Class Parameter

object --+
         |
        Parameter

Known Subclasses:

parameterclasses.BooleanParameter, parameterclasses.CallableParameter, parameterclasses.SelectorParameter, parameterclasses.CompositeParameter, parameterclasses.Dynamic, parameterclasses.Enumeration, parameterclasses.ListParameter, parameterclasses.NumericTuple, parameterclasses.StringParameter, simulation.EventProcessorParameter, misc.filepaths.Filename, boundingregion.BoundingRegionParameter, boundingregion.Cartesian2DPoint

An attribute descriptor for declaring parameters.

Parameters are a special kind of class attribute. Setting a ParameterizedObject class attribute to be a Parameter instance causes that attribute of the class (and the class's instances) to be treated as a Parameter. This allows special behavior, including dynamically generated parameter values, documentation strings, constant and read-only parameters, and type or range checking at assignment time.

** Note that Parameters can only be used when set as class ** ** attributes of ParameterizedObjects. **

For example, suppose someone wants to define two new kinds of objects Foo and Bar, such that Bar has a parameter delta, Foo is a subclass of Bar, and Foo has parameters alpha, sigma, and gamma (and delta inherited from Bar). She would begin her class definitions with something like this:

class Bar(ParameterizedObject):

delta = Parameter(default=0.6, doc='The difference between steps.') ...

class Foo(Bar):

alpha = Parameter(default=0.1, doc='The starting value.') sigma = Parameter(default=0.5, doc='The standard deviation.', constant=True) gamma = Parameter(default=1.0, doc='The ending value.') ...

Class Foo would then have four parameters, with delta defaulting to 0.6.

Parameters have several advantages over plain attributes:

1. Parameters can be set automatically when an instance is constructed: The default constructor for Foo (and Bar) will accept arbitrary keyword arguments, each of which can be used to specify the value of a Parameter of Foo (or any of Foo's superclasses). E.g., if a script does this:

   myfoo = Foo(alpha=0.5)

   myfoo.alpha will return 0.5, without the Foo constructor needing special code to set alpha.

   If Foo implements its own constructor, keyword arguments will still be accepted if the constructor accepts a dictionary of keyword arguments (as in def __init__(self,**params):), and then each class calls its superclass (as in super(Foo,self).__init__(**params)) so that the ParameterizedObject constructor will process the keywords.

2. A ParameterizedObject need specify only the attributes of a Parameter whose values differ from those declared in superclasses of the ParameterizedObject; the other values will be inherited. E.g. if Foo declares

   delta = Parameter(default=0.2)

   the default value of 0.2 will override the 0.6 inherited from Bar, but the doc will be inherited from Bar.

3. The Parameter descriptor class can be subclassed to provide more complex behavior, allowing special types of parameters that, for example, require their values to be numbers in certain ranges, generate their values dynamically from a random distribution, or read their values from a file or other external source.

4. The attributes associated with Parameters provide enough information for automatically generating property sheets in graphical user interfaces, to allow ParameterizedObjects to be edited by users.