diff --git a/src/zope/security/_compat.py b/src/zope/security/_compat.py
index 6de4758..f3e441f 100644
--- a/src/zope/security/_compat.py
+++ b/src/zope/security/_compat.py
@@ -14,11 +14,13 @@
 """ Python 2 / 3 compatibility
 """
 import platform
+import os
 import sys
 import types
 
 py_impl = getattr(platform, 'python_implementation', lambda: None)
 PYPY = py_impl() == 'PyPy'
+PURE_PYTHON = os.environ.get('PURE_PYTHON', False)
 
 if sys.version_info[0] < 3: #pragma NO COVER
 
diff --git a/src/zope/security/checker.py b/src/zope/security/checker.py
index 061cc44..89ae214 100644
--- a/src/zope/security/checker.py
+++ b/src/zope/security/checker.py
@@ -177,6 +177,8 @@ def __init__(self, get_permissions, set_permissions=None):
         if set_permissions is not None:
             if not isinstance(set_permissions, dict):
                 raise TypeError('set_permissions must be a dict')
+        else:
+            set_permissions = {}
         self.set_permissions = set_permissions
 
     def permission_id(self, name):
diff --git a/src/zope/security/protectclass.py b/src/zope/security/protectclass.py
index 57effb3..7e91f22 100644
--- a/src/zope/security/protectclass.py
+++ b/src/zope/security/protectclass.py
@@ -38,7 +38,6 @@ def protectName(class_, name, permission):
 
 def protectSetAttribute(class_, name, permission):
     """Set a permission on a particular name."""
-
     checker = getCheckerForInstancesOf(class_)
     if checker is None:
         checker = Checker({}, {})
diff --git a/src/zope/security/proxy.py b/src/zope/security/proxy.py
index 974b923..7f7d626 100644
--- a/src/zope/security/proxy.py
+++ b/src/zope/security/proxy.py
@@ -114,13 +114,19 @@ def __delattr__(self, name):
 
     @_check_name
     def __getslice__(self, start, end):
-        getitem = PyProxyBase.__getattribute__(self, '__getitem__')
-        return getitem(slice(start, end))
+        try:
+            return self._wrapped.__getslice__(start, end)
+        except:
+            getitem = PyProxyBase.__getattribute__(self, '__getitem__')
+            return getitem(slice(start, end))
 
     @_check_name
     def __setslice__(self, i, j, value):
-        setitem = PyProxyBase.__getattribute__(self, '__setitem__')
-        return setitem(slice(i, j), value)
+        try:
+            return self._wrapped.__setslice__(i, j, value)
+        except:
+            setitem = PyProxyBase.__getattribute__(self, '__setitem__')
+            return setitem(slice(i, j), value)
 
     def __cmp__(self, other):
         # no check
@@ -172,17 +178,14 @@ def __coerce__(self, other):
         # For some reason _check_name does not work for coerce()
         wrapped = super(PyProxyBase, self).__getattribute__('_wrapped')
         checker = super(PyProxyBase, self).__getattribute__('_checker')
-        checker.check_getattr(wrapped, '__coerce__')
-        # Re-implement __coerce__(), so we do not depend on self._wrapped
-        left, right = coerce(wrapped, other)
-        if left == wrapped and type(left) is type(wrapped):
-            left = self
-        return left, right
+        checker.check(wrapped, '__coerce__')
+        return super(ProxyPy, self).__coerce__(other)
 
     def __str__(self):
         try:
             return _check_name(PyProxyBase.__str__)(self)
-        except ForbiddenAttribute:
+        except:
+            # The C implementation catches all exceptions.
             wrapped = super(PyProxyBase, self).__getattribute__('_wrapped')
             return '<security proxied %s.%s instance at %s>' %(
                 wrapped.__class__.__module__, wrapped.__class__.__name__,
@@ -191,7 +194,8 @@ def __str__(self):
     def __repr__(self):
         try:
             return _check_name(PyProxyBase.__repr__)(self)
-        except ForbiddenAttribute:
+        except:
+            # The C implementation catches all exceptions.
             wrapped = super(PyProxyBase, self).__getattribute__('_wrapped')
             return '<security proxied %s.%s instance at %s>' %(
                 wrapped.__class__.__module__, wrapped.__class__.__name__,
diff --git a/src/zope/security/tests/test_proxy.py b/src/zope/security/tests/test_proxy.py
index 974c26f..f73990d 100644
--- a/src/zope/security/tests/test_proxy.py
+++ b/src/zope/security/tests/test_proxy.py
@@ -16,7 +16,7 @@
 import unittest
 import sys
 
-from zope.security._compat import PYTHON2, PYPY
+from zope.security._compat import PYTHON2, PYPY, PURE_PYTHON
 
 def _skip_if_not_Py2(testfunc):
     from functools import update_wrapper
@@ -1946,6 +1946,6 @@ def test_suite():
         unittest.makeSuite(ProxyTests),
         unittest.makeSuite(LocationProxySecurityCheckerTests),
     ))
-    if not PYPY:
+    if not (PYPY or PURE_PYTHON):
         suite.addTest(unittest.makeSuite(ProxyCTests))
     return suite