add TensorNetwork.cut_bond

docs/changelog.md

@@ -2,6 +2,16 @@
 
 Release notes for `quimb`.
 
+
+(whats-new-1-7-2)=
+## v1.7.2 (unreleased)
+
+**Enhancements:**
+
+- add [`TensorNetwork.cut_bond`](quimb.tensor.tensor_core.TensorNetwork.cut_bond)
+  for cutting a bond index
+
+
 (whats-new-1-7-1)=
 ## v1.7.1 (2024-01-30)
 

quimb/tensor/tensor_core.py

@@ -7974,6 +7974,31 @@ def cut_between(self, left_tags, right_tags, left_ind, right_ind):
         (tid2,) = self._get_tids_from_tags(right_tags)
         self._cut_between_tids(tid1, tid2, left_ind, right_ind)
 
+    def cut_bond(self, bond, new_left_ind=None, new_right_ind=None):
+        """Cut the bond index specified by ``bond`` between the tensors it
+        connects. Use ``cut_between`` for control over which tensor gets which
+        new index ``new_left_ind``  or ``new_right_ind``. The index must
+        connect exactly two tensors.
+
+        Parameters
+        ----------
+        bond : str
+            The index to cut.
+        new_left_ind : str, optional
+            The new index to give to the left tensor (lowest ``tid`` value).
+        new_right_ind : str, optional
+            The new index to give to the right tensor (highest ``tid`` value).
+        """
+        tid1, tid2 = sorted(self.ind_map[bond])
+        tl, tr = self._tids_get(tid1, tid2)
+        if new_left_ind is None:
+            new_left_ind = rand_uuid()
+        if new_right_ind is None:
+            new_right_ind = rand_uuid()
+        tl.reindex_({bond: new_left_ind})
+        tr.reindex_({bond: new_right_ind})
+        return new_left_ind, new_right_ind
+
     def isel(self, selectors, inplace=False):
         """Select specific values for some dimensions/indices of this tensor
         network, thereby removing them.

tests/test_tensor/test_tensor_core.py

@@ -1595,6 +1595,14 @@ def test_fuse_multibonds(self):
         tn.fuse_multibonds(inplace=True)
         assert len(tn.inner_inds()) == 3
 
+    def test_cut_bond(self):
+        ta = qtn.rand_tensor((2, 2, 2), inds="abc", tags="A")
+        tb = qtn.rand_tensor((2, 2, 2), inds="cde", tags="B")
+        tn = ta | tb
+        tn.cut_bond('c', new_left_ind='l', new_right_ind='r')
+        assert ta.inds == ('a', 'b', 'l')
+        assert tb.inds == ('r', 'd', 'e')
+
     def test_draw(self):
         import matplotlib
         from matplotlib import pyplot as plt