Fix GlobalObject breaking with just a .

Fixes #35.

CHANGES.rst

@@ -4,7 +4,9 @@ Changes
 4.2.1 (unreleased)
 ------------------
 
-- Nothing changed yet.
+- Fix ``GlobalObject`` (and ``GlobalInterface``) no longer allowing
+  just a single '.'. See `issue 35
+  <https://github.com/zopefoundation/zope.configuration/issues/35>`_.
 
 
 4.2.0 (2018-09-26)

src/zope/configuration/fields.py

@@ -35,7 +35,11 @@
 
 class PythonIdentifier(schema_PythonIdentifier):
     """
-    This class is like `zope.schema.PythonIdentifier`, but does not allow empty strings.
+    This class is like `zope.schema.PythonIdentifier`.
+
+    .. versionchanged:: 4.2.0
+       Extend `zope.schema.PythonIdentifier`, which implies that `fromUnicode`
+       validates the strings.
     """
 
     def _validate(self, value):
@@ -73,9 +77,12 @@ def fromUnicode(self, value):
 
         try:
             # Leading dots are allowed here to indicate current
-            # package.
+            # package, but not accepted by DottedName. Take care,
+            # though, because a single dot is valid to resolve, but
+            # not valid to pass to DottedName (as an empty string)
             to_validate = name[1:] if name.startswith('.') else name
-            self._DOT_VALIDATOR.validate(to_validate)
+            if to_validate:
+                self._DOT_VALIDATOR.validate(to_validate)
         except ValidationError as v:
             v.with_field_and_value(self, name)
             raise

src/zope/configuration/tests/test_fields.py

@@ -92,9 +92,23 @@ def resolve(self, name):
         context = Context()
         bound = go.bind(context)
         found = bound.fromUnicode('tried')
-        self.assertTrue(found is _target)
+        self.assertIs(found, _target)
         self.assertEqual(context._resolved, 'tried')
 
+    def test_fromUnicode_w_resolve_dot(self):
+        _target = object()
+        class Context(object):
+            _resolved = None
+            def resolve(self, name):
+                self._resolved = name
+                return _target
+        go = self._makeOne()
+        context = Context()
+        bound = go.bind(context)
+        found = bound.fromUnicode('.')
+        self.assertIs(found, _target)
+        self.assertEqual(context._resolved, '.')
+
     def test_fromUnicode_w_resolve_but_validation_fails(self):
         from zope.schema import Text
         from zope.schema import ValidationError