add test for dtype in setup_linsys

kwant/physics/tests/test_leads.py

@@ -297,3 +297,30 @@ def test_for_all_evs_equal():
     assert modes.vecs.shape == (1, 2)
     assert modes.vecslmbdainv.shape == (1, 2)
     assert modes.nmodes == 1
+
+
+def test_dtype_linsys():
+    """Test that setup_linsys stays in real arithmetics when possible."""
+    h_cell = np.array([[2.0, -1.0], [-1.0, 2.0]], dtype=np.float64)
+    h_hop = np.array([[0.0],[-1.0]], dtype=np.float64)
+
+    lsys = kwant.physics.leads.setup_linsys(h_cell - 0.3*np.eye(2),
+                                            h_hop)
+    assert lsys.eigenproblem[0].dtype == np.float64
+
+    lsys = kwant.physics.leads.setup_linsys(h_cell.astype(np.complex128) 
+                                            - 0.3*np.eye(2),
+                                            h_hop.astype(np.complex128))
+    assert lsys.eigenproblem[0].dtype == np.float64
+    
+    # energy=1 is an eigenstate of the isolated cell Hamiltonian,
+    # i.e. a complex self-energy stabilization is necessary
+    lsys = kwant.physics.leads.setup_linsys(h_cell - 1*np.eye(2),
+                                            h_hop)
+    assert lsys.eigenproblem[0].dtype == np.complex128
+
+    # with complex input, output must be complex, too
+    h_hop = np.array([[0.0],[-1.0 + 0.1j]], dtype=np.complex128)
+    lsys = kwant.physics.leads.setup_linsys(h_cell - 0.3*np.eye(2),
+                                            h_hop)
+    assert lsys.eigenproblem[0].dtype == np.complex128