diff --git a/python/mach_commands.py b/python/mach_commands.py
--- a/python/mach_commands.py
+++ b/python/mach_commands.py
@@ -11,28 +11,20 @@
 import tempfile
 from multiprocessing import cpu_count
 
 import six
 
-from concurrent.futures import (
-    ThreadPoolExecutor,
-    as_completed,
-    thread,
-)
+from concurrent.futures import ThreadPoolExecutor, as_completed, thread
 
 import mozinfo
 from mozfile import which
 from manifestparser import TestManifest
 from manifestparser import filters as mpf
 
 from mozbuild.base import MachCommandBase
 
-from mach.decorators import (
-    CommandArgument,
-    CommandProvider,
-    Command,
-)
+from mach.decorators import CommandArgument, CommandProvider, Command
 
 here = os.path.abspath(os.path.dirname(__file__))
 
 
 @CommandProvider
@@ -48,19 +40,18 @@
         "--ipython",
         action="store_true",
         default=False,
         help="Use ipython instead of the default Python REPL.",
     )
+    @CommandArgument("--test", action="store_true")
     @CommandArgument("args", nargs=argparse.REMAINDER)
     def python(self, no_virtualenv, exec_file, ipython, args):
         # Avoid logging the command
         self.log_manager.terminal_handler.setLevel(logging.CRITICAL)
 
         # Note: subprocess requires native strings in os.environ on Windows.
-        append_env = {
-            "PYTHONDONTWRITEBYTECODE": str("1"),
-        }
+        append_env = {"PYTHONDONTWRITEBYTECODE": str("1")}
 
         if no_virtualenv:
             python_path = sys.executable
             append_env["PYTHONPATH"] = os.pathsep.join(sys.path)
         else: