Merge branch 'stable_py2' into stable

kwant/builder.py

@@ -1059,6 +1059,9 @@ class Builder:
         The lead numbering starts from zero and increments from there, i.e.
         the leads are numbered in the order in which they are attached.
         """
+        if self.symmetry.num_directions:
+            raise ValueError("Leads can only be attached to finite systems.")
+
         if add_cells < 0 or int(add_cells) != add_cells:
             raise ValueError('add_cells must be an integer >= 0.')
 

kwant/physics/leads.py

@@ -578,8 +578,8 @@ def modes(h_cell, h_hop, tol=1e6, stabilization=None):
         raise ValueError("Incompatible matrix sizes for h_cell and h_hop.")
 
     if not complex_any(h_hop):
-        v = np.zeros((0, m))
-        return (PropagatingModes(np.zeros((0, n)), np.zeros((0,)),
+        v = np.zeros((m, 0))
+        return (PropagatingModes(np.zeros((n, 0)), np.zeros((0,)),
                                  np.zeros((0,))),
                 StabilizedModes(np.zeros((0, 0)), np.zeros((0, 0)), 0, v))
 

kwant/physics/tests/test_leads.py

@@ -324,3 +324,19 @@ def test_dtype_linsys():
     lsys = kwant.physics.leads.setup_linsys(h_cell - 0.3*np.eye(2),
                                             h_hop)
     assert lsys.eigenproblem[0].dtype == np.complex128
+
+
+def test_zero_hopping():
+    h_cell = np.identity(2)
+    h_hop = np.zeros((2, 1))
+    expected = (leads.PropagatingModes(np.zeros((2, 0)), np.zeros((0,)),
+                                       np.zeros((0,))),
+                leads.StabilizedModes(np.zeros((0, 0)), np.zeros((0, 0)), 0,
+                                      np.zeros((1, 0))))
+    actual = leads.modes(h_cell, h_hop)
+    assert all(np.alltrue(getattr(actual[1], attr) ==
+                          getattr(expected[1], attr)) for attr
+                   in ('vecs', 'vecslmbdainv', 'nmodes', 'sqrt_hop'))
+    assert all(np.alltrue(getattr(actual[0], attr) ==
+                          getattr(expected[0], attr)) for attr
+                   in ('wave_functions', 'velocities', 'momenta'))