add support for conservation laws and discrete symmetries to wraparound

No extra logic is added to wraparound, we just transfer over extra
Builder attributes to the wrapped around system, being careful to
set TR and PH symmetry to None (for the k-space Hamiltonian these
are only satisfied at special points in the Brillouin zone, and
currently there is no way in Kwant to specify that symmetries only
appear for certain parameter values).

kwant/tests/test_wraparound.py

@@ -166,3 +166,29 @@ def test_signatures():
         params, takes_kwargs = get_parameters(wrapped_syst[hopping])
         assert params[2:] == params_should_be
         assert takes_kwargs == should_take_kwargs
+
+
+def test_symmetry():
+    syst = _simple_syst(kwant.lattice.square())
+
+    matrices = [np.random.rand(2, 2) for i in range(4)]
+    laws = (matrices, [(lambda a: m) for m in matrices])
+    for cl, ch, ph, tr in laws:
+        syst.conservation_law = cl
+        syst.chiral = ch
+        syst.particle_hole = ph
+        syst.time_reversal = tr
+
+        wrapped = wraparound(syst)
+
+        assert wrapped.time_reversal is None
+        assert wrapped.particle_hole is None
+        for attr in ('conservation_law', 'chiral'):
+            new = getattr(wrapped, attr)
+            orig = getattr(syst, attr)
+            if callable(orig):
+                params, _ = get_parameters(new)
+                assert params[1:] == ['k_x', 'k_y']
+                assert np.all(orig(None) == new(None, None, None))
+            else:
+                assert np.all(orig == new)

kwant/wraparound.py

@@ -228,6 +228,18 @@ def wraparound(builder, keep=None, *, coordinate_names=('x', 'y', 'z')):
         sym = TranslationalSymmetry(*periods)
         momenta.pop(keep)
 
+    # Wrapped around system retains conservation law and chiral symmetry.
+    # We use 'bind_site' to add the momenta arguments if required.
+    cons = builder.conservation_law
+    ret.conservation_law = bind_site(cons) if callable(cons) else cons
+    chiral = builder.chiral
+    ret.chiral = bind_site(chiral) if callable(chiral) else chiral
+    # Set these to zero, as they can only hold @ k=0, and we currently
+    # have no mechanism for telling the system about the existence
+    # (or not) of a symmetry at different parameter values.
+    ret.particle_hole = None
+    ret.time_reversal = None
+
     sites = {}
     hops = collections.defaultdict(list)