bpo-33504: Migrate configparser from OrderedDict to dict. (#6819)

With 3.7+, dictionary are ordered by design.  Configparser still uses
collections.OrderedDict, which is unnecessary.  This updates the module
to use the standard dict implementation by default, and changes the
docs and tests to match.

5 files changed, 15 insertions, 43 deletions

diff --git a/Doc/library/configparser.rst b/Doc/library/configparser.rst
index f51c580c0d..5a2178f11b 100644
--- a/Doc/library/configparser.rst
+++ b/Doc/library/configparser.rst
@@ -445,20 +445,19 @@ the :meth:`__init__` options:
   Hint: if you want to specify default values for a specific section, use
   :meth:`read_dict` before you read the actual file.
 
-* *dict_type*, default value: :class:`collections.OrderedDict`
+* *dict_type*, default value: :class:`dict`
 
   This option has a major impact on how the mapping protocol will behave and how
-  the written configuration files look.  With the default ordered
-  dictionary, every section is stored in the order they were added to the
-  parser.  Same goes for options within sections.
+  the written configuration files look.  With the standard dictionary, every
+  section is stored in the order they were added to the parser.  Same goes for
+  options within sections.
 
   An alternative dictionary type can be used for example to sort sections and
-  options on write-back.  You can also use a regular dictionary for performance
-  reasons.
+  options on write-back.
 
   Please note: there are ways to add a set of key-value pairs in a single
   operation.  When you use a regular dictionary in those operations, the order
-  of the keys may be random.  For example:
+  of the keys will be ordered.  For example:
 
   .. doctest::
 
@@ -474,40 +473,9 @@ the :meth:`__init__` options:
      ...                                'baz': 'z'}
      ... })
      >>> parser.sections()  # doctest: +SKIP
-     ['section3', 'section2', 'section1']
-     >>> [option for option in parser['section3']] # doctest: +SKIP
-     ['baz', 'foo', 'bar']
-
-  In these operations you need to use an ordered dictionary as well:
-
-  .. doctest::
-
-     >>> from collections import OrderedDict
-     >>> parser = configparser.ConfigParser()
-     >>> parser.read_dict(
-     ...   OrderedDict((
-     ...     ('s1',
-     ...      OrderedDict((
-     ...        ('1', '2'),
-     ...        ('3', '4'),
-     ...        ('5', '6'),
-     ...      ))
-     ...     ),
-     ...     ('s2',
-     ...      OrderedDict((
-     ...        ('a', 'b'),
-     ...        ('c', 'd'),
-     ...        ('e', 'f'),
-     ...      ))
-     ...     ),
-     ...   ))
-     ... )
-     >>> parser.sections()  # doctest: +SKIP
-     ['s1', 's2']
-     >>> [option for option in parser['s1']]  # doctest: +SKIP
-     ['1', '3', '5']
-     >>> [option for option in parser['s2'].values()]  # doctest: +SKIP
-     ['b', 'd', 'f']
+     ['section1', 'section2', 'section3']
+     >>> [option for option in parser['section3']]  # doctest: +SKIP
+     ['foo', 'bar', 'baz']
 
 * *allow_no_value*, default value: ``False``
 
diff --git a/Lib/configparser.py b/Lib/configparser.py
index a681d3990e..c88605feff 100644
--- a/Lib/configparser.py
+++ b/Lib/configparser.py
@@ -139,7 +139,7 @@ ConfigParser -- responsible for parsing a list of
 """
 
 from collections.abc import MutableMapping
-from collections import OrderedDict as _default_dict, ChainMap as _ChainMap
+from collections import ChainMap as _ChainMap
 import functools
 import io
 import itertools
@@ -157,6 +157,7 @@ __all__ = ["NoSectionError", "DuplicateOptionError", "DuplicateSectionError",
            "LegacyInterpolation", "SectionProxy", "ConverterMapping",
            "DEFAULTSECT", "MAX_INTERPOLATION_DEPTH"]
 
+_default_dict = dict
 DEFAULTSECT = "DEFAULT"
 
 MAX_INTERPOLATION_DEPTH = 10
diff --git a/Lib/test/test_configparser.py b/Lib/test/test_configparser.py
index 87b811f09b..f4df622050 100644
--- a/Lib/test/test_configparser.py
+++ b/Lib/test/test_configparser.py
@@ -1109,7 +1109,7 @@ class RawConfigParserTestCase(BasicTestCase, unittest.TestCase):
         self.assertEqual(cf.get(123, 'this is sick'), True)
         if cf._dict is configparser._default_dict:
             # would not work for SortedDict; only checking for the most common
-            # default dictionary (OrderedDict)
+            # default dictionary (dict)
             cf.optionxform = lambda x: x
             cf.set('non-string', 1, 1)
             self.assertEqual(cf.get('non-string', 1), 1)
diff --git a/Misc/ACKS b/Misc/ACKS
index 42f1abc8da..481385f36b 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -1315,6 +1315,7 @@ Marc Recht
 John Redford
 Terry J. Reedy
 Gareth Rees
+John Reese
 Steve Reeves
 Lennart Regebro
 John Regehr
diff --git a/Misc/NEWS.d/next/Library/2018-05-15-12-11-13.bpo-33504.czsHFg.rst b/Misc/NEWS.d/next/Library/2018-05-15-12-11-13.bpo-33504.czsHFg.rst
new file mode 100644
index 0000000000..d5065d9224
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2018-05-15-12-11-13.bpo-33504.czsHFg.rst
@@ -0,0 +1,2 @@
+Switch the default dictionary implementation for :mod:`configparser` from
+:class:`collections.OrderedDict` to the standard :class:`dict` type.