Allow subcommand run parameter defaults

README.md

@@ -42,8 +42,10 @@ If it is a subcommand that is executed:
   The object may register any arguments or subparsers that it needs.
 * The `run` function is called when the subcommand is used by the user.
   The arguments to this function are passed in by name, and the names can
-  be any of the options that the subcommand registered. There are two other
-  special argument names:
+  be any of the options that the subcommand registered. A parameter without
+  a corresponding registered option must have a default value; when the
+  option is absent, Python uses that default. There are also these special
+  argument names:
   * `options` - if specified, this is the Namespace returned by parse_args
   * `robot_class` - if specified, the user's robot.py will be loaded and
     it will be inspected for their robot class, which will be passed in

robotpy/main.py

@@ -296,7 +296,8 @@ def _run(
     for k, v in params.items():
         if v.kind in ok_args:
             # An error here is an error in the command -- should never happen
-            kwargs[k] = getattr(options, k)
+            if v.default is inspect.Parameter.empty or hasattr(options, k):
+                kwargs[k] = getattr(options, k)
         elif v.kind in bad_args:
             raise ValueError(
                 "internal error: subcommands may only have keyword or normal arguments"

tests/test_argparse.py

@@ -35,6 +35,22 @@ def run(self, count):
     assert received == {"count": 3}
 
 
+def test_run_parameter_without_parser_argument_uses_default():
+    received = []
+
+    class Command:
+        def __init__(self, parser):
+            pass
+
+        def run(self, mode="default-mode"):
+            received.append(mode)
+
+    exit_code = main._run(["sample"], [("sample", Command)])
+
+    assert exit_code == 0
+    assert received == ["default-mode"]
+
+
 def test_nested_subcommand_is_dispatched():
     received = []