ENH: implement Graph.__repr__ (#711)

* ENH: implement Graph.__repr__

* update the formatting

libpysal/graph/base.py

@@ -151,6 +151,16 @@ def __getitem__(self, item):
             )
         return self._adjacency.loc[item]
 
+    def __repr__(self):
+        if len(self.unique_ids) > 5:
+            unique_ids = f"{str(self.unique_ids[:5].tolist())[:-1]}, ...]"
+        else:
+            unique_ids = self.unique_ids.tolist()
+        return (
+            f"<Graph of {self.n} nodes and {self.nonzero} nonzero edges indexed by\n"
+            f" {unique_ids}>"
+        )
+
     def copy(self, deep=True):
         """Make a copy of this Graph's adjacency table and transformation
 

libpysal/graph/tests/test_base.py

@@ -138,6 +138,26 @@ def test_init(self):
         with pytest.raises(ValueError, match="'transformation' needs to be"):
             graph.Graph(self.adjacency_int_binary, transformation="foo")
 
+    def test___repr__(self):
+        expected = (
+            "<Graph of 10 nodes and 25 nonzero edges indexed by\n"
+            " [0, 1, 2, 3, 4, ...]>"
+        )
+        assert repr(self.g_int) == expected
+
+        expected = (
+            "<Graph of 10 nodes and 25 nonzero edges indexed by\n"
+            " ['a', 'b', 'c', 'd', 'e', ...]>"
+        )
+        assert repr(self.g_str) == expected
+
+        nybb = graph.Graph.build_contiguity(self.nybb)
+        expected = (
+            "<Graph of 5 nodes and 10 nonzero edges indexed by\n"
+            " ['Staten Island', 'Queens', 'Brooklyn', 'Manhattan', 'Bronx']>"
+        )
+        assert repr(nybb) == expected
+
     def test_copy(self):
         g_copy = self.g_str.copy()
         assert g_copy == self.g_str