Issue 24190: Add inspect.BoundArguments.apply_defaults() method.

2 files changed, 65 insertions, 0 deletions

diff --git a/Lib/inspect.py b/Lib/inspect.py
index 9389f3b..8d2920a 100644
--- a/Lib/inspect.py
+++ b/Lib/inspect.py
@@ -2443,6 +2443,36 @@ class BoundArguments:
 
         return kwargs
 
+    def apply_defaults(self):
+        """Set default values for missing arguments.
+
+        For variable-positional arguments (*args) the default is an
+        empty tuple.
+
+        For variable-keyword arguments (**kwargs) the default is an
+        empty dict.
+        """
+        arguments = self.arguments
+        if not arguments:
+            return
+        new_arguments = []
+        for name, param in self._signature.parameters.items():
+            try:
+                new_arguments.append((name, arguments[name]))
+            except KeyError:
+                if param.default is not _empty:
+                    val = param.default
+                elif param.kind is _VAR_POSITIONAL:
+                    val = ()
+                elif param.kind is _VAR_KEYWORD:
+                    val = {}
+                else:
+                    # This BoundArguments was likely produced by
+                    # Signature.bind_partial().
+                    continue
+                new_arguments.append((name, val))
+        self.arguments = OrderedDict(new_arguments)
+
     def __eq__(self, other):
         return (self is other or
                     (issubclass(other.__class__, BoundArguments) and
diff --git a/Lib/test/test_inspect.py b/Lib/test/test_inspect.py
index bed3bad..e712efb 100644
--- a/Lib/test/test_inspect.py
+++ b/Lib/test/test_inspect.py
@@ -3153,6 +3153,41 @@ class TestBoundArguments(unittest.TestCase):
         ba = sig.bind(20, 30, z={})
         self.assertRegex(repr(ba), r'<BoundArguments \(a=20,.*\}\}\)>')
 
+    def test_signature_bound_arguments_apply_defaults(self):
+        def foo(a, b=1, *args, c:1={}, **kw): pass
+        sig = inspect.signature(foo)
+
+        ba = sig.bind(20)
+        ba.apply_defaults()
+        self.assertEqual(
+            list(ba.arguments.items()),
+            [('a', 20), ('b', 1), ('args', ()), ('c', {}), ('kw', {})])
+
+        # Make sure that we preserve the order:
+        # i.e. 'c' should be *before* 'kw'.
+        ba = sig.bind(10, 20, 30, d=1)
+        ba.apply_defaults()
+        self.assertEqual(
+            list(ba.arguments.items()),
+            [('a', 10), ('b', 20), ('args', (30,)), ('c', {}), ('kw', {'d':1})])
+
+        # Make sure that BoundArguments produced by bind_partial()
+        # are supported.
+        def foo(a, b): pass
+        sig = inspect.signature(foo)
+        ba = sig.bind_partial(20)
+        ba.apply_defaults()
+        self.assertEqual(
+            list(ba.arguments.items()),
+            [('a', 20)])
+
+        # Test no args
+        def foo(): pass
+        sig = inspect.signature(foo)
+        ba = sig.bind()
+        ba.apply_defaults()
+        self.assertEqual(list(ba.arguments.items()), [])
+
 
 class TestSignaturePrivateHelpers(unittest.TestCase):
     def test_signature_get_bound_param(self):