Importing tomography with cvxpy installed fails

[mtreinish]

Informations

• Qiskit Ignis version: 0.3.2
• Python version: any
• Operating system: any

What is the current behavior?

import qiskit.ignis.verification

raise an ImportErrorif cvxpy isn't installed:

ImportError: cannot import name 'cvxpy' from 'qiskit.ignis.verification.tomography.fitters.cvx_fit' (/home/mtreinish/git/qiskit/qiskit/.tox/docs/lib/python3.8/site-packages/qiskit/ignis/verification/tomography/fitters/cvx_fit.py)

It looks like an edge case was missed in #422. We don't have a ci job that doesn't install cvxpy so this is never tested and thus this slipped through

Steps to reproduce the problem

import qiskit.ignis.verification

What is the expected behavior?

You can import ignis without an optional dependency.

Suggested solutions

Fix the import in process tomography and add a test job.

[gadial]

This was easy enough to fix (#433 ) - the crash resulted from Process tomography explicitly testing cvxpy (and if it is None, acting upon it), but this assignment was removed in #422 .

I was unable, however, to add a corresponding test; I tried something along the lines of

with unittest.mock.patch.dict(sys.modules, {'cvxpy': None}):
             del sys.modules['qiskit.ignis.verification.tomography']
             import qiskit.ignis.verification.tomography

but I couldn't get it to crash (while switching to a cvxpy-less Python worked, of course). @mtreinish , can you help me with this? What is the standard way in Python to test in the mocked presence of missing dependancies?

[mtreinish]

@gadial mocking imports is not always straightforward because there is a multiple pieces of state to track and also module vs global scoping. You can see an example of what I did for a similar case in: https://github.com/Qiskit/qiskit-terra/pull/4296/files (in that case I wasn't trying to mock it away but replace a module with a mock).

That being said I already have a fix up in #431 which makes the docs job run without cvxpy which should catch this case in the future. But I also am going to push a new CI job that runs without cvxpy (and without matplotlib), it's the only way to reliably test the absence of a dependency across the board. We can mock it away in one place, but there might be unseen consequences somewhere else in the code (cvxpy's usage is pretty well isolated though).