bpo-33197: Add description property for _ParameterKind

Doc/library/inspect.rst

@@ -752,6 +752,23 @@ function.
          ...         print('Parameter:', param)
          Parameter: c
 
+   .. attribute:: Parameter.kind.description
+
+      Describes a enum value of Parameter.kind.
+
+      Example: print all descriptions of arguments::
+
+         >>> def foo(a, b, *, c, d=10):
+         ...     pass
+
+         >>> sig = signature(foo)
+         >>> for param in sig.parameters.values():
+         ...     print(param.kind.description)
+         positional or keyword
+         positional or keyword
+         keyword-only
+         keyword-only
+
    .. method:: Parameter.replace(*[, name][, kind][, default][, annotation])
 
       Create a new Parameter instance based on the instance replaced was invoked

Lib/inspect.py

@@ -2395,6 +2395,9 @@ class _ParameterKind(enum.IntEnum):
     def __str__(self):
         return self._name_
 
+    @property
+    def description(self):
+        return _PARAM_NAME_MAPPING[self]
 
 _POSITIONAL_ONLY         = _ParameterKind.POSITIONAL_ONLY
 _POSITIONAL_OR_KEYWORD   = _ParameterKind.POSITIONAL_OR_KEYWORD
@@ -2410,8 +2413,6 @@ def __str__(self):
     _VAR_KEYWORD: 'variadic keyword'
 }
 
-_get_paramkind_descr = _PARAM_NAME_MAPPING.__getitem__
-
 
 class Parameter:
     """Represents a parameter in a function signature.
@@ -2453,7 +2454,7 @@ def __init__(self, name, kind, *, default=_empty, annotation=_empty):
         if default is not _empty:
             if self._kind in (_VAR_POSITIONAL, _VAR_KEYWORD):
                 msg = '{} parameters cannot have default values'
-                msg = msg.format(_get_paramkind_descr(self._kind))
+                msg = msg.format(self._kind.description)
                 raise ValueError(msg)
         self._default = default
         self._annotation = annotation
@@ -2475,7 +2476,7 @@ def __init__(self, name, kind, *, default=_empty, annotation=_empty):
                     'implicit arguments must be passed as '
                     'positional or keyword arguments, not {}'
                 )
-                msg = msg.format(_get_paramkind_descr(self._kind))
+                msg = msg.format(self._kind.description)
                 raise ValueError(msg)
             self._kind = _POSITIONAL_ONLY
             name = 'implicit{}'.format(name[1:])
@@ -2751,8 +2752,8 @@ def __init__(self, parameters=None, *, return_annotation=_empty,
                             'wrong parameter order: {} parameter before {} '
                             'parameter'
                         )
-                        msg = msg.format(_get_paramkind_descr(top_kind),
-                                         _get_paramkind_descr(kind))
+                        msg = msg.format(top_kind.description,
+                                         kind.description)
                         raise ValueError(msg)
                     elif kind > top_kind:
                         kind_defaults = False

Misc/NEWS.d/next/Library/2018-05-30-00-26-05.bpo-33197.XkE2kL.rst

@@ -0,0 +1 @@
+Add description property for _ParameterKind