The sherpa.optmethods module

Optimization classes.

The OptMethod class provides an interface to a number of optimisers. When creating an optimizer an optional name can be added; this name is only used in string representations of the class:

>>> from sherpa.optmethods import NelderMead
>>> opt = NelderMead()
>>> print(opt)
name         = simplex
ftol         = 1.1920928955078125e-07
maxfev       = None
initsimplex  = 0
finalsimplex = 9
step         = None
iquad        = 1
verbose      = 0
reflect      = True

A model is fit by providing the fit method a callback, the starting point (parameter values), and parameter ranges. The callback should match the StatFunc type:

callback(pars)

and return the statistic value to minimise along with the per-bin statistic values (note that per-bin here refers to the independent axis of the data being fit, and not the number of parameters being fit).

Changed in version 4.18.0: The callback is no-longer sent extra information (that is, the statargs and statkwargs values) and is just sent an array of parameter values.

Notes

Each optimizer has certain classes of problem where it is more, or less, successful. For instance, the NelderMead class should only be used with chi-square based statistics.

Examples

Using Sherpa classes for data, models, and statistics we can create a callback, in this case using a least-squared statistic to fit a constant model to a 1D dataset (we do not need to send any extra arguments to the callback other than the parameter values in this case):

>>> import numpy as np
>>> from sherpa.data import Data1D
>>> from sherpa.models.basic import Const1D
>>> from sherpa.stats import LeastSq
>>> x = np.asarray([1, 2, 5])
>>> y = np.asarray([3, 2, 7])
>>> d = Data1D('data', x, y)
>>> mdl = Const1D()
>>> stat = LeastSq()
>>> def cb(pars):
...     mdl.thawedpars = pars
...     return stat.calc_stat(d, mdl)

We can check the model before the optimisaton run:

>>> print(mdl)
const1d
   Param        Type          Value          Min          Max      Units
   -----        ----          -----          ---          ---      -----
   const1d.c0   thawed            1 -3.40282e+38  3.40282e+38

The model can be fit using the fit method:

>>> from sherpa.optmethods import NelderMead
>>> opt = NelderMead()
>>> res = opt.fit(cb, mdl.thawedpars, mdl.thawedparmins, mdl.thawedparmaxes)

The return from fit is a tuple where the first element indicates whether the fit was successful, then the best-fit parameters, the best-fit statistic, a string message, along with a dictionary depending on the optimiser:

>>> print(res)
(True, array([4.]), 14.0, 'Optimization terminated successfully', {'info': True, 'nfev': 98})
>>> print(f"Best-fit value: {res[1][0]}")
Best-fit value: 4.0

We can see that the model has been updated thanks to the use of cb, which sets the thawedpars attribute of mdl:

>>> print(mdl)
const1d
   Param        Type          Value          Min          Max      Units
   -----        ----          -----          ---          ---      -----
   const1d.c0   thawed            4 -3.40282e+38  3.40282e+38

Classes

OptMethod(name, optfunc)	Base class for the optimisers.
LevMar([name])	Levenberg-Marquardt optimization method.
NelderMead([name])	Nelder-Mead Simplex optimization method.
MonCar([name])	Monte Carlo optimization method.
GridSearch([name])	Grid Search optimization method.

Class Inheritance Diagram