diff --git a/Lib/idlelib/idle_test/test_pyparse.py b/Lib/idlelib/idle_test/test_pyparse.py
index c7b645caa6a85d..a94192801095df 100644
--- a/Lib/idlelib/idle_test/test_pyparse.py
+++ b/Lib/idlelib/idle_test/test_pyparse.py
@@ -8,46 +8,6 @@
 from idlelib import pyparse
 
 
-class StringTranslatePseudoMappingTest(unittest.TestCase):
-
-    @classmethod
-    def setUpClass(cls):
-        whitespace_chars = ' \t\n\r'
-        cls.preserve_dict = {ord(c): ord(c) for c in whitespace_chars}
-        cls.default = ord('x')
-        cls.mapping = pyparse.StringTranslatePseudoMapping(
-                                cls.preserve_dict, default_value=ord('x'))
-
-    @classmethod
-    def tearDownClass(cls):
-        del cls.preserve_dict, cls.default, cls.mapping
-
-    def test__init__(self):
-        m = self.mapping
-        self.assertEqual(m._non_defaults, self.preserve_dict)
-        self.assertEqual(m._default_value, self.default)
-
-    def test__get_item__(self):
-        self.assertEqual(self.mapping[ord('\t')], ord('\t'))
-        self.assertEqual(self.mapping[ord('a')], self.default)
-
-    def test__len__(self):
-        self.assertEqual(len(self.mapping), len(self.preserve_dict))
-
-    def test__iter__(self):
-        count = 0
-        for key, value in self.mapping.items():
-            self.assertIn(key, self.preserve_dict)
-            count += 1
-        self.assertEqual(count, len(self.mapping))
-
-    def test_get(self):
-        self.assertEqual(self.mapping.get(ord('\t')), ord('\t'))
-        self.assertEqual(self.mapping.get('a'), self.default)
-        # Default is a parameter, but it isn't used.
-        self.assertEqual(self.mapping.get('a', default=500), self.default)
-
-
 class PyParseTest(unittest.TestCase):
 
     @classmethod
diff --git a/Lib/idlelib/pyparse.py b/Lib/idlelib/pyparse.py
index 443d198d1efe4d..fa8cba72ecb530 100644
--- a/Lib/idlelib/pyparse.py
+++ b/Lib/idlelib/pyparse.py
@@ -10,7 +10,7 @@
 _closere - line that must be followed by dedent.
 _chew_ordinaryre - non-special characters.
 """
-from collections.abc import Mapping
+from collections import defaultdict
 import re
 import sys
 
@@ -101,48 +101,6 @@
 """, re.VERBOSE).match
 
 
-class StringTranslatePseudoMapping(Mapping):
-    r"""Utility class to be used with str.translate()
-
-    This Mapping class wraps a given dict. When a value for a key is
-    requested via __getitem__() or get(), the key is looked up in the
-    given dict. If found there, the value from the dict is returned.
-    Otherwise, the default value given upon initialization is returned.
-
-    This allows using str.translate() to make some replacements, and to
-    replace all characters for which no replacement was specified with
-    a given character instead of leaving them as-is.
-
-    For example, to replace everything except whitespace with 'x':
-
-    >>> whitespace_chars = ' \t\n\r'
-    >>> preserve_dict = {ord(c): ord(c) for c in whitespace_chars}
-    >>> mapping = StringTranslatePseudoMapping(preserve_dict, ord('x'))
-    >>> text = "a + b\tc\nd"
-    >>> text.translate(mapping)
-    'x x x\tx\nx'
-    """
-    def __init__(self, non_defaults, default_value):
-        self._non_defaults = non_defaults
-        self._default_value = default_value
-
-        def _get(key, _get=non_defaults.get, _default=default_value):
-            return _get(key, _default)
-        self._get = _get
-
-    def __getitem__(self, item):
-        return self._get(item)
-
-    def __len__(self):
-        return len(self._non_defaults)
-
-    def __iter__(self):
-        return iter(self._non_defaults)
-
-    def get(self, key, default=None):
-        return self._get(key)
-
-
 class Parser:
 
     def __init__(self, indentwidth, tabwidth):
@@ -228,11 +186,10 @@ def set_lo(self, lo):
     # brackets to '(', close brackets to ')' while preserving quotes,
     # backslashes, newlines and hashes. This is to be passed to
     # str.translate() in _study1().
-    _tran = {}
+    _tran = defaultdict(lambda: 'x')
     _tran.update((ord(c), ord('(')) for c in "({[")
     _tran.update((ord(c), ord(')')) for c in ")}]")
     _tran.update((ord(c), ord(c)) for c in "\"'\\\n#")
-    _tran = StringTranslatePseudoMapping(_tran, default_value=ord('x'))
 
     def _study1(self):
         """Find the line numbers of non-continuation lines.
diff --git a/Misc/NEWS.d/next/IDLE/2018-02-24-12-39-29.bpo-32940.hT65Wf.rst b/Misc/NEWS.d/next/IDLE/2018-02-24-12-39-29.bpo-32940.hT65Wf.rst
new file mode 100644
index 00000000000000..82003be29201ca
--- /dev/null
+++ b/Misc/NEWS.d/next/IDLE/2018-02-24-12-39-29.bpo-32940.hT65Wf.rst
@@ -0,0 +1 @@
+Replace StringTranslatePseudoMapping with a defaultdict.