define ordering for `SiteFamily` and add a test for this

kwant/builder.py

@@ -12,6 +12,7 @@ __all__ = ['Builder', 'Site', 'SiteFamily', 'SimpleSiteFamily', 'Symmetry',
 import abc
 import warnings
 import operator
+from functools import total_ordering
 from itertools import islice, chain
 import tinyarray as ta
 import numpy as np
@@ -82,6 +83,7 @@ class Site(tuple):
         return self.family.pos(self.tag)
 
 
+@total_ordering
 class SiteFamily(metaclass=abc.ABCMeta):
     """Abstract base class for site families.
 
@@ -154,6 +156,12 @@ class SiteFamily(metaclass=abc.ABCMeta):
         except AttributeError:
             return True
 
+    def __lt__(self, other):
+        try:
+            return self.canonical_repr < other.canonical_repr
+        except AttributeError:
+            return False
+
     @abc.abstractmethod
     def normalize_tag(self, tag):
         """Return a normalized version of the tag.

kwant/tests/test_builder.py

@@ -111,6 +111,23 @@ def test_site_families():
     assert_not_equal(fam, 'a')
 
 
+def test_site_families_sorting():
+    fam1 = builder.SimpleSiteFamily('fam1')
+    fam2 = builder.SimpleSiteFamily('fam2')
+
+    rng = Random(123)
+
+    tags = [(rng.randint(0,10), rng.randint(0,10)) for i in range(10)]
+    tags_sorted = [t for t in sorted(tags)]
+
+    sites = [fam1(*t) for t in tags] + [fam2(*t) for t in tags]
+    rng.shuffle(sites)
+    sites_sorted = ([fam1(*t) for t in tags_sorted] +
+                    [fam2(*t) for t in tags_sorted])
+
+    assert_equal(list(sorted(sites)), sites_sorted)
+
+
 class VerySimpleSymmetry(builder.Symmetry):
     def __init__(self, period):
         self.period = period