Minor fixes for better scalar behavior

hg hash: 016f609813fe95864d10f2a0ad38f6fca5b23c4f

dedalus/core/field.py

@@ -550,14 +550,17 @@ def as_ncc_operator(self, frozen_arg_basis_meta, cutoff, max_terms, cacheid=None
             if basis.separable:
                 if not self.meta[basis.name]['constant']:
                     raise ValueError("{} is non-constant along separable direction '{}'.".format(self, basis.name))
+        # Scatter transverse-constant coefficients
         basis = domain.bases[-1]
         coeffs = np.zeros(basis.coeff_size, dtype=basis.coeff_dtype)
-        # Scatter transverse-constant coefficients
         self.require_coeff_space()
         if domain.dist.rank == 0:
             select = (0,) * (domain.dim - 1)
             np.copyto(coeffs, self.data[select])
         domain.dist.comm_cart.Bcast(coeffs, root=0)
+        # Revert to scalar behavior for constants
+        if self.meta[-1]['constant']:
+            return coeffs[0]
         # Build matrix
         ncc_basis_meta = self.meta[-1]
         arg_basis_meta = dict(frozen_arg_basis_meta)

dedalus/core/operators.py

@@ -1080,6 +1080,12 @@ class TimeDerivative(LinearOperator, FutureField):
 
     name = 'dt'
 
+    def __new__(cls, arg0, **kw):
+        if not isinstance(arg0, Operand):
+            return 0
+        else:
+            return object.__new__(cls)
+
     @property
     def base(self):
         return TimeDerivative

dedalus/tests/test_ivp.py

@@ -30,10 +30,11 @@ def test_heat_1d_periodic(benchmark, x_basis_class, Nx, timestepper, dtype):
     x = domain.grid(0)
     F['g'] = -np.sin(x)
     # Problem
-    problem = de.IVP(domain, variables=['u'])
+    problem = de.IVP(domain, variables=['u','ux'])
     problem.meta['u']['x']['parity'] = -1
     problem.parameters['F'] = F
-    problem.add_equation("-dt(u) + dx(dx(u)) = F")
+    problem.add_equation("-dt(u) + dx(ux) = F")
+    problem.add_equation("ux - dx(u) = 0")
     # Solver
     solver = problem.build_solver(timestepper)
     dt = 1e-5