LeastSquares.visualize fails for model functions which accept arrays of parameters

[jlabounty]

Hello,

After the quick bug fix regarding pickling, I was excited to upgrade to the latest version and try the new interactive fitting. However, I noticed that the visualize() method for the LeastSquares cost function only works when each parameter is an individual argument rather than a numpy array

def func1(x,p): #fails
    return x*p[1] + p[0]

def func2(x,p0,p1): #works great
    return x*p1 + p0

The traceback I get when executing this python code

cost1 = LeastSquares(xs,ys,np.ones_like(xs),func1)
m1 = iminuit.Minuit(cost1, [2,2])

m1.migrad()
cost1.visualize([2,2])

is:

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
/home/jlab/github/g2_analysis/analysis/interactive.ipynb Cell 7 in <cell line: 1>()
----> [1](vscode-notebook-cell://wsl%2Bubuntu-20.04/home/jlab/github/g2_analysis/analysis/interactive.ipynb#ch0000010vscode-remote?line=0) cost1.visualize([2,2])

File ~/miniconda3/envs/gm2/lib/python3.10/site-packages/iminuit/cost.py:1447, in LeastSquares.visualize(self, args, model_points)
   1445     ym = self.model(xm, *args)
   1446 else:
-> 1447     xm, ym = _smart_sampling(lambda x: self.model(x, *args), x[0], x[-1])
   1448 plt.plot(xm, ym)

File ~/miniconda3/envs/gm2/lib/python3.10/site-packages/iminuit/util.py:1453, in _smart_sampling(f, xmin, xmax, start, tol)
   1451 def _smart_sampling(f, xmin, xmax, start=5, tol=5e-3):
   1452     x = np.linspace(xmin, xmax, start)
-> 1453     ynew = f(x)
   1454     ymin = np.min(ynew)
   1455     ymax = np.max(ynew)

File ~/miniconda3/envs/gm2/lib/python3.10/site-packages/iminuit/cost.py:1447, in LeastSquares.visualize.<locals>.<lambda>(x)
   1445     ym = self.model(xm, *args)
   1446 else:
-> 1447     xm, ym = _smart_sampling(lambda x: self.model(x, *args), x[0], x[-1])
   1448 plt.plot(xm, ym)

TypeError: func1() takes 2 positional arguments but 3 were given

Because of this, the visualization in Minuit.interactive() also fails. I've uploaded a notebook here showing the issue in more detail.

Thanks for all the work that goes into maintaining this project!

[jlabounty]

I should add that I am running iminuit version 2.15.1

[HDembinski]

Thanks for the report. Yes, visualize currently does not support cost functions that accept an array, but this can and should be implemented. That limitation should have been documented.