3 files changed, 119 insertions, 19 deletions
diff --git a/src/zope/schema/_field.py b/src/zope/schema/_field.py
index 1414b47..0444656 100644
--- a/src/zope/schema/_field.py
+++ b/src/zope/schema/_field.py
@@ -65,6 +65,8 @@ from zope.schema.interfaces import IMinMaxLen
 from zope.schema.interfaces import IMapping
 from zope.schema.interfaces import IMutableMapping
 from zope.schema.interfaces import IMutableSequence
+from zope.schema.interfaces import INativeString
+from zope.schema.interfaces import INativeStringLine
 from zope.schema.interfaces import IObject
 from zope.schema.interfaces import INumber
 from zope.schema.interfaces import IPassword
@@ -170,11 +172,27 @@ class Bytes(MinMaxLen, Field):
         self.validate(value)
         return value
 
-# for things which are of the str type on both Python 2 and 3
-if PY3:  # pragma: no cover
-    NativeString = Text
-else:  # pragma: no cover
-    NativeString = Bytes
+
+@implementer(INativeString, IFromUnicode, IFromBytes)
+class NativeString(Text if PY3 else Bytes):
+    """
+    A native string is always the type `str`.
+
+    In addition to :class:`~zope.schema.interfaces.INativeString`,
+    this implements :class:`~zope.schema.interfaces.IFromUnicode` and
+    :class:`~zope.schema.interfaces.IFromBytes`.
+
+    .. versionchanged:: 4.9.0
+       This is now a distinct type instead of an alias for either `Text` or `Bytes`,
+       depending on the platform.
+    """
+    _type = str
+
+    if PY3:
+        def fromBytes(self, value):
+            value = value.decode('utf-8')
+            self.validate(value)
+            return value
 
 
 @implementer(IASCII)
@@ -191,17 +209,34 @@ class ASCII(NativeString):
 
 @implementer(IBytesLine)
 class BytesLine(Bytes):
-    """A Text field with no newlines."""
+    """A `Bytes` field with no newlines."""
 
     def constraint(self, value):
         # TODO: we should probably use a more general definition of newlines
         return b'\n' not in value
 
-# for things which are of the str type on both Python 2 and 3
-if PY3:  # pragma: no cover
-    NativeStringLine = TextLine
-else:  # pragma: no cover
-    NativeStringLine = BytesLine
+
+@implementer(INativeStringLine, IFromUnicode, IFromBytes)
+class NativeStringLine(TextLine if PY3 else BytesLine):
+    """
+    A native string is always the type `str`; this field excludes
+    newlines.
+
+    In addition to :class:`~zope.schema.interfaces.INativeStringLine`,
+    this implements :class:`~zope.schema.interfaces.IFromUnicode` and
+    :class:`~zope.schema.interfaces.IFromBytes`.
+
+    .. versionchanged:: 4.9.0
+       This is now a distinct type instead of an alias for either `TextLine`
+       or `BytesLine`, depending on the platform.
+    """
+    _type = str
+
+    if PY3:
+        def fromBytes(self, value):
+            value = value.decode('utf-8')
+            self.validate(value)
+            return value
 
 
 @implementer(IASCIILine)
diff --git a/src/zope/schema/interfaces.py b/src/zope/schema/interfaces.py
index c0ac93b..c6c7aea 100644
--- a/src/zope/schema/interfaces.py
+++ b/src/zope/schema/interfaces.py
@@ -416,10 +416,14 @@ class IText(IMinMaxLen, IIterable, IField):
 
 
 # for things which are of the str type on both Python 2 and 3
-if PY3:  # pragma: no cover
-    INativeString = IText
-else:  # pragma: no cover
-    INativeString = IBytes
+class INativeString(IText if PY3 else IBytes):
+    """
+    A field that always contains the native `str` type.
+
+    .. versionchanged:: 4.9.0
+       This is now a distinct type instead of an alias for either `IText`
+       or `IBytes`, depending on the platform.
+    """
 
 
 class IASCII(INativeString):
@@ -446,10 +450,14 @@ class ITextLine(IText):
     """Field containing a unicode string without newlines."""
 
 
-if PY3:  # pragma: no cover
-    INativeStringLine = ITextLine
-else:  # pragma: no cover
-    INativeStringLine = IBytesLine
+class INativeStringLine(ITextLine if PY3 else IBytesLine):
+    """
+    A field that always contains the native `str` type, without any newlines.
+
+    .. versionchanged:: 4.9.0
+       This is now a distinct type instead of an alias for either `ITextLine`
+       or `IBytesLine`, depending on the platform.
+    """
 
 
 class IPassword(ITextLine):
diff --git a/src/zope/schema/tests/test__field.py b/src/zope/schema/tests/test__field.py
index 0eb02ff..4f99d69 100644
--- a/src/zope/schema/tests/test__field.py
+++ b/src/zope/schema/tests/test__field.py
@@ -1702,6 +1702,63 @@ class DictTests(MutableMappingTests):
         with self.assertRaises(WrongType):
             super(DictTests, self).test_mutable_mapping()
 
+class NativeStringTests(EqualityTestsMixin,
+                        WrongTypeTestsMixin,
+                        unittest.TestCase):
+
+    def _getTargetClass(self):
+        from zope.schema._field import NativeString
+        return NativeString
+
+    def _getTargetInterface(self):
+        from zope.schema.interfaces import INativeString
+        return INativeString
+
+    def _getTargetInterfaces(self):
+        from zope.schema.interfaces import IFromUnicode
+        from zope.schema.interfaces import IFromBytes
+        return [self._getTargetInterface(), IFromUnicode, IFromBytes]
+
+    def test_fromBytes(self):
+        field = self._makeOne()
+        self.assertEqual(field.fromBytes(b''), '')
+        self.assertEqual(field.fromBytes(b'DEADBEEF'), 'DEADBEEF')
+
+    def test_fromUnicode(self):
+        field = self._makeOne()
+        self.assertIsInstance(field.fromUnicode(u''), str)
+        self.assertEqual(field.fromUnicode(u''), '')
+        self.assertEqual(field.fromUnicode(u'DEADBEEF'), 'DEADBEEF')
+
+
+class NativeStringLineTests(EqualityTestsMixin,
+                            WrongTypeTestsMixin,
+                            unittest.TestCase):
+
+    def _getTargetClass(self):
+        from zope.schema._field import NativeStringLine
+        return NativeStringLine
+
+    def _getTargetInterface(self):
+        from zope.schema.interfaces import INativeStringLine
+        return INativeStringLine
+
+    def _getTargetInterfaces(self):
+        from zope.schema.interfaces import IFromUnicode
+        from zope.schema.interfaces import IFromBytes
+        return [self._getTargetInterface(), IFromUnicode, IFromBytes]
+
+    def test_fromBytes(self):
+        field = self._makeOne()
+        self.assertEqual(field.fromBytes(b''), '')
+        self.assertEqual(field.fromBytes(b'DEADBEEF'), 'DEADBEEF')
+
+    def test_fromUnicode(self):
+        field = self._makeOne()
+        self.assertIsInstance(field.fromUnicode(u''), str)
+        self.assertEqual(field.fromUnicode(u''), '')
+        self.assertEqual(field.fromUnicode(u'DEADBEEF'), 'DEADBEEF')
+
 
 def _makeSampleVocabulary():
     from zope.interface import implementer