Variant of patch #1478292. doctest.register_optionflag(name)

shouldn't create a new flag when `name` is already the name of
an option flag.

Lib/doctest.py

@@ -129,9 +129,8 @@ warnings.filterwarnings("ignore", "is_private", DeprecationWarning,
 
 OPTIONFLAGS_BY_NAME = {}
 def register_optionflag(name):
-    flag = 1 << len(OPTIONFLAGS_BY_NAME)
-    OPTIONFLAGS_BY_NAME[name] = flag
-    return flag
+    # Create a new flag unless `name` is already known.
+    return OPTIONFLAGS_BY_NAME.setdefault(name, 1 << len(OPTIONFLAGS_BY_NAME))
 
 DONT_ACCEPT_TRUE_FOR_1 = register_optionflag('DONT_ACCEPT_TRUE_FOR_1')
 DONT_ACCEPT_BLANKLINE = register_optionflag('DONT_ACCEPT_BLANKLINE')

Lib/test/test_doctest.py

@@ -1300,6 +1300,26 @@ count as failures:
         ValueError: 2
     (3, 5)
 
+New option flags can also be registered, via register_optionflag().  Here
+we reach into doctest's internals a bit.
+
+    >>> unlikely = "UNLIKELY_OPTION_NAME"
+    >>> unlikely in doctest.OPTIONFLAGS_BY_NAME
+    False
+    >>> new_flag_value = doctest.register_optionflag(unlikely)
+    >>> unlikely in doctest.OPTIONFLAGS_BY_NAME
+    True
+
+Before 2.4.4/2.5, registering a name more than once erroneously created
+more than one flag value.  Here we verify that's fixed:
+
+    >>> redundant_flag_value = doctest.register_optionflag(unlikely)
+    >>> redundant_flag_value == new_flag_value
+    True
+
+Clean up.
+    >>> del doctest.OPTIONFLAGS_BY_NAME[unlikely]
+
     """
 
     def option_directives(): r"""

Misc/NEWS

@@ -67,7 +67,7 @@ Core and builtins
 Extension Modules
 -----------------
 
-- Use Win32 API to implement os.{access,chdir,chmod,mkdir,remove,rename,rmdir,utime}. 
+- Use Win32 API to implement os.{access,chdir,chmod,mkdir,remove,rename,rmdir,utime}.
   As a result, these functions now raise WindowsError instead of OSError.
 
 - Calling Tk_Init twice is refused if the first call failed as that
@@ -96,6 +96,9 @@ Extension Modules
 Library
 -------
 
+- Patch #1478292. ``doctest.register_optionflag(name)`` shouldn't create a
+  new flag when ``name`` is already the name of an option flag.
+
 - Bug #1385040: don't allow "def foo(a=1, b): pass" in the compiler
   package.