small tweaks

nifty5/operators/scaling_operator.py

@@ -35,14 +35,6 @@ class ScalingOperator(EndomorphicOperator):
     -----
     :class:`Operator` supports the multiplication with a scalar. So one does
     not need instantiate :class:`ScalingOperator` explicitly in most cases.
-
-    Formally, this operator always supports all operation modes (times,
-    adjoint_times, inverse_times and inverse_adjoint_times), even if `factor`
-    is 0 or infinity. It is the user's responsibility to apply the operator
-    only in appropriate ways (e.g. call inverse_times only if `factor` is
-    nonzero).
-
-    This shortcoming will hopefully be fixed in the future.
     """
 
     def __init__(self, factor, domain):
@@ -52,7 +44,10 @@ class ScalingOperator(EndomorphicOperator):
             raise TypeError("Scalar required")
         self._factor = factor
         self._domain = makeDomain(domain)
-        self._capability = self._all_ops
+        if self._factor == 0.:
+            self._capability = self.TIMES | self.ADJOINT_TIMES
+        else:
+            self._capability = self._all_ops
 
     def apply(self, x, mode):
         self._check_input(x, mode)

nifty5/operators/sum_operator.py

@@ -23,6 +23,7 @@ from ..sugar import domain_union
 from ..utilities import indent
 from .block_diagonal_operator import BlockDiagonalOperator
 from .linear_operator import LinearOperator
+from .simple_linear_operators import NullOperator
 
 
 class SumOperator(LinearOperator):
@@ -59,6 +60,8 @@ class SumOperator(LinearOperator):
                     negnew += [not n for n in op._neg]
                 else:
                     negnew += list(op._neg)
+            elif isinstance(op, NullOperator):
+                pass
             else:
                 opsnew.append(op)
                 negnew.append(ng)