diff --git a/src/operators/diagonal_operator.py b/src/operators/diagonal_operator.py
index dc39968c21451a656e5e2d2dbf8a450ffe01045c..0367c8edb490a04e2ca60583044aa37de1e29af6 100644
--- a/src/operators/diagonal_operator.py
+++ b/src/operators/diagonal_operator.py
@@ -15,9 +15,6 @@
 #
 # NIFTy is being developed at the Max-Planck-Institut fuer Astrophysik.
 
-from functools import partial
-from operator import mul
-
 import numpy as np
 
 from .. import utilities
@@ -72,8 +69,8 @@ class DiagonalOperator(EndomorphicOperator):
     """
 
     def __init__(self, diagonal, domain=None, spaces=None, sampling_dtype=None,
-                 trafo=0):
-# MR: trafo is more or les deliberately undocumened, since it is not supposed
+                 _trafo=0):
+# MR: _trafo is more or less deliberately undocumened, since it is not supposed
 # to be necessary for "end users". It describes the type of transform for which
 # the diagonal can be used without modification
 # (0:TIMES, 1:ADJOINT, 2:INVERSE, 3:ADJOINT_INVERSE)
@@ -81,7 +78,7 @@ class DiagonalOperator(EndomorphicOperator):
             raise TypeError("Field object required")
         utilities.check_dtype_or_none(sampling_dtype)
         self._dtype = sampling_dtype
-        self._trafo = trafo
+        self._trafo = _trafo
         if domain is None:
             self._domain = diagonal.domain
         else:
@@ -141,7 +138,7 @@ class DiagonalOperator(EndomorphicOperator):
             return 1./self._ldiag
         if self._trafo == 3:
             return np.conj(1./self._ldiag) if self._complex else 1./self._ldiag
-        
+
     def _scale(self, fct):
         if not np.isscalar(fct):
             raise TypeError("scalar value required")