update identity contraction handling

cotengra/interface.py

@@ -539,9 +539,18 @@ def _build_expression(
  # 'contraction' is a no-op
 
  def fn(*arrays, backend=None):
+ # this is surprisingly complicated 'to do nothing', since we
+ # have to respect potential backend conversion
+ x = arrays[0]
  if backend is None:
- return arrays[0]
- return ar.do("array", arrays[0], like=backend)
+ # haven't requested a specific backend, just return
+ return x
+ elif backend == ar.infer_backend(x):
+ # requested backend is the same as the input, just return
+ return x
+ else:
+ # requested backend is different, convert
+ return ar.do("array", arrays[0], like=backend)
 
  elif len(term) == len(output):
  # 'contraction' is just a transposition

setup.py

@@ -30,7 +30,7 @@
  ],
  extras_require={
  "recommended": [
- "cotengrust",
+ "cotengrust>=0.1.3",
  "cytoolz",
  "kahypar",
  "networkx",
@@ -76,6 +76,7 @@
  "Programming Language :: Python :: 3.9",
  "Programming Language :: Python :: 3.10",
  "Programming Language :: Python :: 3.11",
+ "Programming Language :: Python :: 3.12",
  ],
  keywords="tensor network contraction graph hypergraph partition einsum",
 )