Respect PURE_PYTHON at runtime. Partial fix for #33.

CHANGES.rst

@@ -32,6 +32,9 @@ Changes
 - Simplify the internal ``_compat.py`` module now that we only run on
   newer Python versions. See `PR 32 <https://github.com/zopefoundation/zope.security/pull/32>`_.
 
+- Respect ``PURE_PYTHON`` at runtime. See `issue 33
+  <https://github.com/zopefoundation/zope.security/issues/33>`_.
+
 4.1.1 (2017-05-17)
 ------------------
 

src/zope/security/checker.py

@@ -49,6 +49,7 @@
 from zope.security._definitions import thread_local
 from zope.security._compat import CLASS_TYPES
 from zope.security._compat import PYTHON2
+from zope.security._compat import PURE_PYTHON
 from zope.security.proxy import Proxy
 from zope.security.proxy import getChecker
 
@@ -438,11 +439,14 @@ def undefineChecker(type_):
 _available_by_default = []
 
 # Get optimized versions
-try:
-    import zope.security._zope_security_checker
-except (ImportError, AttributeError): #pragma NO COVER PyPy / PURE_PYTHON
-    pass
-else:
+_c_available = not PURE_PYTHON
+if _c_available:
+    try:
+        import zope.security._zope_security_checker
+    except (ImportError, AttributeError): #pragma NO COVER PyPy / PURE_PYTHON
+        _c_available = False
+
+if _c_available:
     from zope.security._zope_security_checker import _checkers, selectChecker
     from zope.security._zope_security_checker import NoProxy, Checker
     from zope.security._zope_security_checker import _defaultChecker

src/zope/security/proxy.py

@@ -17,7 +17,7 @@
 import sys
 
 from zope.proxy import PyProxyBase
-from zope.security._compat import PYPY
+from zope.security._compat import PYPY, PURE_PYTHON
 from zope.security.interfaces import ForbiddenAttribute
 
 def _check_name(meth, wrap_result=True):
@@ -346,9 +346,14 @@ def getObjectPy(proxy):
         return proxy
     return super(PyProxyBase, proxy).__getattribute__('_wrapped')
 
-try:
-    from zope.security._proxy import _Proxy
-except (ImportError, AttributeError): #pragma NO COVER PyPy / PURE_PYTHON
+_c_available = not PURE_PYTHON
+if _c_available:
+    try:
+        from zope.security._proxy import _Proxy
+    except (ImportError, AttributeError): #pragma NO COVER PyPy / PURE_PYTHON
+        _c_available = False
+
+if not _c_available:
     getChecker = getCheckerPy
     getObject = getObjectPy
     Proxy = ProxyPy