plot_bloch_vector (and others) no longer directly importable from qiskit.tools.visualization

[dtmcclure]

Informations

• Qiskit Terra version: 0.5.7
• Python version: 3.6.5
• Operating system: Windows 7

What is the current behavior?

The command from qiskit.tools.visualization import plot_bloch_vector no longer works now that this function is defined in the file _state_visualization.py in the visualization folder, rather than in the file visualization.py. Perhaps this is intentional, but in that case then the file qiskit/python/quantum_phase_bloch.py in the repository ibmqx-user-guides should be updated accordingly.

Steps to reproduce the problem

1. Start python
2. Run from qiskit.tools.visualization import plot_bloch_vector
3. Observe error message

What is the expected behavior?

1. I expect it to import the function and not give an error message.

Suggested solutions

1. Add ", plot_bloch_vector" (and possibly other functions) to the end of line 12 in qiskit.tools.visualization.__init__.py.
2. Alternatively, change line 5 of qiskit/python/quantum_phase_bloch.py in the ibmqx-user-guides repository to read "from qiskit.tools.visualization._state_visualization import plot_bloch_vector"

[diego-plan9]

Nice catch @dtmcclure ! I'm actually unsure about the best solution: perhaps the best way is to update the ibmqx-user-guides file, but instead of modifying the import for pointing to the specific function (as technically modules with an underscore are "private"), make the file use plot_state(rho, method='bloch') instead?

The reason would be that plot_state acts as a "dispatcher" function of sorts - any other state visualization can be invoked through it ... and it seems nor the tutorials or the user guides are using any other function defined in _state_visualization. I'm unsure if this was by convention, though: probably a question for the tutorials/user guides?

[jaygambetta]

We want the plot_bloch_vector as a function.

[ajavadia]

This was done to give users one unified interface for plotting density matrices:

plot_state(rho, method)

where method can be "bloch", "city", "qsphere" or "paulivec". The idea is that these all plot the same thing, in different ways.

So I would also vote for keeping that the "public" interface, and the underlying methods as private.

[jaygambetta]

yeah, I agree but the point is you can plot a Bloch sphere with much less information than a state as it is not informationally complete. I am fine with the others not being accessed unless using state but this one I think should be special. I can't believe i'm defending the Bloch sphere when I think this is the worst thing physicists ever got used to helping explain quantum computing.

The plot_state method should only use rho but the plot_vector uses the smaller vector format. The reason is also when using it for education i dont want to make a rho when we dont need to.

[ajavadia]

Thanks for the clarification. I made a PR to make the plot_bloch_vector method public, but the rest remain accessible through plot_state.

[jaygambetta]

@ajavadia we can close this correct?