options 1.0.1

Usage

from options import Options, attrs

class Shape(object):

    options = Options(
        name   = None,
        color  = 'white',
        height = 10,
        width  = 10,
    )

    def __init__(self, **kwargs):
        self.options = Shape.options.push(kwargs)

    def draw(self, **kwargs):
        opts = self.options.push(kwargs)
        print attrs(opts)

one = Shape(name='one')
one.draw()
one.draw(color='red')
one.draw(color='green', width=22)

yielding:

color='white', width=10, name='one', height=10
color='red', width=10, name='one', height=10
color='green', width=22, name='one', height=10

So far we could do this with instance variables and standard arguments. It might look a bit like this:

class ClassicShape(object):

    def __init__(self, name=None, color='white', height=10, width=10):
        self.name   = name
        self.color  = color
        self.height = height
        self.width  = width

but when we got to the draw method, things would be quite a bit messier.:

def draw(self, **kwargs):
    name   = kwargs.get('name',   self.name)
    color  = kwargs.get('color',  self.color)
    height = kwargs.get('height', self.height)
    width  = kwargs.get('width',  self.width)
    print "color='{}', width={}, name='{}', height={}".format(color, width, name, height)

One problem here is that we broke apart the values provided to __init__() into separate instance variables, now we need to re-assemble them into something unified. And we need to explicitly choose between the **kwargs and the instance variables. It gets repetitive, and is not pretty. Another classic alternative, using native keyword arguments, is no better:

def draw2(self, name=None, color=None, height=None, width=None):
    name   = name   or self.name
    color  = color  or self.color
    height = height or self.height
    width  = width  or self.width
    print "color='{}', width={}, name='{}', height={}".format(color, width, name, height)

If we add just a few more instance variables, we have the Mr. Creosote of class design on our hands. Not good. Things get worse if we want to set default values for all shapes in the class. We have to rework every method that uses values, the __init__ method, et cetera. We’ve entered “just one more wafer-thin mint…” territory.

But with options, it’s easy:

Shape.options.set(color='blue')
one.draw()
one.draw(height=100)
one.draw(height=44, color='yellow')

yields:

color='blue', width=10, name='one', height=10
color='blue', width=10, name='one', height=100
color='yellow', width=10, name='one', height=44

In one line, we reset the default for all Shape objects.

The more options and settings a class has, the more unwieldy the class and instance variable approach becomes, and the more desirable the delegation alternative. Inheritance is a great software pattern for many kinds of data and program structures, but it’s a bad pattern for complex option and configuration handling. For richly featured classes, the delegation pattern options proves simpler. Supporting even a large number of options requires almost no additional code and imposes no additional complexity or failure modes. By consolidating options into one place, and by allowing neat, attribute-style access, everything is kept tidy. We can add new options or methods with confidence:

def is_tall(self, **kwargs):
    opts = self.options.push(kwargs)
    return opts.height > 100

Under the covers, options uses a variation on the ChainMap data structure (a multi-layer dictionary) to provide its option stacking. Every option set is stacked on top of previously set option sets, with lower-level values shining through if they’re not set at higher levels. This stacking or overlay model resembles how local and global variables are managed in many programming languages.

For more, please see the full installation and usage documentation on Read the Docs.