************************ Creating model instances ************************ The :mod:`sherpa.models` and :mod:`sherpa.astro.models` namespaces provides a collection of one- and two-dimensional models as a convenience; the actual definition of each particular model depends on its type. .. note:: I have no idea what deep truth I meant by the sentance above. .. note:: To get the link above probably means using :mod:`sherpa.models.model`, but users can get away with just importing ``sherpa.models`` as shown below. The following modules are assumed to have been imported for this section:: >>> import numpy as np >>> import matplotlib.pyplot as plt >>> from sherpa import models Creating a model instance ========================= Models must be created before there parameter values can be set. In this case a one-dimensional gaussian using the :py:class:`~sherpa.models.basic.Gauss1D` class:: >>> g = models.Gauss1D() >>> print(g) gauss1d Param Type Value Min Max Units ----- ---- ----- --- --- ----- gauss1d.fwhm thawed 10 1.17549e-38 3.40282e+38 gauss1d.pos thawed 0 -3.40282e+38 3.40282e+38 gauss1d.ampl thawed 1 -3.40282e+38 3.40282e+38 A description of the model is provided by ``help(g)``. The parameter values have a current value, a valid range (as given by the the minimum and maximum columns in the table above), and a units field. The units field is a string, describing the expected units for the parameter; there is currently *no support* for using `astropy.units `_ to set a parameter value. The "Type" column refers to whether the parameter is fixed, (``frozen``) or can be varied during a fit (``thawed``), as described below, in the :ref:`params-freeze` section. Models can be given a name, to help distinguish multiple versions of the same model type. The default value is the lower-case version of the class name. :: >>> g.name 'gauss1d' >>> h = models.Gauss1D('other') >>> print(h) other Param Type Value Min Max Units ----- ---- ----- --- --- ----- other.fwhm thawed 10 1.17549e-38 3.40282e+38 other.pos thawed 0 -3.40282e+38 3.40282e+38 other.ampl thawed 1 -3.40282e+38 3.40282e+38 >>> h.name 'other' The model classes are expected to derive from the :py:class:`~sherpa.models.model.ArithmeticModel` class. .. _model-combine: Combining models ================ Models can be combined and shared by using the standard Python numerical operators. For instance, a one-dimensional gaussian plus a flat background - using the :py:class:`~sherpa.models.basic.Const1D` class - would be represented by the following model:: >>> src1 = models.Gauss1D('src1') >>> back = models.Const1D('back') >>> mdl1 = src1 + back >>> print(mdl1) (src1 + back) Param Type Value Min Max Units ----- ---- ----- --- --- ----- src1.fwhm thawed 10 1.17549e-38 3.40282e+38 src1.pos thawed 0 -3.40282e+38 3.40282e+38 src1.ampl thawed 1 -3.40282e+38 3.40282e+38 back.c0 thawed 1 -3.40282e+38 3.40282e+38 Now consider fitting a second dataset where it is known that the background is two times higher than the first:: >>> src2 = models.Gauss1D('src2') >>> mdl2 = src2 + 2 * back >>> print(mdl2) (src2 + (2 * back)) Param Type Value Min Max Units ----- ---- ----- --- --- ----- src2.fwhm thawed 10 1.17549e-38 3.40282e+38 src2.pos thawed 0 -3.40282e+38 3.40282e+38 src2.ampl thawed 1 -3.40282e+38 3.40282e+38 back.c0 thawed 1 -3.40282e+38 3.40282e+38 The two models can then be fit separately or simultaneously. In this example the two source models (the Gaussian component) were completely separate, but they could have been identical - in which case ``mdl2 = src1 + 2 * back`` would have been used instead - or :ref:`parameter linking ` could be used to constrain the models. An example of the use of linking would be to force the two FWHM (full-width half-maximum) parameters to be the same but to let the position and amplitude values vary independently. More information is available in the :doc:`combining models <../evaluation/combine>` documentation. Changing a parameter ==================== The parameters of a model - those numeric variables that control the shape of the model, and that can be varied during a fit - can be accesed as attributes, both to read or change the current settings. The :py:attr:`~sherpa.models.parameter.Parameter.val` attribute contains the current value:: >>> print(h.fwhm) val = 10.0 min = 1.17549435082e-38 max = 3.40282346639e+38 units = frozen = False link = None default_val = 10.0 default_min = 1.17549435082e-38 default_max = 3.40282346639e+38 >>> h.fwhm.val 10.0 >>> h.fwhm.min 1.1754943508222875e-38 >>> h.fwhm.val = 15 >>> print(h.fwhm) val = 15.0 min = 1.17549435082e-38 max = 3.40282346639e+38 units = frozen = False link = None default_val = 15.0 default_min = 1.17549435082e-38 default_max = 3.40282346639e+38 Assigning a value to a parameter directly (i.e. without using the ``val`` attribute) also works:: >>> h.fwhm = 12 >>> print(h.fwhm) val = 12.0 min = 1.17549435082e-38 max = 3.40282346639e+38 units = frozen = False link = None default_val = 12.0 default_min = 1.17549435082e-38 default_max = 3.40282346639e+38 .. _params-limits: The soft and hard limits of a parameter ======================================= Each parameter has two sets of limits, which are referred to as "soft" and "hard". The soft limits are shown when the model is displayed, and refer to the :py:attr:`~sherpa.models.parameter.Parameter.min` and :py:attr:`~sherpa.models.parameter.Parameter.max` attributes for the parameter, whereas the hard limits are given by the :py:attr:`~sherpa.models.parameter.Parameter.hard_min` and :py:attr:`~sherpa.models.parameter.Parameter.hard_max` (which are not displayed, and can not be changed). >>> print(h) other Param Type Value Min Max Units ----- ---- ----- --- --- ----- other.fwhm thawed 12 1.17549e-38 3.40282e+38 other.pos thawed 0 -3.40282e+38 3.40282e+38 other.ampl thawed 1 -3.40282e+38 3.40282e+38 >>> print(h.fwhm) val = 12.0 min = 1.17549435082e-38 max = 3.40282346639e+38 units = frozen = False link = None default_val = 12.0 default_min = 1.17549435082e-38 default_max = 3.40282346639e+38 These limits act to bound the acceptable parameter range; this is often because certain values are physically impossible, such as having a negative value for the full-width-half-maxium value of a Gaussian, but can also be used to ensure that the fit is restricted to a meaningful part of the search space. The hard limits are set by the model class, and represent the full valid range of the parameter, whereas the soft limits can be changed by the user, although they often default to the same values as the hard limits. Setting a parameter to a value outside its soft limits will raise a :py:exc:`~sherpa.utils.err.ParameterErr` exception. During a fit the paramater values are bound by the soft limits, and a screen message will be displayed if an attempt to move outside this range was made. During error analysis the parameter values are allowed outside the soft limits, as long as they remain inside the hard limits. .. _params-freeze: Freezing and Thawing parameters =============================== Not all model parameters should be varied during a fit: perhaps the data quality is not sufficient to constrain all the parameters, it is already known, the parameter is highly correlated with another, or perhaps the parameter value controls a behavior of the model that should not vary during a fit (such as the interpolation scheme to use). The :py:attr:`~sherpa.models.parameter.Parameter.frozen` attribute controls whether a fit should vary that parameter or not; it can be changed directly, as shown below:: >>> h.fwhm.frozen False >>> h.fwhm.frozen = True or via the :py:meth:`~sherpa.models.parameter.Parameter.freeze` and :py:meth:`~sherpa.models.parameter.Parameter.thaw` methods for the parameter. :: >>> h.fwhm.thaw() >>> h.fwhm.frozen False There are times when a model parameter should *never* be varied during a fit. In this case the :py:attr:`~sherpa.models.parameter.Parameter.alwaysfrozen` attribute will be set to ``True`` (this particular parameter is read-only). .. _params-link: Linking parameters ================== There are times when it is useful for one parameter to be related to another: this can be equality, such as saying that the width of two model components are the same, or a functional form, such as saying that the position of one component is a certain distance away from another component. This concept is refererred to as linking parameter values. The second case incudes the first - where the functional relationship is equality - but it is treated separately here as it is a common operation. Lnking parameters also reduces the number of free parameters in a fit. The following examples use the same two model components:: >>> g1 = models.Gauss1D('g1') >>> g2 = models.Gauss1D('g2') Linking parameter values requires referring to the parameter, rather than via the :py:attr:`~sherpa.models.parameter.Parameter.val` attribute. The :py:attr:`~sherpa.models.parameter.Parameter.link` attribute is set to the link value (and is ``None`` for parameters that are not linked). Equality -------- After the following, the two gaussian components have the same width:: >>> g2.fwhm.val 10.0 >>> g2.fwhm = g1.fwhm >>> g1.fwhm = 1024 >>> g2.fwhm.val 1024.0 >>> g1.fwhm.link is None True >>> g2.fwhm.link When displaying the model, the value and link expression are included:: >>> print(g2) g2 Param Type Value Min Max Units ----- ---- ----- --- --- ----- g2.fwhm linked 1024 expr: g1.fwhm g2.pos thawed 0 -3.40282e+38 3.40282e+38 g2.ampl thawed 1 -3.40282e+38 3.40282e+38 Functional relationship ----------------------- The link can accept anything that evaluates to a value, such as adding a constant. :: >>> g2.pos = g1.pos + 8234 >>> g1.pos = 1200 >>> g2.pos.val 9434.0 The :py:class:`~sherpa.models.parameter.CompositeParameter` class controls how parameters are combined. In this case the result is a :py:class:`~sherpa.models.parameter.BinaryOpParameter` object. Including another parameter --------------------------- It is possible to include other parameters in a link expression, which can lead to further constraints on the fit. For instance, rather than using a fixed separation, a range can be used. One way to do this is to use a :py:class:`~sherpa.models.basic.Const1D` model, restricting the value its one parameter can vary. :: >>> sep = models.Const1D('sep') >>> print(sep) sep Param Type Value Min Max Units ----- ---- ----- --- --- ----- sep.c0 thawed 1 -3.40282e+38 3.40282e+38 >>> g2.fwhm = g1.fwhm + sep.c0 >>> sep.c0 = 1200 >>> sep.c0.min = 800 >>> sep.c0.max = 1600 In this example, the separation of the two components is restricted to lie in the range 800 to 1600. In order for the optimiser to recognize that it needs to vary the new parameter (``sep.c0``), the component *must* be included in the model expression. As it does not contribute to the model output directly, it should be multiplied by zero. So, for this example the model to be fit would be given by an expression like:: >>> mdl = g1 + g2 + 0 * sep .. _parameter_reset: Resetting parameter values ========================== .. todo:: Needs work The :py:meth:`~sherpa.models.parameter.Parameter.reset` method of a parameter will change the parameter settings (which includes the status of the thawed flag and allowed ranges, as well as the value) to the values they had the last time the parameter was *explicitly* set. That is, it does not restore the initial values used when the model was created, but the last values the user set. The model class has its own :py:meth:`~sherpa.models.model.Model.reset` method which calls reset on the thawed parameters. This can be used to :ref:`change the starting point of a fit ` to see how robust the optimiser is by: - explicitly setting parameter values (or using the default values) - fit the data - call reset - change one or more parameters - refit .. note:: What about the :py:attr:`~sherpa.models.parameter.Parameter.default_val` attribute? Inspecting models and parameters ================================ Models, whether a single component or composite, contain a ``pars`` attribute which is a tuple of all the parameters for that model. This can be used to programatically query or change the parameter values. There are several attributes that return arrays of values for the thawed parameters of the model expression: the most useful is :py:attr:`~sherpa.models.model.Model.thawedpars`, which gives the current values. Composite models can be queried to find the individual components using the ``parts`` attribute, which contains a tuple of the components (these components can themselves be composite objects).