Add 3.5's apply_defaults method to BoundArguments

README.rst

@@ -273,25 +273,8 @@ function.
 
          Arguments for which :meth:`Signature.bind` or
          :meth:`Signature.bind_partial` relied on a default value are skipped.
-         However, if needed, it is easy to include them.
-
-      ::
-
-        >>> def foo(a, b=10):
-        ...     pass
-
-        >>> sig = signature(foo)
-        >>> ba = sig.bind(5)
-
-        >>> ba.args, ba.kwargs
-        ((5,), {})
-
-        >>> for param in sig.parameters.values():
-        ...     if param.name not in ba.arguments:
-        ...         ba.arguments[param.name] = param.default
-
-        >>> ba.args, ba.kwargs
-        ((5, 10), {})
+         However, if needed, use :meth:`BoundArguments.apply_defaults` to add
+         them.
 
 
    .. attribute:: BoundArguments.args
@@ -303,6 +286,24 @@ function.
 
       A dict of keyword arguments values.  Dynamically computed from the
       :attr:`arguments` attribute.
+
+   .. method:: BoundArguments.apply_defaults()
+
+      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.
+
+      ::
+
+        >>> def foo(a, b='ham', *args): pass
+        >>> ba = inspect.signature(foo).bind('spam')
+        >>> ba.apply_defaults()
+        >>> ba.arguments
+        OrderedDict([('a', 'spam'), ('b', 'ham'), ('args', ())])
 
    The :attr:`args` and :attr:`kwargs` properties can be used to invoke
    functions::

funcsigs/__init__.py

@@ -439,6 +439,34 @@ def kwargs(self):
 
         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
+        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 __hash__(self):
         msg = "unhashable type: '{0}'".format(self.__class__.__name__)
         raise TypeError(msg)

tests/test_inspect.py

@@ -1000,3 +1000,58 @@ def foo(a): pass
         def bar(b): pass
         ba4 = inspect.signature(bar).bind(1)
         self.assertNotEqual(ba, ba4)
+
+    if sys.version_info[0] > 2:
+        exec("""
+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})])
+""")
+
+    def test_signature_bound_arguments_apply_defaults_common(self):
+        def foo(a, b=1, *args, **kw): pass
+        sig = inspect.signature(foo)
+
+        ba = sig.bind(20)
+        ba.apply_defaults()
+        self.assertEqual(
+            list(ba.arguments.items()),
+            [('a', 20), ('b', 1), ('args', ()), ('kw', {})])
+
+        # 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()), [])
+
+        # Make sure a no-args binding still acquires proper defaults.
+        def foo(a='spam'): pass
+        sig = inspect.signature(foo)
+        ba = sig.bind()
+        ba.apply_defaults()
+        self.assertEqual(list(ba.arguments.items()), [('a', 'spam')])