Test PURE_PYTHON at runtime.

Turns out that the C extensions build and work fine with PyPy, but don't use them by default. Let them
be forced, though.

Tests needed some refactoring to account for the various permutations.

1 files changed, 108 insertions, 1 deletions

diff --git a/src/zope/interface/_compat.py b/src/zope/interface/_compat.py
index fb61e13..1a834d5 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 +62,104 @@ 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:
+        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:
+            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