change _util.deep_replace_property return value

This patch changes the definition of the function decorated by
_util.deep_replace_property, from returning None if no replacement is made, to
returning the object itself. The new definition is a more natural choice for a
recursive procedure (recursion stops when a trivial loop is reached) and
results in better readable code for the casual observer.

nutils/_util.py

@@ -951,7 +951,7 @@ class deep_replace_property:
     Args
     ----
     func
-        Callable which maps an object onto a new object, or ``None`` if no
+        Callable which maps an object onto a new object, or onto itself if no
         replacement is made. It must have precisely one positional argument for
         the object.
     '''
@@ -983,7 +983,7 @@ def __get__(self, obj, objtype=None):
             if isinstance(obj, self.recreate): # recreate object from rstack
                 f, nargs = obj
                 r = f(*[rstack.pop() for _ in range(nargs)])
-                if isinstance(r, self.owner) and (newr := self.func(r)) is not None:
+                if isinstance(r, self.owner) and (newr := self.func(r)) is not r:
                     fstack.append(newr) # recursion
                 else:
                     rstack.append(r)

nutils/evaluable.py

@@ -293,7 +293,9 @@ def _format_stack(self, values, e):
     @util.deep_replace_property
     def simplified(obj):
         retval = obj._simplified()
-        if retval is not None and isinstance(obj, Array):
+        if retval is None:
+            return obj
+        if isinstance(obj, Array):
             assert isinstance(retval, Array) and equalshape(retval.shape, obj.shape) and retval.dtype == obj.dtype, '{} --simplify--> {}'.format(obj, retval)
         return retval
 
@@ -310,7 +312,9 @@ def optimized_for_numpy(self):
     @util.deep_replace_property
     def _optimized_for_numpy1(obj):
         retval = obj._simplified() or obj._optimized_for_numpy()
-        if retval is not None and isinstance(obj, Array):
+        if retval is None:
+            return obj
+        if isinstance(obj, Array):
             assert isinstance(retval, Array) and equalshape(retval.shape, obj.shape), '{0}._optimized_for_numpy or {0}._simplified resulted in shape change'.format(type(obj).__name__)
         return retval
 

tests/test_util.py

@@ -540,8 +540,10 @@ def simple(self):
             self.called = True
             if isinstance(self, replace.Ten):
                 return replace.Intermediate() # to test recursion
-            if isinstance(self, replace.Intermediate):
+            elif isinstance(self, replace.Intermediate):
                 return 10
+            else:
+                return self
 
     class Ten(Base): pass
     class Intermediate(Base): pass