[BUG] Can't initialize qml.GateFabric with wires as kwargs.

[dwierichs]

Expected behavior

Like any PennyLane operation, qml.GateFabric should be able to initialize with wires given as kwarg, even when not giving init_state as kwarg.

Actual behavior

The GateFabric errors out, unless init_state is provided as kwarg as well.

edit
AllSinglesDoubles has the same problem, with hf_state being the offending positional argument.
BasisRotation has the same problem, with unitary_matrix being the offending positional argument.

Additional information

This error is caused by GateFabric allowing wires as a positional arg but not as the last positional arg, which is a requirement for PennyLane operations.
We should either change the order of the positional args or make init_state a kwarg.

Source code

qml.GateFabric(np.ones((3, 1, 2)), [0, 1, 1, 0], wires=[2, 3, 0, 1])

weights = np.ones(3)
singles = [[0, 2], [1, 3]]
doubles = [[0, 1, 2, 3]]
hf_state = [1, 1, 0, 0]

qml.AllSinglesDoubles(weights, hf_state, wires=[0, 1, 2, 3], singles=singles, double=doubles)

Tracebacks

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[5], line 1
----> 1 qml.GateFabric(np.ones((3, 1, 2)), [0, 1, 1, 0], wires=[2, 3, 0, 1])

File ~/repos/pennylane/pennylane/capture/meta_type.py:151, in PLXPRMeta.__call__(cls, *args, **kwargs)
    145 def __call__(cls, *args, **kwargs):
    146     # this method is called everytime we want to create an instance of the class.
    147     # default behavior uses __new__ then __init__
    148     # when tracing is enabled, we want to
    150     if not plxpr_enabled():
--> 151         return type.__call__(cls, *args, **kwargs)
    152     # use bind to construct the class if we want class construction to add it to the jaxpr
    153     return cls._primitive_bind_call(*args, **kwargs)

TypeError: GateFabric.__init__() got multiple values for argument 'wires'

System information

pl dev

Existing GitHub issues

• I have searched existing GitHub issues to make sure the issue does not already exist.

[PietropaoloFrisoni]

Hi @dwierichs, just one point. If we make init_state a kwarg, this would fulfill the requirement to have wires as the last positional argument, but the example above:

qml.GateFabric(np.ones((3, 1, 2)), [0, 1, 1, 0], wires=[2, 3, 0, 1])

still would not work, so I think the only option would be to change the argument order. If we do that, then the above line of code works fine, but for:

qml.AllSinglesDoubles(weights, hf_state, wires=[0, 1, 2, 3], singles=singles, doubles=doubles)

we get a different error:

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
(...)
AttributeError: 'int' object has no attribute 'dtype'

[dwierichs]

Hi @PietropaoloFrisoni ,thanks for looking into this!

but the example above: [...] still would not work,

I agree, but I think that if we made init_state a kwarg, users would not try the above (because it is clear that Python would not map the init_state correctly to the kwarg), whereas the current syntax/documentation suggests that it is a valid input format.

we get a different error:

Yeah, the input validation is not optimal, it assumes ndarray(int) and does not work with list[int]. But that's independent and also a problem with the current code.

[albi3ro]

While this does break pennylane convention, it doesn't actually break any code. And though a different call signature would be better, it's not worth the problems and costs that accompany a breaking change of this sort.

After discussion with relevant stakeholders, we are marking this as a "wont-fix".