Create a soft_def shim to combine move_to and Getattr (#473)

* Create a soft_def shim to combine move_to and Getattr

* Rename `soft_def` to `register_shim`

dill/_shims.py

@@ -150,11 +150,43 @@ def decorator(func):
         return func
     return decorator
 
+def register_shim(name, default):
+    """
+    A easier to understand and more compact way of "softly" defining a function.
+    These two pieces of code are equivalent:
+
+    if _dill.OLD3X:
+        def _create_class():
+            ...
+    _create_class = register_shim('_create_class', types.new_class)
+
+    if _dill.OLD3X:
+        @move_to(_dill)
+        def _create_class():
+            ...
+    _create_class = Getattr(_dill, '_create_class', types.new_class)
+
+    Intuitively, it creates a function or object in the versions of dill/python
+    that require special reimplementations, and use a core library or default
+    implementation if that function or object does not exist.
+    """
+    func = globals().get(name)
+    if func is not None:
+        _dill.__dict__[name] = func
+        func.__module__ = _dill.__name__
+
+    if default is Getattr.NO_DEFAULT:
+        reduction = (getattr, (_dill, name))
+    else:
+        reduction = (getattr, (_dill, name, default))
+
+    return Reduce(*reduction, is_callable=callable(default))
+
 ######################
 ## Compatibility Shims are defined below
 ######################
 
-_CELL_EMPTY = Getattr(_dill, '_CELL_EMPTY', None)
+_CELL_EMPTY = register_shim('_CELL_EMPTY', None)
 
 if _dill.OLD37:
     if _dill.HAS_CTYPES and hasattr(_dill.ctypes, 'pythonapi') and hasattr(_dill.ctypes.pythonapi, 'PyCell_Set'):
@@ -163,15 +195,13 @@ def decorator(func):
 
         _PyCell_Set = ctypes.pythonapi.PyCell_Set
 
-        @move_to(_dill)
         def _setattr(object, name, value):
             if type(object) is _dill.CellType and name == 'cell_contents':
                 _PyCell_Set.argtypes = (ctypes.py_object, ctypes.py_object)
                 _PyCell_Set(object, value)
             else:
                 setattr(object, name, value)
 
-        @move_to(_dill)
         def _delattr(object, name):
             if type(object) is _dill.CellType and name == 'cell_contents':
                 _PyCell_Set.argtypes = (ctypes.py_object, ctypes.c_void_p)
@@ -195,7 +225,6 @@ def cell_setter(value):
                 func(value)
             else:
                 setattr(cell, name, value)''' % __nonlocal)
-        move_to(_dill)(_setattr)
 
         exec('''def _delattr(cell, name):
             if type(cell) is _dill.CellType and name == 'cell_contents':
@@ -210,7 +239,6 @@ def cell_deleter():
                 func()
             else:
                 delattr(cell, name)''' % __nonlocal)
-        move_to(_dill)(_delattr)
 
     else:
         # Likely PyPy 2.7. Simulate the nonlocal keyword with bytecode
@@ -221,7 +249,6 @@ def cell_deleter():
         # Copyright (c) 2012, Regents of the University of California.
         # Copyright (c) 2009 `PiCloud, Inc. <http://www.picloud.com>`_.
         # License: https://github.com/cloudpipe/cloudpickle/blob/master/LICENSE
-        @move_to(_dill)
         def _setattr(cell, name, value):
             if type(cell) is _dill.CellType and name == 'cell_contents':
                 _cell_set = _dill.FunctionType(
@@ -255,12 +282,11 @@ def _cell_set_factory(value):
 
         del co
 
-        @move_to(_dill)
         def _delattr(cell, name):
             if type(cell) is _dill.CellType and name == 'cell_contents':
                 pass
             else:
                 delattr(cell, name)
 
-_setattr = Getattr(_dill, '_setattr', setattr)
-_delattr = Getattr(_dill, '_delattr', delattr)
+_setattr = register_shim('_setattr', setattr)
+_delattr = register_shim('_delattr', delattr)