Magic ParametersΒΆ

Python’s *args variable-number of arguments and **kwargs keyword arguments are sometimes called “magic” arguments. options takes this up a notch, allowing setters much like Python’s property function or @property decorator. This allows arguments to be interpreted on the fly. This is useful, for instance, to provide relative rather than just absolute values. As an example, say that we added this code after Shape.options was defined:

options.magic(
    height = lambda v, cur: cur.height + int(v) if isinstance(v, str) else v,
    width  = lambda v, cur: cur.width  + int(v) if isinstance(v, str) else v
)

Now, in addition to absolute height and width parameters which are provided by specifying int (integer/numeric) values, your module auto-magically supports relative parameters for height and width.:

one.draw(width='+200')

yields:

color='blue', width=210, name='one', height=10

This can be as fancy as you like, defining an entire domain-specific expression language. But even small functions can give you a great bump in expressive power. For example, add this and you get full relative arithmetic capability (+, -, *, and /):

def relmath(value, currently):
    if isinstance(value, str):
        if value.startswith('*'):
            return currently * int(value[1:])
        elif value.startswith('/'):
            return currently / int(value[1:])
        else:
            return currently + int(value)
    else:
        return value

...

options.magic(
    height = lambda v, cur: relmath(v, cur.height),
    width  = lambda v, cur: relmath(v, cur.width)
)

Then:

one.draw(width='*4', height='/2')

yields:

color='blue', width=40, name='one', height=5

Magically interpreted parameters are the sort of thing that one doesn’t need very often or for every parameter–but when they’re useful, they’re enormously useful and highly leveraged, leading to much simpler, much higher function APIs.

We call them ‘magical’ here because of the “auto-magical” interpretation, but they are really just analogs of Python object properties. The magic function is basically a “setter” for a dictionary element.