MMMLExpressionToEvent: Add option to skip argument with special value…

… '_'

Before this patch all arguments of a MMML expression needed to be
defined in their given order. This means that if a user wanted to
specify the 5th argument of an expression, it also needed to specify the
four previous arguments even if the user didn't care about them. So for
instance when setting the clef on a rest the user had to write:

    r 1/4 pp arpeggio.direction=None clef.name=bass

So duration, volume and playing indicators needed to be defined,
although they didn't matter. This patch adds the new special argument
value `_`. With this, arguments are ignored, so the previous example
could simply be:

    r 1/4 _ _ clef.name=bass

mutwo/mmml_converters/constants.py

@@ -4,5 +4,8 @@
 ENCODER_REGISTRY = mmml_utilities.EncoderRegistry()
 INDENTATION = r"    "
 
+IGNORE_MAGIC = r"_"
+"""If this is used as an argument, it is ignored"""
+
 
 del mmml_utilities

mutwo/mmml_converters/frontends.py

@@ -35,8 +35,11 @@ class MMMLExpressionToEvent(core_converters.abc.Converter):
 
     def __init__(self, use_defaults: bool = False):
         self._use_defaults = False
+
         self._wrapped_decoder_dict = {}
+
         self.__decoder_default_dict = {}
+        self.__decoder_varnames_dict = {}
 
     def reset_defaults(self):
         self.__decoder_default_dict = {}
@@ -104,14 +107,26 @@ def _wrap_decoder(self, decoder_name: str, function: typing.Callable):
                 n 1/1 c
         """
 
+        varnames = function.__code__.co_varnames
+        self.__decoder_varnames_dict[decoder_name] = varnames
+
         def _(*args):
             if self._use_defaults:
                 self._set_decoder_default_args(decoder_name, args)
                 args = self._get_decoder_default_args(decoder_name, args)
-            return function(*args)
+            kwargs = self._args_to_kwargs(decoder_name, args)
+            return function(**kwargs)
 
         return _
 
+    def _args_to_kwargs(self, decoder_name: str, args: tuple) -> dict:
+        varnames = self.__decoder_varnames_dict[decoder_name]
+        return {
+            name: arg
+            for name, arg in zip(varnames, args)
+            if arg != mmml_converters.constants.IGNORE_MAGIC
+        }
+
     def _set_decoder_default_args(self, decoder_name: str, args: tuple):
         """Set currently defined arguments as new default values for decoder"""
         present_default = self.__decoder_default_dict.get(decoder_name, [])
@@ -138,7 +153,9 @@ def _split_to_header_and_block(expression: str):
     header, block = None, expression
     while not header:
         if not block:
-            raise mmml_utilities.MalformedMMML(f"No MMML expression found in expression '{expression}'")
+            raise mmml_utilities.MalformedMMML(
+                f"No MMML expression found in expression '{expression}'"
+            )
         header, _, block = block.partition("\n")
     return header, block
 

tests/converters_tests.py

@@ -129,6 +129,11 @@ def test_decoder_sim(self):
         # Set tag
         self.assertEqual(sim(tag="abc"), self.c("sim abc"))
 
+    def test_empty_argument(self):
+        """Ensure that MMML takes the decoders default value if magic '_' is given as an argument"""
+        self.assertEqual(n(volume="pppp"), self.c("n _ _ pppp"))
+        self.assertEqual(n(volume="pppp", duration="5/4"), self.c("n 5/4 _ pppp"))
+
 
 class EventToMMMLExpressionTest(unittest.TestCase):
     def setUp(self):