Simplify __init__.py to not repeat itself three times.

There are many fewer 'no cover' lines now.

Remove the call to copy_reg.constructor---it doesn't do anything anymore except check that the object is callable, there is no registry.

Also make timestamp support PURE_PYTHON.

persistent/__init__.py

@@ -15,55 +15,49 @@
 
 Fall back to pure Python implementations.
 """
-import os
 
-PURE_PYTHON = os.environ.get('PURE_PYTHON')
-if not PURE_PYTHON:
-    try:
-        from persistent.cPersistence import Persistent
-        from persistent.cPersistence import GHOST
-        from persistent.cPersistence import UPTODATE
-        from persistent.cPersistence import CHANGED
-        from persistent.cPersistence import STICKY
-        from persistent.cPersistence import simple_new
-    except ImportError: # pragma: no cover
-        from persistent.persistence import Persistent
-        from persistent.persistence import GHOST
-        from persistent.persistence import UPTODATE
-        from persistent.persistence import CHANGED
-        from persistent.persistence import STICKY
-    else:
-        from persistent._compat import copy_reg
-        copy_reg.constructor(simple_new)
-        # Make an interface declaration for Persistent, if zope.interface
-        # is available.  Note that the Python version already does this.
-        try:
+import sys
+
+__all__ = [
+    'IPersistent',
+    'Persistent',
+    'GHOST',
+    'UPTODATE',
+    'CHANGED',
+    'STICKY',
+    'PickleCache',
+    'TimeStamp',
+]
+from persistent._compat import PURE_PYTHON
+from persistent.interfaces import IPersistent
+
+import persistent.timestamp as TimeStamp
+
+from persistent import persistence as pyPersistence
+from persistent import picklecache as pyPickleCache
+
+try:
+    # Be careful not to shadow the modules
+    from persistent import cPersistence as _cPersistence
+    from persistent import cPickleCache as _cPickleCache
+except ImportError: # pragma: no cover
+    _cPersistence = None
+    _cPickleCache = None
+else:
+    # Make an interface declaration for Persistent
+    # Note that the Python version already does this.
     from zope.interface import classImplements
-        except ImportError: # pragma: no cover
-            pass
-        else:
-            from persistent.interfaces import IPersistent
-            classImplements(Persistent, IPersistent)
+    classImplements(_cPersistence.Persistent, IPersistent)
+
+
+_persistence = pyPersistence if PURE_PYTHON or _cPersistence is None else _cPersistence
+_picklecache = pyPickleCache if PURE_PYTHON or _cPickleCache is None else _cPickleCache
 
-    try:
-        from persistent.cPickleCache import PickleCache
-    except ImportError: # pragma: no cover
-        from persistent.picklecache import PickleCache
+Persistent = _persistence.Persistent
+GHOST = _persistence.GHOST
+UPTODATE = _persistence.UPTODATE
+CHANGED = _persistence.CHANGED
+STICKY = _persistence.STICKY
+PickleCache = _picklecache.PickleCache
 
-    try:
-        import persistent.TimeStamp
-    except ImportError: # pragma: no cover
-        import persistent.timestamp as TimeStamp
-        import sys
-        sys.modules['persistent.TimeStamp'
-                   ] = sys.modules['persistent.timestamp']
-else: # pragma: no cover
-    from persistent.persistence import Persistent
-    from persistent.persistence import GHOST
-    from persistent.persistence import UPTODATE
-    from persistent.persistence import CHANGED
-    from persistent.persistence import STICKY
-    from persistent.picklecache import PickleCache
-    import persistent.timestamp as TimeStamp
-    import sys
-    sys.modules['persistent.TimeStamp'] = sys.modules['persistent.timestamp']
+sys.modules['persistent.TimeStamp'] = sys.modules['persistent.timestamp']

persistent/_compat.py

@@ -13,6 +13,9 @@
 ##############################################################################
 
 import sys
+import os
+
+PURE_PYTHON = os.environ.get('PURE_PYTHON')
 
 if sys.version_info[0] > 2: # pragma: no cover
     import copyreg as copy_reg

persistent/timestamp.py

@@ -18,6 +18,8 @@ import math
 import struct
 import sys
 
+from persistent._compat import PURE_PYTHON
+
 _RAWTYPE = bytes
 _MAXINT = sys.maxsize
 
@@ -205,6 +207,8 @@ class pyTimeStamp(object):
 
 
 try:
-    from persistent._timestamp import TimeStamp
+    from persistent._timestamp import TimeStamp as CTimeStamp
 except ImportError: # pragma: no cover
-    TimeStamp = pyTimeStamp
+    CTimeStamp = None
+
+TimeStamp = pyTimeStamp if PURE_PYTHON or CTimeStamp is None else CTimeStamp