Merge pull request #151 from zopefoundation/no-pure-python-build

Remove checking of PURE_PYTHON at build time

18 files changed, 477 insertions, 435 deletions

diff --git a/.travis.yml b/.travis.yml
index 7c596f1..dd4c3e0 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -27,6 +27,14 @@ jobs:
         - sphinx-build -b doctest -d docs/_build/doctrees docs docs/_build/doctest
       after_success:
 
+    - name: PyPy C Extensions
+      env: PURE_PYTHON=0
+      python: pypy
+
+    - name: CPython No C Extension
+      env: PURE_PYTHON=1
+      python: 3.8
+
     # manylinux wheel builds
     - name: 64-bit manylinux wheels (all Pythons)
       services: docker
@@ -80,7 +88,7 @@ install:
 
 script:
   - python --version
-  - coverage run setup.py -q test
+  - coverage run -m unittest discover -s src
   - python setup.py -q bdist_wheel
 
 after_success:
diff --git a/CHANGES.rst b/CHANGES.rst
index f4b5620..0039191 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -1,21 +1,26 @@
-Changes
-=======
+=========
+ Changes
+=========
 
-4.7.2 (unreleased)
-------------------
+4.8.0 (unreleased)
+==================
 
-- Nothing changed yet.
+- Support the ``PURE_PYTHON`` environment variable at runtime instead
+  of just at wheel build time. A value of 0 forces the C extensions to
+  be used (even on PyPy) failing if they aren't present. Any other
+  value forces the Python implementation to be used, ignoring the C
+  extensions.
 
 
 4.7.1 (2019-11-11)
-------------------
+==================
 
 - Use Python 3 syntax in the documentation.  See `issue 119
   <https://github.com/zopefoundation/zope.interface/issues/119>`_.
 
 
 4.7.0 (2019-11-11)
-------------------
+==================
 
 - Drop support for Python 3.4.
 
@@ -27,7 +32,7 @@ Changes
 
 
 4.6.0 (2018-10-23)
-------------------
+==================
 
 - Add support for Python 3.7
 
@@ -37,7 +42,7 @@ Changes
 
 
 4.5.0 (2018-04-19)
-------------------
+==================
 
 - Drop support for 3.3, avoid accidental dependence breakage via setup.py.
   See `PR 110 <https://github.com/zopefoundation/zope.interface/pull/110>`_.
@@ -49,7 +54,7 @@ Changes
 
 
 4.4.3 (2017-09-22)
-------------------
+==================
 
 - Avoid exceptions when the ``__annotations__`` attribute is added to
   interface definitions with Python 3.x type hints. See `issue 98
@@ -60,7 +65,7 @@ Changes
 
 
 4.4.2 (2017-06-14)
-------------------
+==================
 
 - Fix a regression storing
   ``zope.component.persistentregistry.PersistentRegistry`` instances.
@@ -71,7 +76,7 @@ Changes
   <https://github.com/zopefoundation/zope.interface/issues/93>`_.
 
 4.4.1 (2017-05-13)
-------------------
+==================
 
 - Simplify the caching of utility-registration data. In addition to
   simplification, avoids spurious test failures when checking for
@@ -82,7 +87,7 @@ Changes
   methods:  prevents corruption of lookup caches.
 
 4.4.0 (2017-04-21)
-------------------
+==================
 
 - Avoid a warning from the C compiler.
   (https://github.com/zopefoundation/zope.interface/issues/71)
@@ -90,7 +95,7 @@ Changes
 - Add support for Python 3.6.
 
 4.3.3 (2016-12-13)
-------------------
+==================
 
 - Correct typos and ReST formatting errors in documentation.
 
@@ -104,21 +109,21 @@ Changes
 
 
 4.3.2 (2016-09-05)
-------------------
+==================
 
 - Fix equality testing of ``implementedBy`` objects and proxies.
   (https://github.com/zopefoundation/zope.interface/issues/55)
 
 
 4.3.1 (2016-08-31)
-------------------
+==================
 
 - Support Components subclasses that are not hashable.
   (https://github.com/zopefoundation/zope.interface/issues/53)
 
 
 4.3.0 (2016-08-31)
-------------------
+==================
 
 - Add the ability to sort the objects returned by ``implementedBy``.
   This is compatible with the way interface classes sort so they can
@@ -136,7 +141,7 @@ Changes
 
 
 4.2.0 (2016-06-10)
-------------------
+==================
 
 - Add support for Python 3.5
 
@@ -144,14 +149,14 @@ Changes
 
 
 4.1.3 (2015-10-05)
-------------------
+==================
 
 - Fix installation without a C compiler on Python 3.5
   (https://github.com/zopefoundation/zope.interface/issues/24).
 
 
 4.1.2 (2014-12-27)
-------------------
+==================
 
 - Add support for PyPy3.
 
@@ -162,13 +167,13 @@ Changes
 
 
 4.1.1 (2014-03-19)
-------------------
+==================
 
 - Add support for Python 3.4.
 
 
 4.1.0 (2014-02-05)
-------------------
+==================
 
 - Update ``boostrap.py`` to version 2.2.
 
@@ -177,25 +182,25 @@ Changes
 
 
 4.0.5 (2013-02-28)
-------------------
+==================
 
 - Fix a bug where a decorated method caused false positive failures on
   ``verifyClass()``.
 
 
 4.0.4 (2013-02-21)
-------------------
+==================
 
 - Fix a bug that was revealed by porting zope.traversing. During a loop, the
   loop body modified a weakref dict causing a ``RuntimeError`` error.
 
 4.0.3 (2012-12-31)
-------------------
+==================
 
 - Fleshed out PyPI Trove classifiers.
 
 4.0.2 (2012-11-21)
-------------------
+==================
 
 - Add support for Python 3.3.
 
@@ -205,7 +210,7 @@ Changes
   in Python 3.3.
 
 4.0.1 (2012-05-22)
-------------------
+==================
 
 - Drop explicit ``DeprecationWarnings`` for "class advice" APIS (these
   APIs are still deprecated under Python 2.x, and still raise an exception
@@ -213,7 +218,7 @@ Changes
   Python 2.x).
 
 4.0.0 (2012-05-16)
-------------------
+==================
 
 - Automated build of Sphinx HTML docs and running doctest snippets via tox.
 
@@ -252,7 +257,7 @@ Changes
   Pitrou for the patch.
 
 3.8.0 (2011-09-22)
-------------------
+==================
 
 - New module ``zope.interface.registry``.  This is code moved from
   ``zope.component.registry`` which implements a basic nonperistent component
@@ -281,18 +286,18 @@ Changes
 - No longer Python 2.4 compatible (tested under 2.5, 2.6, 2.7, and 3.2).
 
 3.7.0 (2011-08-13)
-------------------
+==================
 
 - Move changes from 3.6.2 - 3.6.5 to a new 3.7.x release line.
 
 3.6.7 (2011-08-20)
-------------------
+==================
 
 - Fix sporadic failures on x86-64 platforms in tests of rich comparisons
   of interfaces.
 
 3.6.6 (2011-08-13)
-------------------
+==================
 
 - LP #570942:  Now correctly compare interfaces  from different modules but
   with the same names.
@@ -304,7 +309,7 @@ Changes
 - Revert to software as released with 3.6.1 for "stable" 3.6 release branch.
 
 3.6.5 (2011-08-11)
-------------------
+==================
 
 - LP #811792:  work around buggy behavior in some subclasses of
   ``zope.interface.interface.InterfaceClass``, which invoke ``__hash__``
@@ -319,18 +324,18 @@ Changes
 - Fix testing deprecation warnings issued when tested under Py3K.
 
 3.6.4 (2011-07-04)
-------------------
+==================
 
 - LP 804951:  InterfaceClass instances were unhashable under Python 3.x.
 
 3.6.3 (2011-05-26)
-------------------
+==================
 
 - LP #570942:  Now correctly compare interfaces  from different modules but
   with the same names.
 
 3.6.2 (2011-05-17)
-------------------
+==================
 
 - Moved detailed documentation out-of-line from PyPI page, linking instead to
   http://docs.zope.org/zope.interface .
@@ -346,7 +351,7 @@ Changes
   running under Python 3.
 
 3.6.1 (2010-05-03)
-------------------
+==================
 
 - A non-ASCII character in the changelog made 3.6.0 uninstallable on
   Python 3 systems with another default encoding than UTF-8.
@@ -354,7 +359,7 @@ Changes
 - Fix compiler warnings under GCC 4.3.3.
 
 3.6.0 (2010-04-29)
-------------------
+==================
 
 - LP #185974:  Clear the cache used by ``Specificaton.get`` inside
   ``Specification.changed``.  Thanks to Jacob Holm for the patch.
@@ -388,21 +393,21 @@ Changes
 
 
 3.5.4 (2009-12-23)
-------------------
+==================
 
 - Use the standard Python doctest module instead of zope.testing.doctest, which
   has been deprecated.
 
 
 3.5.3 (2009-12-08)
-------------------
+==================
 
 - Fix an edge case: make providedBy() work when a class has '__provides__' in
   its __slots__ (see http://thread.gmane.org/gmane.comp.web.zope.devel/22490)
 
 
 3.5.2 (2009-07-01)
-------------------
+==================
 
 - BaseAdapterRegistry.unregister, unsubscribe: Remove empty portions of
   the data structures when something is removed.  This avoids leaving
@@ -411,7 +416,7 @@ Changes
 
 
 3.5.1 (2009-03-18)
-------------------
+==================
 
 - verifyObject: use getattr instead of hasattr to test for object attributes
   in order to let exceptions other than AttributeError raised by properties
@@ -427,7 +432,7 @@ Changes
 
 
 3.5.0 (2008-10-26)
-------------------
+==================
 
 - Fix declaration of _zope_interface_coptimizations, it's not a top level
   package.
@@ -447,20 +452,20 @@ Changes
 
 
 3.4.1 (2007-10-02)
-------------------
+==================
 
 - Fix a setup bug that prevented installation from source on systems
   without setuptools.
 
 
 3.4.0 (2007-07-19)
-------------------
+==================
 
 - Final release for 3.4.0.
 
 
 3.4.0b3 (2007-05-22)
---------------------
+====================
 
 
 - When checking whether an object is already registered, use identity
@@ -468,17 +473,17 @@ Changes
 
 
 3.3.0.1 (2007-01-03)
---------------------
+====================
 
 - Made a reference to OverflowWarning, which disappeared in Python
   2.5, conditional.
 
 
 3.3.0 (2007/01/03)
-------------------
+==================
 
 New Features
-++++++++++++
+------------
 
 - Refactor the adapter-lookup algorithim to make it much simpler and faster.
 
@@ -497,33 +502,33 @@ New Features
   zope.interface.taggedValue).
 
 Bug Fixes
-+++++++++
+---------
 
 - A bug in multi-adapter lookup sometimes caused incorrect adapters to
   be returned.
 
 
 3.2.0.2 (2006-04-15)
---------------------
+====================
 
 - Fix packaging bug:  'package_dir' must be a *relative* path.
 
 
 3.2.0.1 (2006-04-14)
---------------------
+====================
 
 - Packaging change:  suppress inclusion of 'setup.cfg' in 'sdist' builds.
 
 
 3.2.0 (2006-01-05)
-------------------
+==================
 
 - Corresponds to the verison of the zope.interface package shipped as part of
   the Zope 3.2.0 release.
 
 
 3.1.0 (2005-10-03)
-------------------
+==================
 
 - Corresponds to the verison of the zope.interface package shipped as part of
   the Zope 3.1.0 release.
@@ -536,7 +541,7 @@ Bug Fixes
 
 
 3.0.1 (2005-07-27)
-------------------
+==================
 
 - Corresponds to the verison of the zope.interface package shipped as part of
   the Zope X3.0.1 release.
@@ -546,7 +551,7 @@ Bug Fixes
 
 
 3.0.0 (2004-11-07)
-------------------
+==================
 
 - Corresponds to the verison of the zope.interface package shipped as part of
   the Zope X3.0.0 release.
diff --git a/README.rst b/README.rst
index 377c594..966350b 100644
--- a/README.rst
+++ b/README.rst
@@ -1,5 +1,6 @@
-``zope.interface``
-==================
+====================
+ ``zope.interface``
+====================
 
 .. image:: https://img.shields.io/pypi/v/zope.interface.svg
     :target: https://pypi.python.org/pypi/zope.interface/
diff --git a/appveyor.yml b/appveyor.yml
index 078991d..25ab921 100644
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -33,7 +33,7 @@ build_script:
   - python -W ignore setup.py -q bdist_wheel
 
 test_script:
-  - python setup.py -q test
+  - python -m unittest discover -s src
 
 artifacts:
   - path: 'dist\*.whl'
diff --git a/setup.py b/setup.py
index ce66157..d6d0e34 100644
--- a/setup.py
+++ b/setup.py
@@ -20,17 +20,16 @@
 """
 
 import os
-import platform
 import sys
 
-from setuptools import setup, Extension
-from setuptools.command.build_ext import build_ext
-from setuptools import find_packages
-
 from distutils.errors import CCompilerError
 from distutils.errors import DistutilsExecError
 from distutils.errors import DistutilsPlatformError
 
+from setuptools import setup, Extension
+from setuptools.command.build_ext import build_ext
+from setuptools import find_packages
+
 
 class optional_build_ext(build_ext):
     """This class subclasses build_ext and allows
@@ -68,20 +67,20 @@ codeoptimization = [
         [os.path.normcase(codeoptimization_c)]
     ),
 ]
-py_impl = getattr(platform, 'python_implementation', lambda: None)
-is_pypy = py_impl() == 'PyPy'
+
 is_jython = 'java' in sys.platform
-is_pure = 'PURE_PYTHON' in os.environ
 
-# Jython cannot build the C optimizations, while on PyPy they are
-# anti-optimizations (the C extension compatibility layer is known-slow,
-# and defeats JIT opportunities).
-if is_pypy or is_jython or is_pure:
+# Jython cannot build the C optimizations. Everywhere else,
+# including PyPy, defer the decision to runtime.
+if is_jython:
     ext_modules = []
 else:
     ext_modules = codeoptimization
-tests_require = ['zope.event']
-testing_extras = tests_require + ['nose', 'coverage']
+tests_require = [
+    'coverage >= 5.0.3',
+    'zope.event',
+]
+testing_extras = tests_require
 
 
 def read(*rnames):
@@ -96,7 +95,7 @@ long_description = (
         )
 
 setup(name='zope.interface',
-      version='4.7.2.dev0',
+      version='4.8.0.dev0',
       url='https://github.com/zopefoundation/zope.interface',
       license='ZPL 2.1',
       description='Interfaces for Python',
@@ -104,22 +103,22 @@ setup(name='zope.interface',
       author_email='zope-dev@zope.org',
       long_description=long_description,
       classifiers=[
-        "Development Status :: 5 - Production/Stable",
-        "Intended Audience :: Developers",
-        "License :: OSI Approved :: Zope Public License",
-        "Operating System :: OS Independent",
-        "Programming Language :: Python",
-        "Programming Language :: Python :: 2",
-        "Programming Language :: Python :: 2.7",
-        "Programming Language :: Python :: 3",
-        "Programming Language :: Python :: 3.5",
-        "Programming Language :: Python :: 3.6",
-        "Programming Language :: Python :: 3.7",
-        "Programming Language :: Python :: 3.8",
-        "Programming Language :: Python :: Implementation :: CPython",
-        "Programming Language :: Python :: Implementation :: PyPy",
-        "Framework :: Zope :: 3",
-        "Topic :: Software Development :: Libraries :: Python Modules",
+          "Development Status :: 5 - Production/Stable",
+          "Intended Audience :: Developers",
+          "License :: OSI Approved :: Zope Public License",
+          "Operating System :: OS Independent",
+          "Programming Language :: Python",
+          "Programming Language :: Python :: 2",
+          "Programming Language :: Python :: 2.7",
+          "Programming Language :: Python :: 3",
+          "Programming Language :: Python :: 3.5",
+          "Programming Language :: Python :: 3.6",
+          "Programming Language :: Python :: 3.7",
+          "Programming Language :: Python :: 3.8",
+          "Programming Language :: Python :: Implementation :: CPython",
+          "Programming Language :: Python :: Implementation :: PyPy",
+          "Framework :: Zope :: 3",
+          "Topic :: Software Development :: Libraries :: Python Modules",
       ],
       packages=find_packages('src'),
       package_dir={'': 'src'},
diff --git a/src/zope/interface/_compat.py b/src/zope/interface/_compat.py
index fb61e13..f8b7887 100644
--- a/src/zope/interface/_compat.py
+++ b/src/zope/interface/_compat.py
@@ -11,8 +11,14 @@
 # FOR A PARTICULAR PURPOSE.
 #
 ##############################################################################
-"""Basic components support
 """
+Support functions for dealing with differences in platforms, including Python
+versions and implementations.
+
+This file should have no imports from the rest of zope.interface because it is
+used during early bootstrapping.
+"""
+import os
 import sys
 import types
 
@@ -56,3 +62,105 @@ def _skip_under_py3k(test_method):
 def _skip_under_py2(test_method):
     import unittest
     return unittest.skipIf(sys.version_info[0] < 3, "Only on Python 3")(test_method)
+
+
+def _c_optimizations_required():
+    """
+    Return a true value if the C optimizations are required.
+
+    This uses the ``PURE_PYTHON`` variable as documented in `_use_c_impl`.
+    """
+    pure_env = os.environ.get('PURE_PYTHON')
+    require_c = pure_env == "0"
+    return require_c
+
+
+def _c_optimizations_available():
+    """
+    Return the C optimization module, if available, otherwise
+    a false value.
+
+    If the optimizations are required but not available, this
+    raises the ImportError.
+
+    This does not say whether they should be used or not.
+    """
+    catch = () if _c_optimizations_required() else (ImportError,)
+    try:
+        from zope.interface import _zope_interface_coptimizations as c_opt
+        return c_opt
+    except catch: # pragma: no cover (only Jython doesn't build extensions)
+        return False
+
+
+def _c_optimizations_ignored():
+    """
+    The opposite of `_c_optimizations_required`.
+    """
+    pure_env = os.environ.get('PURE_PYTHON')
+    return pure_env is not None and pure_env != "0"
+
+
+def _should_attempt_c_optimizations():
+    """
+    Return a true value if we should attempt to use the C optimizations.
+
+    This takes into account whether we're on PyPy and the value of the
+    ``PURE_PYTHON`` environment variable, as defined in `_use_c_impl`.
+    """
+    is_pypy = hasattr(sys, 'pypy_version_info')
+
+    if _c_optimizations_required():
+        return True
+    if is_pypy:
+        return False
+    return not _c_optimizations_ignored()
+
+
+def _use_c_impl(py_impl, name=None, globs=None):
+    """
+    Decorator. Given an object implemented in Python, with a name like
+    ``Foo``, import the corresponding C implementation from
+    ``zope.interface._zope_interface_coptimizations`` with the name
+    ``Foo`` and use it instead.
+
+    If the ``PURE_PYTHON`` environment variable is set to any value
+    other than ``"0"``, or we're on PyPy, ignore the C implementation
+    and return the Python version. If the C implementation cannot be
+    imported, return the Python version. If ``PURE_PYTHON`` is set to
+    0, *require* the C implementation (let the ImportError propagate);
+    note that PyPy can import the C implementation in this case (and all
+    tests pass).
+
+    In all cases, the Python version is kept available. in the module
+    globals with the name ``FooPy`` and the name ``FooFallback`` (both
+    conventions have been used; the C implementation of some functions
+    looks for the ``Fallback`` version, as do some of the Sphinx
+    documents).
+
+    Example::
+
+        @_use_c_impl
+        class Foo(object):
+            ...
+    """
+    name = name or py_impl.__name__
+    globs = globs or sys._getframe(1).f_globals
+
+    def find_impl():
+        if not _should_attempt_c_optimizations():
+            return py_impl
+
+        c_opt = _c_optimizations_available()
+        if not c_opt: # pragma: no cover (only Jython doesn't build extensions)
+            return py_impl
+
+        __traceback_info__ = c_opt
+        return getattr(c_opt, name)
+
+    c_impl = find_impl()
+    # Always make available by the FooPy name and FooFallback
+    # name (for testing and documentation)
+    globs[name + 'Py'] = py_impl
+    globs[name + 'Fallback'] = py_impl
+    return c_impl
diff --git a/src/zope/interface/_zope_interface_coptimizations.c b/src/zope/interface/_zope_interface_coptimizations.c
index b1e955e..d025a09 100644
--- a/src/zope/interface/_zope_interface_coptimizations.c
+++ b/src/zope/interface/_zope_interface_coptimizations.c
@@ -264,6 +264,7 @@ providedBy(PyObject *ignored, PyObject *ob)
      matter.
 
 */
+#ifndef PYPY_VERSION
 static PyObject *
 inst_attr(PyObject *self, PyObject *name)
 {
@@ -275,6 +276,32 @@ inst_attr(PyObject *self, PyObject *name)
   PyErr_SetObject(PyExc_AttributeError, name);
   return NULL;
 }
+#else
+/*
+  PyPy.
+  _PyObject_GetDictPtr is a private CPython API and
+  using it on PyPy doesn't work as expected. We must use the documented
+  APIs. This has some subtle differences, notably it would use descriptors.
+  But the tests pass.
+*/
+static PyObject*
+inst_attr(PyObject* self, PyObject* name)
+{
+  PyObject* result;
+  result = PyObject_GetAttr(self, name);
+  if (result != NULL) {
+    /*
+        The CPython version returns a borrowed reference.
+        We don't have that ability with the standard API,
+        so we decref here to mimic it. That should be fine if the
+        attribute was really in the dictionary, but it could be an
+        issue if it was a new object from a descriptor.
+    */
+    Py_DECREF(result);
+  }
+  return result;
+}
+#endif
 
 
 static PyObject *
@@ -286,17 +313,9 @@ Spec_extends(PyObject *self, PyObject *other)
   if (implied == NULL)
     return NULL;
 
-#ifdef Py_True
   if (PyDict_GetItem(implied, other) != NULL)
-    {
-      Py_INCREF(Py_True);
-      return Py_True;
-    }
-  Py_INCREF(Py_False);
-  return Py_False;
-#else
-  return PyInt_FromLong(PyDict_GetItem(implied, other) != NULL);
-#endif
+    Py_RETURN_TRUE;
+  Py_RETURN_FALSE;
 }
 
 static char Spec_extends__doc__[] =
@@ -1336,7 +1355,7 @@ verifying_clear(verify *self)
 static void
 verifying_dealloc(verify *self)
 {
-  PyObject_GC_UnTrack((PyObject *)self);   	
+  PyObject_GC_UnTrack((PyObject *)self);
   verifying_clear(self);
   Py_TYPE(self)->tp_free((PyObject*)self);
 }
diff --git a/src/zope/interface/adapter.py b/src/zope/interface/adapter.py
index aae3155..9587f9c 100644
--- a/src/zope/interface/adapter.py
+++ b/src/zope/interface/adapter.py
@@ -23,6 +23,7 @@ from zope.interface.interfaces import IAdapterRegistry
 
 from zope.interface._compat import _normalize_name
 from zope.interface._compat import STRING_TYPES
+from zope.interface._compat import _use_c_impl
 
 _BLANK = u''
 
@@ -298,7 +299,9 @@ class BaseAdapterRegistry(object):
 
 
 _not_in_mapping = object()
-class LookupBaseFallback(object):
+
+@_use_c_impl
+class LookupBase(object):
 
     def __init__(self):
         self._cache = {}
@@ -406,15 +409,9 @@ class LookupBaseFallback(object):
 
         return result
 
-LookupBasePy = LookupBaseFallback # BBB
-
-try:
-    from zope.interface._zope_interface_coptimizations import LookupBase
-except ImportError:
-    LookupBase = LookupBaseFallback
-
 
-class VerifyingBaseFallback(LookupBaseFallback):
+@_use_c_impl
+class VerifyingBase(LookupBaseFallback):
     # Mixin for lookups against registries which "chain" upwards, and
     # whose lookups invalidate their own caches whenever a parent registry
     # bumps its own '_generation' counter.  E.g., used by
@@ -442,13 +439,6 @@ class VerifyingBaseFallback(LookupBaseFallback):
         self._verify()
         return LookupBaseFallback.subscriptions(self, required, provided)
 
-VerifyingBasePy = VerifyingBaseFallback #BBB
-
-try:
-    from zope.interface._zope_interface_coptimizations import VerifyingBase
-except ImportError:
-    VerifyingBase = VerifyingBaseFallback
-
 
 class AdapterLookupBase(object):
 
diff --git a/src/zope/interface/declarations.py b/src/zope/interface/declarations.py
index b80245f..100417f 100644
--- a/src/zope/interface/declarations.py
+++ b/src/zope/interface/declarations.py
@@ -38,6 +38,7 @@ from zope.interface.interface import SpecificationBase
 from zope.interface.interface import Specification
 from zope.interface._compat import CLASS_TYPES as DescriptorAwareMetaClasses
 from zope.interface._compat import PYTHON3
+from zope.interface._compat import _use_c_impl
 
 # Registry of class-implementation specifications
 BuiltinImplementationSpecifications = {}
@@ -57,6 +58,7 @@ class named(object):
         ob.__component_name__ = self.name
         return ob
 
+
 class Declaration(Specification):
     """Interface declarations"""
 
@@ -91,10 +93,10 @@ class Declaration(Specification):
         """
         return Declaration(
             *[i for i in self.interfaces()
-                if not [j for j in other.interfaces()
-                        if i.extends(j, 0)]
-                ]
-                )
+              if not [j for j in other.interfaces()
+                      if i.extends(j, 0)]
+            ]
+        )
 
     def __add__(self, other):
         """Add two specifications or a specification and an interface
@@ -168,7 +170,7 @@ class Implements(Declaration):
             return -1
 
         n1 = (self.__name__, self.__module__)
-        n2 = (getattr(other, '__name__', ''), getattr(other,  '__module__', ''))
+        n2 = (getattr(other, '__name__', ''), getattr(other, '__module__', ''))
 
         # This spelling works under Python3, which doesn't have cmp().
         return (n1 > n2) - (n1 < n2)
@@ -211,7 +213,9 @@ def _implements_name(ob):
     return (getattr(ob, '__module__', '?') or '?') + \
         '.' + (getattr(ob, '__name__', '?') or '?')
 
-def implementedByFallback(cls):
+
+@_use_c_impl
+def implementedBy(cls):
     """Return the interfaces implemented for a class' instances
 
       The value returned is an `~zope.interface.interfaces.IDeclaration`.
@@ -296,7 +300,6 @@ def implementedByFallback(cls):
 
     return spec
 
-implementedBy = implementedByFallback
 
 def classImplementsOnly(cls, *interfaces):
     """Declare the only interfaces implemented by instances of a class
@@ -312,6 +315,7 @@ def classImplementsOnly(cls, *interfaces):
     spec.inherit = None
     classImplements(cls, *interfaces)
 
+
 def classImplements(cls, *interfaces):
     """Declare additional interfaces implemented for instances of a class
 
@@ -574,7 +578,7 @@ def directlyProvides(object, *interfaces):
       replace interfaces previously declared for the object.
     """
     cls = getattr(object, '__class__', None)
-    if cls is not None and getattr(cls,  '__class__', None) is cls:
+    if cls is not None and getattr(cls, '__class__', None) is cls:
         # It's a meta class (well, at least it it could be an extension class)
         # Note that we can't get here from Py3k tests:  there is no normal
         # class which isn't descriptor aware.
@@ -611,6 +615,7 @@ def alsoProvides(object, *interfaces):
     """
     directlyProvides(object, directlyProvidedBy(object), *interfaces)
 
+
 def noLongerProvides(object, interface):
     """ Removes a directly provided interface from an object.
     """
@@ -618,7 +623,9 @@ def noLongerProvides(object, interface):
     if interface.providedBy(object):
         raise ValueError("Can only remove directly provided interfaces.")
 
-class ClassProvidesBaseFallback(object):
+
+@_use_c_impl
+class ClassProvidesBase(object):
 
     def __get__(self, inst, cls):
         if cls is self._cls:
@@ -633,17 +640,6 @@ class ClassProvidesBaseFallback(object):
 
         raise AttributeError('__provides__')
 
-ClassProvidesBasePy = ClassProvidesBaseFallback # BBB
-ClassProvidesBase = ClassProvidesBaseFallback
-
-# Try to get C base:
-try:
-    import zope.interface._zope_interface_coptimizations
-except ImportError:
-    pass
-else:
-    from zope.interface._zope_interface_coptimizations import ClassProvidesBase
-
 
 class ClassProvides(Declaration, ClassProvidesBase):
     """Special descriptor for class ``__provides__``
@@ -664,6 +660,7 @@ class ClassProvides(Declaration, ClassProvidesBase):
     # Copy base-class method for speed
     __get__ = ClassProvidesBase.__get__
 
+
 def directlyProvidedBy(object):
     """Return the interfaces directly provided by the given object
 
@@ -676,12 +673,13 @@ def directlyProvidedBy(object):
         # optimization. If so, it's like having only one base, that we
         # lop off to exclude class-supplied declarations:
         isinstance(provides, Implements)
-        ):
+    ):
         return _empty
 
     # Strip off the class part of the spec:
     return Declaration(provides.__bases__[:-1])
 
+
 def classProvides(*interfaces):
     """Declare interfaces provided directly by a class
 
@@ -740,6 +738,7 @@ def _classProvides_advice(cls):
     directlyProvides(cls, *interfaces)
     return cls
 
+
 class provider(object):
     """Class decorator version of classProvides"""
 
@@ -750,6 +749,7 @@ class provider(object):
         directlyProvides(ob, *self.interfaces)
         return ob
 
+
 def moduleProvides(*interfaces):
     """Declare interfaces provided by a module
 
@@ -788,6 +788,7 @@ def moduleProvides(*interfaces):
     locals["__provides__"] = Provides(ModuleType,
                                       *_normalizeargs(interfaces))
 
+
 ##############################################################################
 #
 # Declaration querying support
@@ -801,7 +802,8 @@ def ObjectSpecification(direct, cls):
     """
     return Provides(cls, direct) # pragma: no cover fossil
 
-def getObjectSpecificationFallback(ob):
+@_use_c_impl
+def getObjectSpecification(ob):
 
     provides = getattr(ob, '__provides__', None)
     if provides is not None:
@@ -816,9 +818,9 @@ def getObjectSpecificationFallback(ob):
 
     return implementedBy(cls)
 
-getObjectSpecification = getObjectSpecificationFallback
 
-def providedByFallback(ob):
+@_use_c_impl
+def providedBy(ob):
 
     # Here we have either a special object, an old-style declaration
     # or a descriptor
@@ -836,7 +838,6 @@ def providedByFallback(ob):
         # descriptors.  We'll make sure we got one by trying to get
         # the only attribute, which all specs have.
         r.extends
-
     except AttributeError:
 
         # The object's class doesn't understand descriptors.
@@ -867,9 +868,10 @@ def providedByFallback(ob):
             return implementedBy(ob.__class__)
 
     return r
-providedBy = providedByFallback
 
-class ObjectSpecificationDescriptorFallback(object):
+
+@_use_c_impl
+class ObjectSpecificationDescriptor(object):
     """Implement the `__providedBy__` attribute
 
     The `__providedBy__` attribute computes the interfaces peovided by
@@ -888,11 +890,10 @@ class ObjectSpecificationDescriptorFallback(object):
 
         return implementedBy(cls)
 
-ObjectSpecificationDescriptor = ObjectSpecificationDescriptorFallback
 
 ##############################################################################
 
-def _normalizeargs(sequence, output = None):
+def _normalizeargs(sequence, output=None):
     """Normalize declaration arguments
 
     Normalization arguments might contain Declarions, tuples, or single
@@ -914,16 +915,4 @@ def _normalizeargs(sequence, output = None):
 
 _empty = Declaration()
 
-try:
-    import zope.interface._zope_interface_coptimizations
-except ImportError:
-    pass
-else:
-    from zope.interface._zope_interface_coptimizations import implementedBy
-    from zope.interface._zope_interface_coptimizations import providedBy
-    from zope.interface._zope_interface_coptimizations import (
-        getObjectSpecification)
-    from zope.interface._zope_interface_coptimizations import (
-        ObjectSpecificationDescriptor)
-
 objectSpecificationDescriptor = ObjectSpecificationDescriptor()
diff --git a/src/zope/interface/interface.py b/src/zope/interface/interface.py
index 9ecdca6..05f5594 100644
--- a/src/zope/interface/interface.py
+++ b/src/zope/interface/interface.py
@@ -13,7 +13,6 @@
 ##############################################################################
 """Interface object implementation
 """
-from __future__ import generators
 
 import sys
 from types import MethodType
@@ -21,6 +20,7 @@ from types import FunctionType
 import warnings
 import weakref
 
+from zope.interface._compat import _use_c_impl
 from zope.interface.exceptions import Invalid
 from zope.interface.ro import ro
 
@@ -30,6 +30,9 @@ CO_VARKEYWORDS = 8
 TAGGED_DATA = '__interface_tagged_values__'
 
 _decorator_non_return = object()
+_marker = object()
+
+
 
 def invariant(call):
     f_locals = sys._getframe(1).f_locals
@@ -62,8 +65,8 @@ class Element(object):
             __doc__ = __name__
             __name__ = None
 
-        self.__name__=__name__
-        self.__doc__=__doc__
+        self.__name__ = __name__
+        self.__doc__ = __doc__
         self.__tagged_values = {}
 
     def getName(self):
@@ -90,7 +93,9 @@ class Element(object):
         """ Associates 'value' with 'key'. """
         self.__tagged_values[tag] = value
 
-class SpecificationBasePy(object):
+
+@_use_c_impl
+class SpecificationBase(object):
 
     def providedBy(self, ob):
         """Is the interface implemented by an object
@@ -113,14 +118,9 @@ class SpecificationBasePy(object):
 
     __call__ = isOrExtends
 
-SpecificationBase = SpecificationBasePy
-try:
-    from zope.interface._zope_interface_coptimizations import SpecificationBase
-except ImportError:
-    pass
 
-_marker = object()
-class InterfaceBasePy(object):
+@_use_c_impl
+class InterfaceBase(object):
     """Base class that wants to be replaced with a C base :)
     """
 
@@ -154,18 +154,7 @@ class InterfaceBasePy(object):
                 return adapter
 
 
-InterfaceBase = InterfaceBasePy
-try:
-    from zope.interface._zope_interface_coptimizations import InterfaceBase
-except ImportError:
-    pass
-
-
-adapter_hooks = []
-try:
-    from zope.interface._zope_interface_coptimizations import adapter_hooks
-except ImportError:
-    pass
+adapter_hooks = _use_c_impl([], 'adapter_hooks')
 
 
 class Specification(SpecificationBase):
@@ -538,8 +527,8 @@ class InterfaceClass(Element, InterfaceBase, Specification):
         if other is None:
             return -1
 
-        n1 = (getattr(self, '__name__', ''), getattr(self,  '__module__', ''))
-        n2 = (getattr(other, '__name__', ''), getattr(other,  '__module__', ''))
+        n1 = (getattr(self, '__name__', ''), getattr(self, '__module__', ''))
+        n2 = (getattr(other, '__name__', ''), getattr(other, '__module__', ''))
 
         # This spelling works under Python3, which doesn't have cmp().
         return (n1 > n2) - (n1 < n2)
@@ -576,7 +565,8 @@ class InterfaceClass(Element, InterfaceBase, Specification):
         return c >= 0
 
 
-Interface = InterfaceClass("Interface", __module__ = 'zope.interface')
+Interface = InterfaceClass("Interface", __module__='zope.interface')
+
 
 class Attribute(Element):
     """Attribute descriptions
@@ -638,6 +628,7 @@ class Method(Attribute):
 
         return "(%s)" % ", ".join(sig)
 
+
 def fromFunction(func, interface=None, imlevel=0, name=None):
     name = name or func.__name__
     method = Method(name, func.__doc__)
@@ -650,7 +641,7 @@ def fromFunction(func, interface=None, imlevel=0, name=None):
     # Number of required arguments
     nr = na-len(defaults)
     if nr < 0:
-        defaults=defaults[-nr:]
+        defaults = defaults[-nr:]
         nr = 0
 
     # Determine the optional arguments.
diff --git a/src/zope/interface/tests/__init__.py b/src/zope/interface/tests/__init__.py
index 15259c1..c37dffc 100644
--- a/src/zope/interface/tests/__init__.py
+++ b/src/zope/interface/tests/__init__.py
@@ -1 +1,33 @@
-# Make this directory a package.
+from zope.interface._compat import _should_attempt_c_optimizations
+
+
+class OptimizationTestMixin(object):
+    """
+    Helper for testing that C optimizations are used
+    when appropriate.
+    """
+
+    def _getTargetClass(self):
+        """
+        Define this to return the implementation in use,
+        without the 'Py' or 'Fallback' suffix.
+        """
+        raise NotImplementedError
+
+    def _getFallbackClass(self):
+        """
+        Define this to return the fallback Python implementation.
+        """
+        # Is there an algorithmic way to do this? The C
+        # objects all come from the same module so I don't see how we can
+        # get the Python object from that.
+        raise NotImplementedError
+
+    def test_optimizations(self):
+        used = self._getTargetClass()
+        fallback = self._getFallbackClass()
+
+        if _should_attempt_c_optimizations():
+            self.assertIsNot(used, fallback)
+        else:
+            self.assertIs(used, fallback)
diff --git a/src/zope/interface/tests/ifoo.py b/src/zope/interface/tests/ifoo.py
deleted file mode 100644
index 29a7877..0000000
--- a/src/zope/interface/tests/ifoo.py
+++ /dev/null
@@ -1,26 +0,0 @@
-##############################################################################
-#
-# Copyright (c) 2001, 2002 Zope Foundation and Contributors.
-# All Rights Reserved.
-#
-# This software is subject to the provisions of the Zope Public License,
-# Version 2.1 (ZPL).  A copy of the ZPL should accompany this distribution.
-# THIS SOFTWARE IS PROVIDED "AS IS" AND ANY AND ALL EXPRESS OR IMPLIED
-# WARRANTIES ARE DISCLAIMED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
-# WARRANTIES OF TITLE, MERCHANTABILITY, AGAINST INFRINGEMENT, AND FITNESS
-# FOR A PARTICULAR PURPOSE.
-#
-##############################################################################
-"""IFoo test module
-"""
-from zope.interface import Interface
-
-class IFoo(Interface):
-    """
-        Dummy interface for unit tests.
-    """
-
-    def bar(baz):
-        """
-            Just a note.
-        """
diff --git a/src/zope/interface/tests/ifoo_other.py b/src/zope/interface/tests/ifoo_other.py
deleted file mode 100644
index 29a7877..0000000
--- a/src/zope/interface/tests/ifoo_other.py
+++ /dev/null
@@ -1,26 +0,0 @@
-##############################################################################
-#
-# Copyright (c) 2001, 2002 Zope Foundation and Contributors.
-# All Rights Reserved.
-#
-# This software is subject to the provisions of the Zope Public License,
-# Version 2.1 (ZPL).  A copy of the ZPL should accompany this distribution.
-# THIS SOFTWARE IS PROVIDED "AS IS" AND ANY AND ALL EXPRESS OR IMPLIED
-# WARRANTIES ARE DISCLAIMED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
-# WARRANTIES OF TITLE, MERCHANTABILITY, AGAINST INFRINGEMENT, AND FITNESS
-# FOR A PARTICULAR PURPOSE.
-#
-##############################################################################
-"""IFoo test module
-"""
-from zope.interface import Interface
-
-class IFoo(Interface):
-    """
-        Dummy interface for unit tests.
-    """
-
-    def bar(baz):
-        """
-            Just a note.
-        """
diff --git a/src/zope/interface/tests/m2.py b/src/zope/interface/tests/m2.py
deleted file mode 100644
index 511cd9c..0000000
--- a/src/zope/interface/tests/m2.py
+++ /dev/null
@@ -1,15 +0,0 @@
-##############################################################################
-#
-# Copyright (c) 2004 Zope Foundation and Contributors.
-# All Rights Reserved.
-#
-# This software is subject to the provisions of the Zope Public License,
-# Version 2.1 (ZPL).  A copy of the ZPL should accompany this distribution.
-# THIS SOFTWARE IS PROVIDED "AS IS" AND ANY AND ALL EXPRESS OR IMPLIED
-# WARRANTIES ARE DISCLAIMED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
-# WARRANTIES OF TITLE, MERCHANTABILITY, AGAINST INFRINGEMENT, AND FITNESS
-# FOR A PARTICULAR PURPOSE.
-#
-##############################################################################
-"""Test module that doesn't declare an interface
-"""
diff --git a/src/zope/interface/tests/test_adapter.py b/src/zope/interface/tests/test_adapter.py
index 41c618c..25cb4bb 100644
--- a/src/zope/interface/tests/test_adapter.py
+++ b/src/zope/interface/tests/test_adapter.py
@@ -15,6 +15,8 @@
 """
 import unittest
 
+from zope.interface.tests import OptimizationTestMixin
+
 
 def _makeInterfaces():
     from zope.interface import Interface
@@ -234,10 +236,10 @@ class BaseAdapterRegistryTests(unittest.TestCase):
         registry.subscribe([IB1], None, orig)
         registry.unsubscribe([IB1], None, nomatch) #doesn't raise
         self.assertEqual(len(registry._subscribers), 2)
-    
+
     def _instance_method_notify_target(self):
         self.fail("Example method, not intended to be called.")
-    
+
     def test_unsubscribe_instance_method(self):
         IB0, IB1, IB2, IB3, IB4, IF0, IF1, IR0, IR1 = _makeInterfaces()
         registry = self._makeOne()
@@ -249,10 +251,12 @@ class BaseAdapterRegistryTests(unittest.TestCase):
 
 class LookupBaseFallbackTests(unittest.TestCase):
 
-    def _getTargetClass(self):
+    def _getFallbackClass(self):
         from zope.interface.adapter import LookupBaseFallback
         return LookupBaseFallback
 
+    _getTargetClass = _getFallbackClass
+
     def _makeOne(self, uc_lookup=None, uc_lookupAll=None,
                  uc_subscriptions=None):
         if uc_lookup is None:
@@ -559,28 +563,22 @@ class LookupBaseFallbackTests(unittest.TestCase):
         self.assertEqual(_called_with, [(('A',), 'B')])
 
 
-class LookupBaseTests(LookupBaseFallbackTests):
+class LookupBaseTests(LookupBaseFallbackTests,
+                      OptimizationTestMixin):
 
     def _getTargetClass(self):
         from zope.interface.adapter import LookupBase
         return LookupBase
 
-    def test_optimizations(self):
-        from zope.interface.adapter import LookupBaseFallback
-        try:
-            import zope.interface._zope_interface_coptimizations
-        except ImportError:
-            self.assertIs(self._getTargetClass(), LookupBaseFallback)
-        else:
-            self.assertIsNot(self._getTargetClass(), LookupBaseFallback)
-
 
 class VerifyingBaseFallbackTests(unittest.TestCase):
 
-    def _getTargetClass(self):
+    def _getFallbackClass(self):
         from zope.interface.adapter import VerifyingBaseFallback
         return VerifyingBaseFallback
 
+    _getTargetClass = _getFallbackClass
+
     def _makeOne(self, registry, uc_lookup=None, uc_lookupAll=None,
                  uc_subscriptions=None):
         if uc_lookup is None:
@@ -730,21 +728,13 @@ class VerifyingBaseFallbackTests(unittest.TestCase):
         self.assertEqual(found, tuple(_results_2))
 
 
-class VerifyingBaseTests(VerifyingBaseFallbackTests):
+class VerifyingBaseTests(VerifyingBaseFallbackTests,
+                         OptimizationTestMixin):
 
     def _getTargetClass(self):
         from zope.interface.adapter import VerifyingBase
         return VerifyingBase
 
-    def test_optimizations(self):
-        from zope.interface.adapter import VerifyingBaseFallback
-        try:
-            import zope.interface._zope_interface_coptimizations
-        except ImportError:
-            self.assertIs(self._getTargetClass(), VerifyingBaseFallback)
-        else:
-            self.assertIsNot(self._getTargetClass(), VerifyingBaseFallback)
-
 
 class AdapterLookupBaseTests(unittest.TestCase):
 
diff --git a/src/zope/interface/tests/test_declarations.py b/src/zope/interface/tests/test_declarations.py
index 43f95c8..a1ebef8 100644
--- a/src/zope/interface/tests/test_declarations.py
+++ b/src/zope/interface/tests/test_declarations.py
@@ -16,6 +16,7 @@
 import unittest
 
 from zope.interface._compat import _skip_under_py3k
+from zope.interface.tests import OptimizationTestMixin
 
 
 class _Py3ClassAdvice(object):
@@ -326,9 +327,14 @@ class TestImplements(unittest.TestCase):
 
 class Test_implementedByFallback(unittest.TestCase):
 
-    def _callFUT(self, *args, **kw):
+    def _getTargetClass(self):
         from zope.interface.declarations import implementedByFallback
-        return implementedByFallback(*args, **kw)
+        return implementedByFallback
+
+    _getFallbackClass = _getTargetClass
+
+    def _callFUT(self, *args, **kw):
+        return self._getTargetClass()(*args, **kw)
 
     def test_dictless_wo_existing_Implements_wo_registrations(self):
         class Foo(object):
@@ -471,22 +477,13 @@ class Test_implementedByFallback(unittest.TestCase):
         self.assertTrue(self._callFUT(Foo) is impl)
 
 
-class Test_implementedBy(Test_implementedByFallback):
+class Test_implementedBy(Test_implementedByFallback,
+                         OptimizationTestMixin):
     # Repeat tests for C optimizations
 
-    def _callFUT(self, *args, **kw):
-        from zope.interface.declarations import implementedBy
-        return implementedBy(*args, **kw)
-
-    def test_optimizations(self):
-        from zope.interface.declarations import implementedByFallback
+    def _getTargetClass(self):
         from zope.interface.declarations import implementedBy
-        try:
-            import zope.interface._zope_interface_coptimizations
-        except ImportError:
-            self.assertIs(implementedBy, implementedByFallback)
-        else:
-            self.assertIsNot(implementedBy, implementedByFallback)
+        return implementedBy
 
 
 class Test_classImplementsOnly(unittest.TestCase):
@@ -1124,21 +1121,17 @@ class ClassProvidesBaseFallbackTests(unittest.TestCase):
         self.assertRaises(AttributeError, getattr, bar, '__provides__')
 
 
-class ClassProvidesBaseTests(ClassProvidesBaseFallbackTests):
+class ClassProvidesBaseTests(OptimizationTestMixin,
+                             ClassProvidesBaseFallbackTests):
     # Repeat tests for C optimizations
 
     def _getTargetClass(self):
         from zope.interface.declarations import ClassProvidesBase
         return ClassProvidesBase
 
-    def test_optimizations(self):
+    def _getFallbackClass(self):
         from zope.interface.declarations import ClassProvidesBaseFallback
-        try:
-            import zope.interface._zope_interface_coptimizations
-        except ImportError:
-            self.assertIs(self._getTargetClass(), ClassProvidesBaseFallback)
-        else:
-            self.assertIsNot(self._getTargetClass(), ClassProvidesBaseFallback)
+        return ClassProvidesBaseFallback
 
 
 class ClassProvidesTests(unittest.TestCase):
@@ -1366,9 +1359,14 @@ class Test_moduleProvides(unittest.TestCase):
 
 class Test_getObjectSpecificationFallback(unittest.TestCase):
 
-    def _callFUT(self, *args, **kw):
+    def _getFallbackClass(self):
         from zope.interface.declarations import getObjectSpecificationFallback
-        return getObjectSpecificationFallback(*args, **kw)
+        return getObjectSpecificationFallback
+
+    _getTargetClass = _getFallbackClass
+
+    def _callFUT(self, *args, **kw):
+        return self._getTargetClass()(*args, **kw)
 
     def test_wo_existing_provides_classless(self):
         the_dict = {}
@@ -1434,31 +1432,25 @@ class Test_getObjectSpecificationFallback(unittest.TestCase):
         self.assertEqual(list(spec), [])
 
 
-class Test_getObjectSpecification(Test_getObjectSpecificationFallback):
+class Test_getObjectSpecification(Test_getObjectSpecificationFallback,
+                                  OptimizationTestMixin):
     # Repeat tests for C optimizations
 
-    def _callFUT(self, *args, **kw):
-        from zope.interface.declarations import getObjectSpecification
-        return getObjectSpecification(*args, **kw)
-
-    def test_optimizations(self):
-        from zope.interface.declarations import getObjectSpecificationFallback
+    def _getTargetClass(self):
         from zope.interface.declarations import getObjectSpecification
-        try:
-            import zope.interface._zope_interface_coptimizations
-        except ImportError:
-            self.assertIs(getObjectSpecification,
-                          getObjectSpecificationFallback)
-        else:
-            self.assertIsNot(getObjectSpecification,
-                             getObjectSpecificationFallback)
+        return getObjectSpecification
 
 
 class Test_providedByFallback(unittest.TestCase):
 
-    def _callFUT(self, *args, **kw):
+    def _getFallbackClass(self):
         from zope.interface.declarations import providedByFallback
-        return providedByFallback(*args, **kw)
+        return providedByFallback
+
+    _getTargetClass = _getFallbackClass
+
+    def _callFUT(self, *args, **kw):
+        return self._getTargetClass()(*args, **kw)
 
     def test_wo_providedBy_on_class_wo_implements(self):
         class Foo(object):
@@ -1531,31 +1523,24 @@ class Test_providedByFallback(unittest.TestCase):
         self.assertEqual(list(spec), [IFoo])
 
 
-class Test_providedBy(Test_providedByFallback):
+class Test_providedBy(Test_providedByFallback,
+                      OptimizationTestMixin):
     # Repeat tests for C optimizations
 
-    def _callFUT(self, *args, **kw):
-        from zope.interface.declarations import providedBy
-        return providedBy(*args, **kw)
-
-    def test_optimizations(self):
-        from zope.interface.declarations import providedByFallback
+    def _getTargetClass(self):
         from zope.interface.declarations import providedBy
-        try:
-            import zope.interface._zope_interface_coptimizations
-        except ImportError:
-            self.assertIs(providedBy, providedByFallback)
-        else:
-            self.assertIsNot(providedBy, providedByFallback)
+        return providedBy
 
 
 class ObjectSpecificationDescriptorFallbackTests(unittest.TestCase):
 
-    def _getTargetClass(self):
+    def _getFallbackClass(self):
         from zope.interface.declarations \
             import ObjectSpecificationDescriptorFallback
         return ObjectSpecificationDescriptorFallback
 
+    _getTargetClass = _getFallbackClass
+
     def _makeOne(self, *args, **kw):
         return self._getTargetClass()(*args, **kw)
 
@@ -1602,25 +1587,14 @@ class ObjectSpecificationDescriptorFallbackTests(unittest.TestCase):
 
 
 class ObjectSpecificationDescriptorTests(
-                ObjectSpecificationDescriptorFallbackTests):
+        ObjectSpecificationDescriptorFallbackTests,
+        OptimizationTestMixin):
     # Repeat tests for C optimizations
 
     def _getTargetClass(self):
         from zope.interface.declarations import ObjectSpecificationDescriptor
         return ObjectSpecificationDescriptor
 
-    def test_optimizations(self):
-        from zope.interface.declarations import (
-            ObjectSpecificationDescriptorFallback)
-        try:
-            import zope.interface._zope_interface_coptimizations
-        except ImportError:
-            self.assertIs(self._getTargetClass(),
-                          ObjectSpecificationDescriptorFallback)
-        else:
-            self.assertIsNot(self._getTargetClass(),
-                             ObjectSpecificationDescriptorFallback)
-
 
 # Test _normalizeargs through its callers.
 
diff --git a/src/zope/interface/tests/test_interface.py b/src/zope/interface/tests/test_interface.py
index 387897b..69b1d47 100644
--- a/src/zope/interface/tests/test_interface.py
+++ b/src/zope/interface/tests/test_interface.py
@@ -17,6 +17,7 @@
 import unittest
 
 from zope.interface._compat import _skip_under_py3k
+from zope.interface.tests import OptimizationTestMixin
 
 _marker = object()
 
@@ -136,12 +137,14 @@ class ElementTests(unittest.TestCase):
         self.assertEqual(element.queryTaggedValue('foo'), 'bar')
 
 
-class SpecificationBasePyTests(unittest.TestCase):
-
-    def _getTargetClass(self):
+class GenericSpecificationBaseTests(unittest.TestCase):
+    # Tests that work with both implementations
+    def _getFallbackClass(self):
         from zope.interface.interface import SpecificationBasePy
         return SpecificationBasePy
 
+    _getTargetClass = _getFallbackClass
+
     def _makeOne(self):
         return self._getTargetClass()()
 
@@ -154,16 +157,6 @@ class SpecificationBasePyTests(unittest.TestCase):
         with _Monkey(interface, providedBy=_providedBy):
             self.assertFalse(sb.providedBy(object()))
 
-    def test_providedBy_hit(self):
-        from zope.interface import interface
-        sb = self._makeOne()
-        class _Decl(object):
-            _implied = {sb: {},}
-        def _providedBy(obj):
-            return _Decl()
-        with _Monkey(interface, providedBy=_providedBy):
-            self.assertTrue(sb.providedBy(object()))
-
     def test_implementedBy_miss(self):
         from zope.interface import interface
         from zope.interface.declarations import _empty
@@ -173,61 +166,70 @@ class SpecificationBasePyTests(unittest.TestCase):
         with _Monkey(interface, implementedBy=_implementedBy):
             self.assertFalse(sb.implementedBy(object()))
 
-    def test_implementedBy_hit(self):
-        from zope.interface import interface
-        sb = self._makeOne()
-        class _Decl(object):
-            _implied = {sb: {},}
-        def _implementedBy(obj):
-            return _Decl()
-        with _Monkey(interface, implementedBy=_implementedBy):
-            self.assertTrue(sb.implementedBy(object()))
 
-    def test_isOrExtends_miss(self):
+class SpecificationBaseTests(GenericSpecificationBaseTests,
+                             OptimizationTestMixin):
+    # Tests that use the C implementation
+
+    def _getTargetClass(self):
+        from zope.interface.interface import SpecificationBase
+        return SpecificationBase
+
+class SpecificationBasePyTests(GenericSpecificationBaseTests):
+    # Tests that only work with the Python implementation
+
+    def test___call___miss(self):
         sb = self._makeOne()
         sb._implied = {}  # not defined by SpecificationBasePy
         self.assertFalse(sb.isOrExtends(object()))
 
-    def test_isOrExtends_hit(self):
+    def test___call___hit(self):
         sb = self._makeOne()
         testing = object()
         sb._implied = {testing: {}}  # not defined by SpecificationBasePy
         self.assertTrue(sb(testing))
 
-    def test___call___miss(self):
+    def test_isOrExtends_miss(self):
         sb = self._makeOne()
         sb._implied = {}  # not defined by SpecificationBasePy
         self.assertFalse(sb.isOrExtends(object()))
 
-    def test___call___hit(self):
+    def test_isOrExtends_hit(self):
         sb = self._makeOne()
         testing = object()
         sb._implied = {testing: {}}  # not defined by SpecificationBasePy
         self.assertTrue(sb(testing))
 
+    def test_implementedBy_hit(self):
+        from zope.interface import interface
+        sb = self._makeOne()
+        class _Decl(object):
+            _implied = {sb: {},}
+        def _implementedBy(obj):
+            return _Decl()
+        with _Monkey(interface, implementedBy=_implementedBy):
+            self.assertTrue(sb.implementedBy(object()))
 
-class SpecificationBaseTests(unittest.TestCase):
-
-    def _getTargetClass(self):
-        from zope.interface.interface import SpecificationBase
-        return SpecificationBase
-
-    def test_optimizations(self):
-        from zope.interface.interface import SpecificationBasePy
-        try:
-            import zope.interface._zope_interface_coptimizations
-        except ImportError:
-            self.assertIs(self._getTargetClass(), SpecificationBasePy)
-        else:
-            self.assertIsNot(self._getTargetClass(), SpecificationBasePy)
+    def test_providedBy_hit(self):
+        from zope.interface import interface
+        sb = self._makeOne()
+        class _Decl(object):
+            _implied = {sb: {},}
+        def _providedBy(obj):
+            return _Decl()
+        with _Monkey(interface, providedBy=_providedBy):
+            self.assertTrue(sb.providedBy(object()))
 
 
-class InterfaceBasePyTests(unittest.TestCase):
+class GenericInterfaceBaseTests(unittest.TestCase):
+    # Tests for both C and Python implementation
 
-    def _getTargetClass(self):
+    def _getFallbackClass(self):
         from zope.interface.interface import InterfaceBasePy
         return InterfaceBasePy
 
+    _getTargetClass = _getFallbackClass
+
     def _makeOne(self, object_should_provide):
         class IB(self._getTargetClass()):
             def _call_conform(self, conform):
@@ -244,29 +246,42 @@ class InterfaceBasePyTests(unittest.TestCase):
                 return conformed
         self.assertTrue(ib(_Adapted()) is conformed)
 
-    def test___call___w___conform___miss_ob_provides(self):
-        ib = self._makeOne(True)
-        class _Adapted(object):
-            def __conform__(self, iface):
-                return None
-        adapted = _Adapted()
-        self.assertTrue(ib(adapted) is adapted)
-
     def test___call___wo___conform___ob_no_provides_w_alternate(self):
         ib = self._makeOne(False)
         adapted = object()
         alternate = object()
-        self.assertTrue(ib(adapted, alternate) is alternate)
+        self.assertIs(ib(adapted, alternate), alternate)
 
     def test___call___w___conform___ob_no_provides_wo_alternate(self):
         ib = self._makeOne(False)
         adapted = object()
         self.assertRaises(TypeError, ib, adapted)
 
+
+
+class InterfaceBaseTests(GenericInterfaceBaseTests,
+                         OptimizationTestMixin):
+    # Tests that work with the C implementation
+    def _getTargetClass(self):
+        from zope.interface.interface import InterfaceBase
+        return InterfaceBase
+
+
+class InterfaceBasePyTests(GenericInterfaceBaseTests):
+    # Tests that only work with the Python implementation
+
+    def test___call___w___conform___miss_ob_provides(self):
+        ib = self._makeOne(True)
+        class _Adapted(object):
+            def __conform__(self, iface):
+                return None
+        adapted = _Adapted()
+        self.assertIs(ib(adapted), adapted)
+
     def test___adapt___ob_provides(self):
         ib = self._makeOne(True)
         adapted = object()
-        self.assertTrue(ib.__adapt__(adapted) is adapted)
+        self.assertIs(ib.__adapt__(adapted), adapted)
 
     def test___adapt___ob_no_provides_uses_hooks(self):
         from zope.interface import interface
@@ -279,26 +294,9 @@ class InterfaceBasePyTests(unittest.TestCase):
         def _hook_hit(iface, obj):
             return obj
         with _Monkey(interface, adapter_hooks=[_hook_miss, _hook_hit]):
-            self.assertTrue(ib.__adapt__(adapted) is adapted)
+            self.assertIs(ib.__adapt__(adapted), adapted)
             self.assertEqual(_missed, [(ib, adapted)])
 
-
-class InterfaceBaseTests(unittest.TestCase):
-
-    def _getTargetClass(self):
-        from zope.interface.interface import InterfaceBase
-        return InterfaceBase
-
-    def test_optimizations(self):
-        from zope.interface.interface import InterfaceBasePy
-        try:
-            import zope.interface._zope_interface_coptimizations
-        except ImportError:
-            self.assertIs(self._getTargetClass(), InterfaceBasePy)
-        else:
-            self.assertIsNot(self._getTargetClass(), InterfaceBasePy)
-
-
 class SpecificationTests(unittest.TestCase):
 
     def _getTargetClass(self):
diff --git a/tox.ini b/tox.ini
index 941e14c..2c60a50 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,27 +1,33 @@
 [tox]
 envlist =
-    py27,py27-pure,py35,py35-pure,py36,py37,py38,pypy,pypy3,coverage,docs
+    py27,py27-pure,py35,py35-pure,py36,py37,py38,py38-cext,pypy,pypy3,coverage,docs
 # NB: if you add new environments, please also add them to depends in
 # testenv:coverage so that parallel runs (tox -p auto) will work correctly
 
 [testenv]
+# ``usedevelop`` is required otherwise unittest complains that it
+# discovers a file in src/... but imports it from .tox/.../
+# ``skip_install`` also basically works, but that causes the ``extras``
+# not to be installed (though ``deps`` still are), and doesn't
+# rebuild C extensions.
+usedevelop = true
 commands =
-    coverage run setup.py -q test -q {posargs}
-deps =
-    .[test]
-    coverage
-setenv =
-   COVERAGE_FILE=.coverage.{envname}
+    coverage run -p -m unittest discover -s src
+extras = test
+
 
 [testenv:py27-pure]
 setenv =
     PURE_PYTHON=1
-    PIP_CACHE_DIR = {envdir}/.cache
 
 [testenv:py35-pure]
 setenv =
     PURE_PYTHON=1
-    PIP_CACHE_DIR = {envdir}/.cache
+
+[testenv:py38-cext]
+# Require the C extension
+setenv =
+    PURE_PYTHON=0
 
 [testenv:py]
 commands =
@@ -29,16 +35,15 @@ commands =
     {[testenv]commands}
 
 [testenv:coverage]
-setenv =
-  COVERAGE_FILE=.coverage
-skip_install = true
+# The -i/--ignore arg is necessary; we get
+# No source for code: '//.tox/pypy/site-packages/zope/interface/__init__.py'
+# without it.
 commands =
-    coverage erase
     coverage combine
-    coverage report
-    coverage html
-    coverage xml
-depends = py27,py27-pure,py35,py35-pure,py36,py37,py38,pypy,pypy3
+    coverage report -i
+    coverage html -i
+    coverage xml -i
+depends = py27,py27-pure,py35,py35-pure,py36,py37,py38,py38-cext,pypy,pypy3
 parallel_show_output = true
 
 [testenv:docs]