ENH/TST/STY: mainly change gradient in Bregman distance.

If a subgradient opertor is given expicitly, this one is
used if the user calls bregman_dist.gradient(x).

Also minor style fix in bregman distance functional.

Author: aringh

odl/solvers/functional/default_functionals.py

@@ -2551,14 +2551,14 @@ def __init__(self, functional, point, subgradient_op=None):
                                 '``Operator``'.format(subgradient_op))
             if not self.__functional.domain == subgradient_op.domain:
                 raise ValueError('`functional.domain` {} is not the same as '
-                                 '`subgradient_op.domain` {}'.format(
-                                        self.__functional.domain,
-                                        subgradient_op.domain))
+                                 '`subgradient_op.domain` {}'
+                                 ''.format(self.__functional.domain,
+                                           subgradient_op.domain))
             if not self.__functional.domain == subgradient_op.range:
                 raise ValueError('`functional.domain` {} is not the same as '
-                                 '`subgradient_op.range` {}'.format(
-                                        self.__functional.domain,
-                                        subgradient_op.range))
+                                 '`subgradient_op.range` {}'
+                                 ''.format(self.__functional.domain,
+                                           subgradient_op.range))
             self.__subgradient_op = subgradient_op
 
         self.__subgrad_eval = self.__subgradient_op(self.__point)
@@ -2602,7 +2602,7 @@ def proximal(self):
     @property
     def gradient(self):
         """Gradient operator of the functional."""
-        return self.__bregman_dist.gradient
+        return self.subgradient_op - ConstantOperator(self.__subgrad_eval)
 
     def __repr__(self):
         '''Return ``repr(self)``.'''

odl/test/solvers/functional/default_functionals_test.py

@@ -516,12 +516,11 @@ def test_moreau_envelope_l2_sq(space, sigma):
                             x * 2 / (1 + 2 * sigma))
 
 
-def test_bregman_functional_no_gradient():
+def test_bregman_functional_no_gradient(space):
     """Test that the Bregman distance functional fails if the underlying
     functional does not have a gradient and no subgradient operator is
     given."""
 
-    space = odl.uniform_discr(0, 1, 3)
     ind_func = odl.solvers.IndicatorNonnegativity(space)
     point = noise_element(space)
 
@@ -533,7 +532,13 @@ def test_bregman_functional_no_gradient():
     # If a subgradient operator is given separately, it is possible to create
     # an instance of the functional
     subgrad_op = odl.IdentityOperator(space)
-    odl.solvers.BregmanDistance(ind_func, point, subgrad_op)
+    bregman_dist = odl.solvers.BregmanDistance(ind_func, point, subgrad_op)
+
+    # In this case we should be able to call the gradient of the bregman
+    # distance, which would give us a subgradient
+    x = np.abs(noise_element(space))
+    expected_result = subgrad_op(x) - subgrad_op(point)
+    assert all_almost_equal(bregman_dist.gradient(x), expected_result)
 
 
 def test_bregman_functional_l2_squared(space, sigma):