know whether to use error dialog or
     # not
     main.no_term = args.no_term

tests/cli/test_main.py

@@ -1,4 +1,5 @@
 import os
+import sys
 import shutil
 from pathlib import Path
 
@@ -334,15 +335,6 @@ def test_run_winetricks_game_not_found(
 
         assert "Steam app with the given app ID could not be found" in result
 
-    def test_run_no_command(self, cli):
-        """
-        Run only the 'protontricks' command.
-        """
-        result = cli([])
-
-        # Help will be printed if no specific command is given
-        assert result.startswith("usage: ")
-
     @pytest.mark.usefixtures("default_proton")
     def test_run_returncode_passed(self, cli, steam_app_factory):
         """
@@ -780,6 +772,23 @@ def test_run_gui_proton_incomplete(
 
         assert "Proton installation is incomplete" in result
 
+    @pytest.mark.usefixtures("default_proton", "gui_provider")
+    def test_run_no_args(
+            self, cli, steam_app_factory, command_mock, gui_provider,
+            monkeypatch):
+        """
+        Run only the 'protontricks' command. This will default to GUI.
+        """
+        # Monkeypatch 'sys.argv', as that seems to be the only way to determine
+        # whether no arguments were provided
+        monkeypatch.setattr(sys, "argv", ["protontricks"])
+        steam_app_factory(name="Fake game", appid=10)
+
+        result = cli([], expect_returncode=1)
+
+        # Help will be printed if no specific command is given
+        assert "No game was selected" in result
+
 
 class TestCLICommand:
     def test_run_command(