fix and test insert_compressor_between_regions when insert_into=None

Author: jcmgray

.gitattributes

@@ -19,3 +19,5 @@ quimb/_version.py export-subst
 
 # ensure quimb appears as a python project on github
 *.ipynb linguist-language=Python
+# SCM syntax highlighting & preventing 3-way merges
+pixi.lock merge=binary linguist-language=YAML linguist-generated=true

.gitignore

@@ -127,3 +127,6 @@ docs/autoapi
 # quimb specific
 ctg_cache
 quimb/_version.py
+# pixi environments
+.pixi/*
+!.pixi/config.toml

docs/changelog.md

@@ -14,6 +14,11 @@ Release notes for `quimb`.
 - refactor [`TEBDGen`](quimb.tensor.tensor_arbgeom_tebd.TEBDGen) and [`SimpleUpdateGen`](quimb.tensor.tensor_arbgeom_tebd.SimpleUpdateGen)
 - [`tn.draw()`](quimb.tensor.drawing.draw_tn): show abelian signature if using `symmray` arrays.
 
+**Bug fixes:**
+
+- fix [`insert_compressor_between_regions`](quimb.tensor.tensor_core.TensorNetwork.insert_compressor_between_regions) when `insert_into is None`.
+
+
 (whats-new-1-11-2)=
 ## v1.11.2 (2025-07-30)
 

quimb/tensor/tensor_core.py

@@ -9802,7 +9802,7 @@ def insert_compressor_between_regions(
 
         # get the connecting indices and corresponding sizes
         bix = bonds(ltn, rtn)
-        bix_sizes = [tn.ind_size(ix) for ix in bix]
+        bix_sizes = [ltn.ind_size(ix) for ix in bix]
 
         if mode == "nystrom":
             # only use
@@ -9887,8 +9887,8 @@ def insert_compressor_between_regions(
 
         if insert_into is not None:
             tn = insert_into
-            ltn = tn.select(ltags, which=select_which)
-            rtn = tn.select(rtags, which=select_which)
+        ltn = tn.select(ltags, which=select_which)
+        rtn = tn.select(rtags, which=select_which)
 
         # finally cut the bonds
         new_lix = [rand_uuid() for _ in bix]

tests/test_tensor/test_tensor_core.py

@@ -1868,6 +1868,55 @@ def test_tn_isel_rand(self):
         xs = mps.to_dense().ravel()
         assert not any(np.allclose(rx, x) for x in xs)
 
+    def test_insert_compressor_between_regions(self):
+        inputs = ["abgl", "gfhim", "bcdfe", "iekj"]
+        tags = ["A", "C", "B", "D"]
+        size_dict = {ix: 2 for ix in set("".join(inputs))}
+        ts = [
+            qtn.rand_tensor(
+                [size_dict[k] for k in term], inds=term, tags=[tag]
+            )
+            for term, tag in zip(inputs, tags)
+        ]
+        tn = qtn.TensorNetwork(ts)
+        gh = tn.geometry_hash()
+
+        # compute the svd via explicit contraction of the whole tensor
+        td = tn.contract()
+        tn_ex = td.split(["a", "l", "c", "d"], max_bond=4)
+        d1 = tn_ex.distance(tn)
+
+        # test inserting into parallel network
+        tn_other = tn.copy()
+        tn.insert_compressor_between_regions_(
+            ["A", "B"], ["C", "D"], max_bond=4, insert_into=tn_other
+        )
+        assert tn.geometry_hash() == gh
+        assert tn_other.num_tensors == 6
+        d = tn_other.distance(tn)
+        assert d == pytest.approx(d1)
+
+        # test not-inplace
+        tn_other = tn.insert_compressor_between_regions(
+            ["A", "B"],
+            ["C", "D"],
+            max_bond=4,
+        )
+        assert tn.geometry_hash() == gh
+        assert tn_other.num_tensors == 6
+        d = tn_other.distance(tn)
+        assert d == pytest.approx(d1)
+
+        # test inplace
+        tn_other = tn.copy()
+        tn_other.insert_compressor_between_regions_(
+            ["A", "B"], ["C", "D"], max_bond=4
+        )
+        assert tn_other.geometry_hash() != gh
+        assert tn_other.num_tensors == 6
+        d = tn_other.distance(tn)
+        assert d == pytest.approx(d1)
+
 
 class TestTensorNetworkSimplifications:
     def test_rank_simplify(self):