Python Cookbook

Managing Options

Credit: Sébastien Keim

Problem

You have classes that need vast numbers of options to be passed to their constructors for configuration purposes. This often happens with GUI toolkits in particular.

Solution

We can model the options with a suitable class:

class Options:
    def _ _init_ _(self, **kw):
        self._ _dict_ _.update(kw)

    def _ _lshift_ _(self, other):
        """ overloading operator << """
        s = self._ _copy_ _(  )
        s._ _dict_ _.update(other._ _dict_ _)
        return s

    def _ _copy_ _(self):
        return self._ _class_ _(**self._ _dict_ _)

and then have all classes using options inherit from the following class:

class OptionsUser:
    """ Base class for classes that need to use options """

    class OptionError(AttributeError): pass

    def initOptions(self, option, kw):
        """ To be called from the derived class constructor.
        Puts the options into object scope. """
        for k, v in option._ _dict_ _.items() + kw.items(  ):
            if not hasattr(self._ _class_ _, k):
                raise self.OptionError, "invalid option " + k
            setattr(self, k, v)

    def reconfigure(self, option=Options(  ), **kw):
        """ used to change options during object life """
        self.initOptions(option, kw)
        self.onReconfigure(self)

    def onReconfigure(self):
        """ To be overloaded by derived classes. Called by the reconfigure
        method or from outside after direct changes to option attributes. """
        pass

Discussion

To explain why you need this recipe, let’s start with an example:

class TextBlock: def _ _init_ _ (self, font='Times', size=14, color=(0,0,0), height=0, width=0, align='LEFT', ...