cvxpy needed by noise transformations but not installed

[nonhermitian]

Informations

• Qiskit Aer version: 0.3.4
• Python version: 3.7
• Operating system: Win

What is the current behavior?

cvxpy is needed for noise transformations but is not installed when installing Aer with the Qiskit meta-package. It is in the requirements-dev, but not in the setup.

Steps to reproduce the problem

What is the expected behavior?

Suggested solutions

[chriseclectic]

Is it not installed by qiskit-ignis either?

[nonhermitian]

Nope. See qiskit-community/qiskit-ignis#312, qiskit-community/qiskit-ignis#313

[atilag]

Is it probably a transitive dependency? I've never needed to install it.
Anyway, yes, we have to add it to the setup.py.

[chriseclectic]

It was always intended as an optional dependenc since it is only used by one not frequently used function. That is is why the import is handled only when that function is called. I'm not sure what the best way to handle these are. Should we add a warning for that function, something like:

def function_that_uses_cvxpy():
    try:
        import cvxpy
    except <whatever exception for module not found is>:
        logger.warn("<this function> requires CVXPy. Please install with `pip install cvxpy`")
        throw NoiseError("CVXPy package not installed.")

[atilag]

Fixed in #520