Support np.sign for parameter

[nkanazawa1989]

What should we add?

I sometime want to check sign of parameter value, but this is not currently supported.

from qiskit.circuit import Parameter
import numpy as np

v = Parameter("value")
np.sign(v)

(temporary hack)

from qiskit.circuit import ParameterExpression
import symengine as sym

sign_of_v = ParameterExpression(
    symbol_map={v: v._symbol_expr},
    expr=sym.sign(v._symbol_expr),
)

It would be great if Qiskit can support this without above ugly hack.

[SamD-1998]

Hey, I can work on this. Can you assign it to me?

[SamD-1998]

I can add a method inside the function definition of Parameter. Perhaps that's simpler and prevents you from importing numpy. Would you want that?

[wshanks]

I wouldn't use numpy. I would do something similar to what is done for other functions like this one:

https://github.com/Qiskit/qiskit-terra/blob/2b225d3713a8ef160a38e8fbfbc7d08a0b8a4682/qiskit/circuit/parameterexpression.py#L377-L386

The main thing is to add a sign method so that the user does not need to access the private _symbol_expr attribute.

[SamD-1998]

Hi, I think I have a working code that implements this. The contribution guidelines say that I have to open a Draft pull request and prepend the title with the WIP tag. How do I do this?

[wshanks]

Opening a draft pull request instead of a normal pull request just requires selecting an option from a drop down on open pull request page. See the screenshot on this page.

Prepending the title just means putting the text "WIP" at the beginning of the title text box when opening the pull request.

Opening a pull request is a little more involved and there are several ways to do it. Maybe this page from GitHub is a good starting point.

[SamD-1998]

Hi, I tried pushing my branch to a new remote branch in this qiskit-terra repo but I got a permission denied error. What should I do?

[wshanks]

Yes, only a limited set of people can modify the Qiskit/qiskit-terra repo. You need to create a fork of the repo in your account, add your branch to your repo, and then create the pull request in the Qiskit/qiskit-terra repo from your fork (probably called SamD-1998/qiskit-terra). There is some description of forks here and about creating a pull request across forks here.

[SamD-1998]

I've initiated a PR in the base repository: Qiskit/qiskit-terra and base: main. I might have to make a note somewhere about how to use this feature. Do I need to include a docstring somewhere to mention it to a user?

[wshanks]

Yes, adding a docstring is good. We can move the discussion to #10571.