Add is_dataclass().

pep-0557.rst

@@ -623,6 +623,16 @@ Module level helper functions
   ``replace()`` (or similarly named) method which handles instance
   copying.
 
+- ``is_dataclass(class_or_instance)``: Returns True if its parameter
+  is a dataclass or an instance of one, otherwise returns False.
+
+  If you need to know if a class is an instance of a dataclass (and
+  not a dataclass itself), then add a further check for ``not
+  isinstance(obj, type)``:
+
+    def is_dataclass_instance(obj):
+        return is_dataclass(obj) and not isinstance(obj, type)
+
 .. _discussion:
 
 Discussion
@@ -816,27 +826,6 @@ decision is to disallow the 3 known built-in mutable types: list,
 dict, and set.  For a complete discussion of this and other options,
 see [#]_.
 
-Providing isdataclass()
------------------------
-
-An earlier version of this PEP defined an ``isdataclass(obj)`` helper
-function.  However, there was no known use case for this, and there
-was debate on whether it should return ``True`` for instances or
-classes or both.  In the end, ``isdataclass()`` was removed.
-
-The supported way of writing a function that checks if an object is a
-dataclass instance or class is::
-
-  def isdataclass(obj):
-      try:
-          dataclasses.fields(obj)
-          return True
-      except TypeError:
-          return False
-
-If needed, a further check for ``isinstance(obj, type)`` can be added
-to discern if ``obj`` is a class.
-
 Examples
 ========