Print full help message for incorrect parameters

The usage message printed automatically by argparse is terse and
confusing. We also already print the full help message in other cases.

Fixes #56

CHANGELOG.md

@@ -8,6 +8,9 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 ### Added
  - Print a warning if both `steamapps` and `SteamApps` directories are found inside the same library directory
 
+### Changed
+ - Print full help message when incorrect parameters are provided.
+
 ## [1.4.1] - 2020-02-17
 ### Fixed
  - Fixed crash caused by Steam library paths containing special characters

src/protontricks/cli.py

@@ -35,24 +35,36 @@ def enable_logging(info=False):
         format="%(name)s (%(levelname)s): %(message)s")
 
 
+class CustomArgumentParser(argparse.ArgumentParser):
+    """
+    Custom argument parser that prints the full help message
+    when incorrect parameters are provided
+    """
+    def error(self, message):
+        self.print_help(sys.stderr)
+        args = {'prog': self.prog, 'message': message}
+        self.exit(2, '%(prog)s: error: %(message)s\n' % args)
+
+
 def main(args=None):
     """
     'protontricks' script entrypoint
     """
-    parser = argparse.ArgumentParser(
+    parser = CustomArgumentParser(
         description=(
             "Wrapper for running Winetricks commands for "
             "Steam Play/Proton games.\n"
             "\n"
             "Usage:\n"
             "\n"
-            "Run winetricks for game with APPID\n"
+            "Run winetricks for game with APPID. "
+            "COMMAND is passed directly to winetricks as-is.\n"
             "$ protontricks APPID COMMAND\n"
             "\n"
             "Search installed games to find the APPID\n"
             "$ protontricks -s GAME_NAME\n"
             "\n"
-            "Launch the Protontricks GUI\n"
+            "Use Protontricks GUI to select the game\n"
             "$ protontricks --gui\n"
             "\n"
             "Environment variables:\n"

tests/test_cli.py

@@ -381,3 +381,16 @@ def test_search_shortcut(
 
         assert "Non-Steam shortcut: fakegame.exe (4149337689)" in result
         assert "Non-Steam shortcut: fakegame.exe (4136117770)" in result
+
+
+def test_cli_error_help(cli):
+    """
+    Ensure that the full help message is printed when an incorrect argument
+    is provided
+    """
+    _, stderr = cli(["--nothing"], expect_exit=True, include_stderr=True)
+
+    # Usage message
+    assert "[-h] [--verbose]" in stderr
+    # Help message
+    assert "positional arguments:" in stderr