Removed isdataclass().

pep-0557.rst

@@ -6,7 +6,7 @@ Type: Standards Track
 Content-Type: text/x-rst
 Created: 02-Jun-2017
 Python-Version: 3.7
-Post-History: 08-Sep-2017, 25-Nov-2017, 30-Nov-2017
+Post-History: 08-Sep-2017, 25-Nov-2017, 30-Nov-2017, 01-Dec-2017
 
 Notice for Reviewers
 ====================
@@ -570,9 +570,6 @@ Module level helper functions
 
   Raises ``TypeError`` if ``instance`` is not a Data Class instance.
 
-- ``isdataclass(instance)``: Returns ``True`` if ``instance`` is an
-  instance of a Data Class, otherwise returns ``False``.
-
 - ``make_dataclass(cls_name, fields, *, bases=(), namespace=None)``:
   Creates a new Data Class with name ``cls_name``, fields as defined
   in ``fields``, base classes as given in ``bases``, and initialized
@@ -798,6 +795,27 @@ 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
 ========