interpreterbase: Add support for optional arguments to typed_pos_args

This allows representing functions like assert(), which take optional
positional arguments, which are not variadic. More importnatly you can
represent a function like (* means optional, but possitional):
```txt
func(str, *int, *str)
```

typed_pos_args will check that all of your types are correct, and if not
provide None, which allow simplifying a number of implementation details

2 files changed, 98 insertions, 2 deletions

diff --git a/mesonbuild/interpreterbase.py b/mesonbuild/interpreterbase.py
index 081a31d..0dadad2 100644
--- a/mesonbuild/interpreterbase.py
+++ b/mesonbuild/interpreterbase.py
@@ -232,9 +232,27 @@ class permittedKwargs:
 
 def typed_pos_args(name: str, *types: T.Union[T.Type, T.Tuple[T.Type, ...]],
                    varargs: T.Optional[T.Union[T.Type, T.Tuple[T.Type]]] = None,
+                   optargs: T.Optional[T.List[T.Union[T.Type, T.Tuple[T.Type]]]] = None,
                    min_varargs: int = 0, max_varargs: int = 0) -> T.Callable[..., T.Any]:
     """Decorator that types type checking of positional arguments.
 
+    This supports two different models of optional aguments, the first is the
+    variadic argument model. Variadic arguments are a possibly bounded,
+    possibly unbounded number of arguments of the same type (unions are
+    supported). The second is the standard default value model, in this case
+    a number of optional arguments may be provided, but they are still
+    ordered, and they may have different types.
+
+    This function does not support mixing variadic and default arguments.
+
+    :name: The name of the decorated function (as displayed in error messages)
+    :varargs: They type(s) of any variadic arguments the function takes. If
+        None the function takes no variadic args
+    :min_varargs: the minimum number of variadic arguments taken
+    :max_varargs: the maximum number of variadic arguments taken. 0 means unlimited
+    :optargs: The types of any optional arguments parameters taken. If None
+        then no optional paramters are taken.
+
     allows replacing this:
     ```python
     def func(self, node, args, kwargs):
@@ -268,9 +286,12 @@ def typed_pos_args(name: str, *types: T.Union[T.Type, T.Tuple[T.Type, ...]],
             assert isinstance(args, list), args
             assert max_varargs >= 0, 'max_varrags cannot be negative'
             assert min_varargs >= 0, 'min_varrags cannot be negative'
+            assert optargs is None or varargs is None, \
+                'varargs and optargs not supported together as this would be ambiguous'
 
             num_args = len(args)
             num_types = len(types)
+            a_types = types
 
             if varargs:
                 num_types += min_varargs
@@ -278,10 +299,19 @@ def typed_pos_args(name: str, *types: T.Union[T.Type, T.Tuple[T.Type, ...]],
                     raise InvalidArguments(f'{name} takes at least {num_types} arguments, but got {num_args}.')
                 elif max_varargs != 0 and (num_args < num_types or num_args > num_types + max_varargs - min_varargs):
                     raise InvalidArguments(f'{name} takes between {num_types} and {num_types + max_varargs - min_varargs} arguments, but got {len(args)}.')
+            elif optargs:
+                if num_args < num_types:
+                    raise InvalidArguments(f'{name} takes at least {num_types} arguments, but got {num_args}.')
+                elif num_args > num_types + len(optargs):
+                    raise InvalidArguments(f'{name} takes at most {num_types + len(optargs)} arguments, but got {num_args}.')
+                # Add the number of positional arguments required
+                if num_args > num_types:
+                    diff = num_args - num_types
+                    a_types = tuple(list(types) + list(optargs[:diff]))
             elif num_args != num_types:
                 raise InvalidArguments(f'{name} takes exactly {num_types} arguments, but got {num_args}.')
 
-            for i, (arg, type_) in enumerate(itertools.zip_longest(args, types, fillvalue=varargs), start=1):
+            for i, (arg, type_) in enumerate(itertools.zip_longest(args, a_types, fillvalue=varargs), start=1):
                 if not isinstance(arg, type_):
                     if isinstance(type_, tuple):
                         shouldbe = 'one of: {}'.format(", ".join(f'"{t.__name__}"' for t in type_))
@@ -298,11 +328,17 @@ def typed_pos_args(name: str, *types: T.Union[T.Type, T.Tuple[T.Type, ...]],
                 # if we have varargs we need to split them into a separate
                 # tuple, as python's typing doesn't understand tuples with
                 # fixed elements and variadic elements, only one or the other.
-                # so in that cas we need T.Tuple[int, str, float, T.Tuple[str, ...]]
+                # so in that case we need T.Tuple[int, str, float, T.Tuple[str, ...]]
                 pos = args[:len(types)]
                 var = list(args[len(types):])
                 pos.append(var)
                 nargs[i] = tuple(pos)
+            elif optargs:
+                if num_args < num_types + len(optargs):
+                    diff =  num_types + len(optargs) - num_args
+                    nargs[i] = tuple(list(args) + [None] * diff)
+                else:
+                    nargs[i] = args
             else:
                 nargs[i] = tuple(args)
             return f(*nargs, **wrapped_kwargs)
diff --git a/run_unittests.py b/run_unittests.py
index 8dfb394..b2e7338 100755
--- a/run_unittests.py
+++ b/run_unittests.py
@@ -1422,6 +1422,66 @@ class InternalTests(unittest.TestCase):
             _(None, mock.Mock(), ['string'], None)
         self.assertEqual(str(cm.exception), 'foo takes between 2 and 3 arguments, but got 1.')
 
+    def test_typed_pos_args_variadic_and_optional(self) -> None:
+        @typed_pos_args('foo', str, optargs=[str], varargs=str, min_varargs=0)
+        def _(obj, node, args: T.Tuple[str, T.List[str]], kwargs) -> None:
+            self.assertTrue(False)  # should not be reachable
+
+        with self.assertRaises(AssertionError) as cm:
+            _(None, mock.Mock(), ['string'], None)
+        self.assertEqual(
+            str(cm.exception),
+            'varargs and optargs not supported together as this would be ambiguous')
+
+    def test_typed_pos_args_min_optargs_not_met(self) -> None:
+        @typed_pos_args('foo', str, str, optargs=[str])
+        def _(obj, node, args: T.Tuple[str, T.Optional[str]], kwargs) -> None:
+            self.assertTrue(False)  # should not be reachable
+
+        with self.assertRaises(InvalidArguments) as cm:
+            _(None, mock.Mock(), ['string'], None)
+        self.assertEqual(str(cm.exception), 'foo takes at least 2 arguments, but got 1.')
+
+    def test_typed_pos_args_min_optargs_max_exceeded(self) -> None:
+        @typed_pos_args('foo', str, optargs=[str])
+        def _(obj, node, args: T.Tuple[str, T.Optional[str]], kwargs) -> None:
+            self.assertTrue(False)  # should not be reachable
+
+        with self.assertRaises(InvalidArguments) as cm:
+            _(None, mock.Mock(), ['string', '1', '2'], None)
+        self.assertEqual(str(cm.exception), 'foo takes at most 2 arguments, but got 3.')
+
+    def test_typed_pos_args_optargs_not_given(self) -> None:
+        @typed_pos_args('foo', str, optargs=[str])
+        def _(obj, node, args: T.Tuple[str, T.Optional[str]], kwargs) -> None:
+            self.assertEqual(len(args), 2)
+            self.assertIsInstance(args[0], str)
+            self.assertEqual(args[0], 'string')
+            self.assertIsNone(args[1])
+
+        _(None, mock.Mock(), ['string'], None)
+
+    def test_typed_pos_args_optargs_some_given(self) -> None:
+        @typed_pos_args('foo', str, optargs=[str, int])
+        def _(obj, node, args: T.Tuple[str, T.Optional[str], T.Optional[int]], kwargs) -> None:
+            self.assertEqual(len(args), 3)
+            self.assertIsInstance(args[0], str)
+            self.assertEqual(args[0], 'string')
+            self.assertIsInstance(args[1], str)
+            self.assertEqual(args[1], '1')
+            self.assertIsNone(args[2])
+
+        _(None, mock.Mock(), ['string', '1'], None)
+
+    def test_typed_pos_args_optargs_all_given(self) -> None:
+        @typed_pos_args('foo', str, optargs=[str])
+        def _(obj, node, args: T.Tuple[str, T.Optional[str]], kwargs) -> None:
+            self.assertEqual(len(args), 2)
+            self.assertIsInstance(args[0], str)
+            self.assertEqual(args[0], 'string')
+            self.assertIsInstance(args[1], str)
+
+        _(None, mock.Mock(), ['string', '1'], None)
 
 @unittest.skipIf(is_tarball(), 'Skipping because this is a tarball release')
 class DataTests(unittest.TestCase):