LMfit-py
========

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.. _LMfit mailing list: https://groups.google.com/group/lmfit-py


Overview
---------

LMfit-py provides a Least-Squares Minimization routine and class with a simple,
flexible approach to parameterizing a model for fitting to data.

LMfit is a pure Python package, and so easy to install from source or with
``pip install lmfit``.

For questions, comments, and suggestions, please use the `LMfit mailing list`_.
Using the bug tracking software in GitHub Issues is encouraged for known
problems and bug reports. Please read
`Contributing.md <.github/CONTRIBUTING.md>`_ before creating an Issue.


Parameters and Fitting
-------------------------

LMfit-py provides a Least-Squares Minimization routine and class with a simple,
flexible approach to parameterizing a model for fitting to data. Named
Parameters can be held fixed or freely adjusted in the fit, or held between
lower and upper bounds. In addition, parameters can be constrained as a simple
mathematical expression of other Parameters.

To do this, the programmer defines a Parameters object, an enhanced dictionary,
containing named parameters::

    fit_params = Parameters()
    fit_params['amp'] = Parameter(value=1.2, min=0.1, max=1000)
    fit_params['cen'] = Parameter(value=40.0, vary=False)
    fit_params['wid'] = Parameter(value=4, min=0)

or using the equivalent::

    fit_params = Parameters()
    fit_params.add('amp', value=1.2, min=0.1, max=1000)
    fit_params.add('cen', value=40.0, vary=False)
    fit_params.add('wid', value=4, min=0)

The programmer will also write a function to be minimized (in the least-squares
sense) with its first argument being this Parameters object, and additional
positional and keyword arguments as desired::

    def myfunc(params, x, data, someflag=True):
        amp = params['amp'].value
        cen = params['cen'].value
        wid = params['wid'].value
        ...
        return residual_array

For each call of this function, the values for the params may have changed,
subject to the bounds and constraint settings for each Parameter. The function
should return the residual (i.e., data-model) array to be minimized.

The advantage here is that the function to be minimized does not have to be
changed if different bounds or constraints are placed on the fitting Parameters.
The fitting model (as described in myfunc) is instead written in terms of
physical parameters of the system, and remains remains independent of what is
actually varied in the fit. In addition, which parameters are adjusted and which
are fixed happens at run-time, so that changing what is varied and what
constraints are placed on the parameters can easily be modified by the user in
real-time data analysis.

To perform the fit, the user calls::

    result = minimize(myfunc, fit_params, args=(x, data), kws={'someflag':True}, ....)

After the fit, a ``MinimizerResult`` class is returned that holds the results
the fit (e.g., fitting statistics and optimized parameters). The dictionary
``result.params`` contains the best-fit values, estimated standard deviations,
and correlations with other variables in the fit.

By default, the underlying fit algorithm is the Levenberg-Marquart algorithm
with numerically-calculated derivatives from MINPACK's lmdif function, as used
by ``scipy.optimize.leastsq``. Most other solvers that are present in ``scipy``
(e.g., Nelder-Mead, differential_evolution, basinhopping, etctera) are also
supported.