`AttributeError` in `pxo.kron`: `'SquaredOp' object has no attribute 'shape'

[NinoHerve]

Hello Pyxu Team,

I’ve been enjoying using the library for some M/EEG inverse modeling, but I think I’ve stumbled upon a small bug in the Kronecker product implementation.

Description: It appears that pxo.kron still relies on the .shape attribute internally, which causes a failure when used with operators like IdentityOp which have .codim_shape and .dim_shape instead.

Minimal Reproducible Example:

import pyxu.abc as pxa
import pyxu.operator as pxo

ones = pxa.LinOp.from_array(np.ones((2,2)))
eye = pxo.IdentityOp(dim_shape=2)
kron = pxo.kron(eye, ones)

Environment:

• Pyxu version: 2.0.3
• Python version: 3.11

Traceback:

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
Cell In[75], [line 6](vscode-notebook-cell:?execution_count=75&line=6)
      4 ones = pxa.LinOp.from_array(np.ones((2,2)))
      5 eye = pxo.IdentityOp(dim_shape=2)
----> [6](vscode-notebook-cell:?execution_count=75&line=6) kron = pxo.kron(eye, ones)

File ~/miniconda3/envs/mindmap/lib/python3.11/site-packages/pyxu/operator/linop/kron.py:188, in kron(A, B)
    186 _A = A.squeeze()
    187 _B = B.squeeze()
--> [188](https://file+.vscode-resource.vscode-cdn.net/home/nino/Code/mindmap/tutorials/analysis/inverse/~/miniconda3/envs/mindmap/lib/python3.11/site-packages/pyxu/operator/linop/kron.py:188) assert (klass := _infer_op_klass(_A, _B)).has(pxa.Property.LINEAR)
    189 is_scalar = lambda _: _.shape == (1, 1)
    190 if is_scalar(_A) and is_scalar(_B):

File ~/miniconda3/envs/mindmap/lib/python3.11/site-packages/pyxu/operator/linop/kron.py:78, in kron.<locals>._infer_op_klass(A, B)
     68 def _infer_op_klass(A: pxt.OpT, B: pxt.OpT) -> pxt.OpC:
     69     # linear \kron linear -> linear
     70     # square (if output square)
   (...)     75     # idemp \kron idemp -> idemp
     76     # func \kron func -> func
     77     properties = set(A.properties() & B.properties())
---> [78](https://file+.vscode-resource.vscode-cdn.net/home/nino/Code/mindmap/tutorials/analysis/inverse/~/miniconda3/envs/mindmap/lib/python3.11/site-packages/pyxu/operator/linop/kron.py:78)     sh = _infer_op_shape(A.shape, B.shape)
     79     if sh[0] == sh[1]:
     80         properties.add(pxa.Property.LINEAR_SQUARE)

AttributeError: 'SquareOp' object has no attribute 'shape'