Saving BinaryQuadraticModel with save_qubo() not preserve variables defined as Python tuples

[vsergeyev]

This issue occurs when using save_qubo function to save BinaryQuadraticModel.

It saves model good, except for a Python tuples converted to JSON arrays. Need to note, this is normal behavior for JSON format.

One gotcha maybe when it will be required to use tuple format for BQM variables.

Below is a slightly modified example from dimod's BinaryQuadraticModel documentation with demo of issue:

>>> bqm = dimod.BinaryQuadraticModel({(0, 0): 1, (1, 0): -1, (2, 0): .5}, {((0, 0), (1, 0)): .5, ((1, 0), (2, 0)): 1.5}, 1.4, dimod.Vartype.SPIN)
>>> bqm
BinaryQuadraticModel({(0, 0): 1.0, (1, 0): -1.0, (2, 0): 0.5}, {((0, 0), (1, 0)): 0.5, ((1, 0), (2, 0)): 1.5}, 1.4, 'SPIN')
>>> save_qubo(bqm, '42.json')
>>> load_qubo('42.json')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/vova/z-quantum-qubo/src/python/zquantum/qubo/io.py", line 68, in load_qubo
    return bqm_from_serializable(qubo_dict)
  File "/home/vova/z-quantum-qubo/src/python/zquantum/qubo/io.py", line 53, in bqm_from_serializable
    {i: coef for i, coef in serializable["linear"]},
  File "/home/vova/z-quantum-qubo/src/python/zquantum/qubo/io.py", line 53, in <dictcomp>
    {i: coef for i, coef in serializable["linear"]},
TypeError: unhashable type: 'list'
>>> with open('42.json') as f:
...     print(f.read())
... 
{"linear": [[[0, 0], 1.0], [[1, 0], -1.0], [[2, 0], 0.5]], "quadratic": [[[0, 0], [1, 0], 0.5], [[1, 0], [2, 0], 1.5]], "offset": 1.4, "vartype": "SPIN", "schema": "zapata-v1-qubo"}