Allow empty edges

tests/core/test_dihypergraph.py

@@ -127,15 +127,12 @@ def test_add_edge_accepts_different_types():
         assert H.edges.head(dtype=dict) == {0: {4}}
 
 
-def test_add_edge_raises_with_empty_edges():
+def test_add_empty_edges():
     H = xgi.DiHypergraph()
-    for edge in [[], {}, iter([])]:
-        with pytest.raises(XGIError):
-            H.add_edge(edge)
-
     for edge in [[[], []], (set(), set())]:
-        with pytest.raises(XGIError):
-            H.add_edge(edge)
+        H.add_edge(edge)
+
+    assert H.edges.size.asdict() == {0: 0, 1: 0}
 
 
 def test_add_edge_rejects_set():
@@ -415,7 +412,14 @@ def test_remove_node_weak(diedgelist1, diedgelist2):
     H.remove_node(3)
     H.remove_node(4)
 
-    assert 0 not in H.edges
+    # test keeping empty edges
+    H = xgi.DiHypergraph(diedgelist1)
+    H.remove_node(1)
+    H.remove_node(2)
+    H.remove_node(3)
+    H.remove_node(4, remove_empty=False)
+    assert 0 in H.edges
+    assert H.edges.size[0] == 0
 
     # test multiple edge removal with a single node.
     H = xgi.DiHypergraph(diedgelist2)
@@ -456,9 +460,9 @@ def test_remove_node_strong(diedgelist1):
     # node in both head and tail
     assert 6 in H
     H.remove_node(6, strong=True)
-    # assert 6 not in H
+    assert 6 not in H
 
-    # assert 1 not in H.edges
+    assert 1 not in H.edges
 
 
 def test_remove_nodes_from(diedgelist1):
@@ -470,6 +474,16 @@ def test_remove_nodes_from(diedgelist1):
     with pytest.warns(Warning):
         H.remove_nodes_from([1, 2, 3])
 
+    H = xgi.DiHypergraph(diedgelist1)
+
+    H.remove_nodes_from([1, 5], strong=True)
+    assert len(H.edges) == 0
+
+    H = xgi.DiHypergraph(diedgelist1)
+    H.remove_nodes_from([1, 2, 3, 4], remove_empty=False)
+    assert 0 in H.edges
+    assert H.edges.size[0] == 0
+
 
 def test_pickle(diedgelist1):
     _, filename = tempfile.mkstemp()

tests/core/test_hypergraph.py

@@ -181,10 +181,12 @@ def test_add_edge():
         assert {1, 2, 3} == H.edges.members(0)
         assert H.edges.members(dtype=dict) == {0: {1, 2, 3}}
 
+    # test adding empty edges
     H = xgi.Hypergraph()
     for edge in [[], set(), iter([])]:
-        with pytest.raises(XGIError):
-            H.add_edge(edge)
+        H.add_edge(edge)
+
+    assert H.edges.size.asdict() == {0: 0, 1: 0, 2: 0}
 
     # check that uid works correctly
     H1 = xgi.Hypergraph()
@@ -583,6 +585,15 @@ def test_remove_node_weak(edgelist1):
     with pytest.raises(IDNotFound):
         H.remove_node(10)
 
+    # test keeping empty edges
+    H = xgi.Hypergraph(edgelist1)
+    H.remove_node(1)
+    H.remove_node(2)
+    H.remove_node(3, remove_empty=False)
+    H.remove_node(4, remove_empty=False)
+    assert 0 in H.edges and 1 in H.edges
+    assert H.edges.size[0] == 0 and H.edges.size[1] == 0
+
 
 def test_remove_node_strong(edgelist1):
     H = xgi.Hypergraph(edgelist1)
@@ -592,6 +603,27 @@ def test_remove_node_strong(edgelist1):
     assert 0 not in H.edges
 
 
+def test_remove_nodes_from(edgelist1):
+    H = xgi.Hypergraph(edgelist1)
+
+    H.remove_nodes_from([1, 2, 3])
+    assert 1 not in H and 2 not in H and 3 not in H
+    assert 0 not in H.edges
+
+    with pytest.warns(Warning):
+        H.remove_nodes_from([1, 2, 3])
+
+    H = xgi.Hypergraph(edgelist1)
+
+    H.remove_nodes_from([1, 4], strong=True)
+    assert 0 not in H.edges and 1 not in H.edges
+
+    H = xgi.Hypergraph(edgelist1)
+    H.remove_nodes_from([1, 2, 3, 4], remove_empty=False)
+    assert 0 in H.edges and 1 in H.edges
+    assert H.edges.size[0] == 0 and H.edges.size[1] == 0
+
+
 def test_issue_445(edgelist1):
     H = xgi.Hypergraph(edgelist1)
     assert 1 in H

xgi/core/dihypergraph.py

@@ -363,7 +363,7 @@ def add_nodes_from(self, nodes_for_adding, **attr):
                 self._node_attr[n] = self._node_attr_dict_factory()
             self._node_attr[n].update(newdict)
 
-    def remove_node(self, n, strong=False):
+    def remove_node(self, n, strong=False, remove_empty=True):
         """Remove a single node.
 
         The removal may be weak (default) or strong.  In weak removal, the node is
@@ -375,9 +375,11 @@ def remove_node(self, n, strong=False):
         ----------
         n : node
             A node in the dihypergraph
-
         strong : bool (default False)
             Whether to execute weak or strong removal.
+        remove_empty : bool, optional
+            Whether to remove empty edges (0 members in both head and tail).
+            By default, False.
 
         Raises
         ------
@@ -406,17 +408,26 @@ def remove_node(self, n, strong=False):
 
             # remove empty edges
             for edge in edge_neighbors["in"].union(edge_neighbors["out"]):
-                if not self._edge[edge]["in"] and not self._edge[edge]["out"]:
+                if (
+                    not self._edge[edge]["in"]
+                    and not self._edge[edge]["out"]
+                    and remove_empty
+                ):
                     del self._edge[edge]
                     del self._edge_attr[edge]
 
-    def remove_nodes_from(self, nodes):
+    def remove_nodes_from(self, nodes, strong=False, remove_empty=True):
         """Remove multiple nodes.
 
         Parameters
         ----------
         nodes : iterable
             An iterable of nodes.
+        strong : bool (default False)
+            Whether to execute weak or strong removal.
+        remove_empty : bool, optional
+            Whether to remove empty edges (0 members in both head and tail).
+            By default, False.
 
         See Also
         --------
@@ -427,7 +438,7 @@ def remove_nodes_from(self, nodes):
             if n not in self._node:
                 warn(f"Node {n} not in dihypergraph")
                 continue
-            self.remove_node(n)
+            self.remove_node(n, strong=strong, remove_empty=remove_empty)
 
     def set_node_attributes(self, values, name=None):
         """Sets node attributes from a given value or dictionary of values.
@@ -506,11 +517,6 @@ def add_edge(self, members, id=None, **attr):
         **attr : dict, optional
             Attributes of the new edge.
 
-        Raises
-        -----
-        XGIError
-            If `members` is empty or is not a list or tuple.
-
         See Also
         --------
         add_edges_from : Add a collection of edges.
@@ -525,18 +531,12 @@ def add_edge(self, members, id=None, **attr):
         >>> DH.add_edge(([1, 2, 3], [2, 3, 4]))
         >>> DH.add_edge(([3, 4], set()), id='myedge')
         """
-        if not members:
-            raise XGIError("Cannot add an empty edge")
-
         if isinstance(members, (tuple, list)):
             tail = members[0]
             head = members[1]
         else:
             raise XGIError("Directed edge must be a list or tuple!")
 
-        if not head and not tail:
-            raise XGIError("Cannot add an empty edge")
-
         uid = next(self._edge_uid) if id is None else id
 
         if id in self._edge.keys():  # check that uid is not present yet
@@ -983,11 +983,8 @@ def freeze(self):
         self.remove_nodes_from = frozen
         self.add_edge = frozen
         self.add_edges_from = frozen
-        self.add_weighted_edges_from = frozen
         self.remove_edge = frozen
         self.remove_edges_from = frozen
-        self.add_node_to_edge = frozen
-        self.remove_node_from_edge = frozen
         self.clear = frozen
         self.frozen = True
 

xgi/core/hypergraph.py

@@ -405,7 +405,7 @@ def add_nodes_from(self, nodes_for_adding, **attr):
                 self._node_attr[n] = self._node_attr_dict_factory()
             self._node_attr[n].update(newdict)
 
-    def remove_node(self, n, strong=False):
+    def remove_node(self, n, strong=False, remove_empty=True):
         """Remove a single node.
 
         The removal may be weak (default) or strong.  In weak removal, the node is
@@ -417,9 +417,10 @@ def remove_node(self, n, strong=False):
         ----------
         n : node
             A node in the hypergraph
-
         strong : bool, optional
             Whether to execute weak or strong removal. By default, False.
+        remove_empty : bool, optional
+            Whether to remove empty edges. By default, True.
 
         Raises
         ------
@@ -445,17 +446,21 @@ def remove_node(self, n, strong=False):
         else:  # weak removal
             for edge in edge_neighbors:
                 self._edge[edge].remove(n)
-                if not self._edge[edge]:
+                if not self._edge[edge] and remove_empty:
                     del self._edge[edge]
                     del self._edge_attr[edge]
 
-    def remove_nodes_from(self, nodes):
+    def remove_nodes_from(self, nodes, strong=False, remove_empty=True):
         """Remove multiple nodes.
 
         Parameters
         ----------
         nodes : iterable
             An iterable of nodes.
+        strong : bool, optional
+            Whether to execute weak or strong removal. By default, False.
+        remove_empty : bool, optional
+            Whether to remove empty edges. By default, True.
 
         See Also
         --------
@@ -466,7 +471,7 @@ def remove_nodes_from(self, nodes):
             if n not in self:
                 warn(f"Node {n} not in hypergraph")
                 continue
-            self.remove_node(n)
+            self.remove_node(n, strong=strong, remove_empty=remove_empty)
 
     def set_node_attributes(self, values, name=None):
         """Sets node attributes from a given value or dictionary of values.
@@ -577,8 +582,6 @@ def add_edge(self, members, id=None, **attr):
 
         """
         members = set(members)
-        if not members:
-            raise XGIError("Cannot add an empty edge")
 
         if id in self._edge.keys():  # check that uid is not present yet
             warn(f"uid {id} already exists, cannot add edge {members}")
@@ -1127,7 +1130,7 @@ def remove_edges_from(self, ebunch):
             del self._edge[id]
             del self._edge_attr[id]
 
-    def remove_node_from_edge(self, edge, node):
+    def remove_node_from_edge(self, edge, node, remove_empty=False):
         """Remove a node from an existing edge.
 
         Parameters
@@ -1136,6 +1139,8 @@ def remove_node_from_edge(self, edge, node):
             The edge ID
         node : hashable
             The node ID
+        remove_empty : bool, optional
+            Whether empty edges are removed. By default, True.
 
         Raises
         ------
@@ -1168,7 +1173,7 @@ def remove_node_from_edge(self, edge, node):
         except ValueError as e:
             raise XGIError(f"Edge {edge} does not contain node {node}") from e
 
-        if not self._edge[edge]:
+        if not self._edge[edge] and remove_empty:
             del self._edge[edge]
             del self._edge_attr[edge]
 
@@ -1434,6 +1439,7 @@ def cleanup(
         self,
         isolates=False,
         singletons=False,
+        empty_edges=False,
         multiedges=False,
         connected=True,
         relabel=True,
@@ -1447,6 +1453,8 @@ def cleanup(
             Whether isolated nodes are allowed, by default False.
         singletons : bool, optional
             Whether singleton edges are allowed, by default False.
+        empty_edges : bool, optional
+            Whether empty edges are allowed, by default False.
         multiedges : bool, optional
             Whether multiedges are allowed, by default False.
         connected : bool, optional
@@ -1466,6 +1474,8 @@ def cleanup(
                 self.merge_duplicate_edges()
             if not singletons:
                 self.remove_edges_from(self.edges.singletons())
+            if not empty_edges:
+                self.remove_edges_from(self.edges.empty())
             if not isolates:
                 self.remove_nodes_from(self.nodes.isolates())
             if connected:
@@ -1493,6 +1503,8 @@ def cleanup(
                 H.merge_duplicate_edges()
             if not singletons:
                 H.remove_edges_from(H.edges.singletons())
+            if not empty_edges:
+                H.remove_edges_from(H.edges.empty())
             if not isolates:
                 H.remove_nodes_from(H.nodes.isolates())
             if connected:

xgi/core/simplicialcomplex.py

@@ -268,11 +268,6 @@ def add_simplex(self, members, id=None, **attr):
         **attr : dict, optional
             Attributes of the new simplex.
 
-        Raises
-        -----
-        XGIError
-            If `members` is empty.
-
         See Also
         --------
         add_simplices_from : Add a collection of simplices.
@@ -313,9 +308,6 @@ def add_simplex(self, members, id=None, **attr):
         except TypeError:
             raise XGIError("The simplex cannot be cast to a frozenset.")
 
-        if not members:
-            raise XGIError("Cannot add an empty edge")
-
         if self.has_simplex(members):
             return
 

xgi/core/views.py

@@ -721,6 +721,20 @@ def singletons(self):
         """
         return self.filterby("size", 1)
 
+    def empty(self):
+        """Edges that contain no nodes.
+
+        Returns
+        -------
+        EdgeView containing the empty edges.
+
+        See Also
+        --------
+        :meth:`NodeView.isolates`
+
+        """
+        return self.filterby("size", 0)
+
     def maximal(self, strict=False):
         """Returns the maximal edges as an EdgeView.