diff -r a36d469f31c1 Lib/lib2to3/fixes/fix_string.py
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/Lib/lib2to3/fixes/fix_string.py	Tue Sep 09 16:06:34 2014 -0400
@@ -0,0 +1,41 @@
+"""Fixer for string constants.
+
+This prefixes lowercase, uppercase, and letters with ascii_
+"""
+
+# Local imports
+from .. import fixer_base
+from ..fixer_util import Name, token, find_binding, syms
+
+
+class FixString(fixer_base.BaseFix):
+    BM_compatible = True
+    constants = "('uppercase'|'lowercase'|'letters')"
+    PATTERN = """
+              const=%(constants)s 
+              """ % (locals())
+
+    # Needs to be run before fix_string_imports
+    run_order = 4
+
+    def start_tree(self, tree, filename):
+        super(FixString, self).start_tree(tree, filename)
+        warning = "%s appears to be user-defined, not renaming to %s"
+
+        # check whether the name has been defined by the user
+        # currently only detects top level definitions - see tests
+        self.user_defs = set()
+        for name in ["uppercase","lowercase","letters"]:
+            binding = find_binding(name, tree)
+            if binding:
+                if (binding.type == syms.import_from
+                    and binding.children[1].value == "string"):
+                    continue
+                self.warning(binding, warning % (name, "ascii_" + name))
+                self.user_defs.add(name)
+
+    def transform(self, node, results):
+        const = results['const'][0]
+        if const.value not in self.user_defs:
+            assert const.type == token.NAME
+            const.replace(Name("ascii_" + const.value, prefix=node.prefix))
diff -r a36d469f31c1 Lib/lib2to3/fixes/fix_string_imports.py
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/Lib/lib2to3/fixes/fix_string_imports.py	Tue Sep 09 16:06:34 2014 -0400
@@ -0,0 +1,45 @@
+""" Remove duplicate imports which may be created by FixString """
+
+# Local imports
+from lib2to3 import fixer_base
+from lib2to3.fixer_util import syms, token
+
+
+class FixStringImports(fixer_base.BaseFix):
+    BM_compatible = True
+    PATTERN = """
+              import_from< 'from' 'string' 'import' imports=any >
+              """ % (locals())
+
+    def transform(self, node, results):
+        imports = results['imports']
+        children = imports.children[:] or [imports]
+
+        if children[0].type == token.STAR:
+            return
+
+        found = set()
+        prev = None
+
+        for child in children:
+            if child.type == token.COMMA:
+                prev = child
+                continue
+
+            if child.type == token.NAME:
+                name_node = child
+            else:
+                assert child.type == syms.import_as_name
+                name_node = child.children[0]
+            
+            if name_node.value in found:
+                child.value = None
+                child.remove()
+                if prev and prev.type == token.COMMA:
+                    prev.value = None
+                    prev.remove()
+
+            found.add(name_node.value)
+            prev = child
+
+
diff -r a36d469f31c1 Lib/lib2to3/tests/test_fixers.py
--- a/Lib/lib2to3/tests/test_fixers.py	Sun Aug 03 15:26:32 2014 -0400
+++ b/Lib/lib2to3/tests/test_fixers.py	Tue Sep 09 16:06:34 2014 -0400
@@ -3647,6 +3647,88 @@
     def test_run_order(self):
         self.assert_runs_after('print')
 
+
+class Test_string_imports(FixerTestCase):
+    fixer = "string_imports"
+
+    def test_simple(self):
+        s = "from string import lowercase, uppercase, letters"
+        self.unchanged(s)
+
+    def test_duplicate(self):
+        b = "from string import ascii_lowercase, foo, ascii_lowercase"
+        a = "from string import ascii_lowercase, foo"
+        self.check(b, a)
+
+    def test_as(self):
+        b = "from string import uppercase as foo, uppercase as bar"
+        a = "from string import uppercase as foo"
+        self.check(b, a)
+
+    def test_star(self):
+        s = "from string import *"
+        self.unchanged(s)
+
+
+class Test_string(FixerTestCase):
+    fixer = "string"
+
+    def test_import(self):
+        b = "from string import lowercase, uppercase, letters"
+        a = "from string import ascii_lowercase, ascii_uppercase, ascii_letters"
+        self.check(b, a)
+
+    def test_import_as(self):
+        b = "from string import uppercase as foo"
+        a = "from string import ascii_uppercase as foo"
+        self.check(b, a)
+
+    def test_replace(self):
+        b = "from string import lowercase\nprint lowercase"
+        a = "from string import ascii_lowercase\nprint ascii_lowercase"
+        self.check(b,a)
+
+    def test_star(self):
+        b = "from string import *\nprint letters"
+        a = "from string import *\nprint ascii_letters"
+        self.check(b,a)
+
+    def test_qualified(self):
+        b = "print string.lowercase"
+        a = "print string.ascii_lowercase"
+        self.check(b,a)
+
+    def test_user_def(self):
+        s = "lowercase = 'foo'\nprint lowercase"
+        self.warns_unchanged(s, "lowercase appears to be user-defined, not renaming to ascii_lowercase")
+
+    def test_nonstring_import(self):
+        s = "from mystringpackage import uppercase"
+        self.warns_unchanged(s, "uppercase appears to be user-defined, not renaming to ascii_uppercase")
+
+    def test_import_def(self):
+        s = "from foo import bar as letters\nprint letters"
+        self.warns_unchanged(s, "letters appears to be user-defined, not renaming to ascii_letters")
+
+    def test_functions(self):
+        s = "def letters():\n  return 'abc'\nletters()"
+        self.warns_unchanged(s, "letters appears to be user-defined, not renaming to ascii_letters")
+
+    # lowercase shouldn't get changed here but it currently does
+    def XXX_test_class(self):
+        s = """
+class Foo:
+    def __init__(self): 
+        self.lowercase = "blargh"
+
+f = Foo()
+f.__init__()
+print f.lowercase
+
+"""
+        self.warns_unchanged(s, "lowercase appears to be user-defined, not renaming to ascii_lowercase")
+
+
 class Test_itertools(FixerTestCase):
     fixer = "itertools"