saving constructor arguments automatically

Question

Frequently the constructor of a class will take it's arguments and save them on the instance. For example:

class Example(object):
    def __init__(self, title='',backtitle='', height=20, width=50):
        self.title = title
        self.backtitle = backtitle
        self.height = height
        self.width = width

This is repetitious so I made a helper function to do this automatically:

from inspect import getargspec
def save_args(values):
    for i in getargspec(values['self'].__init__).args[1:]:
        values['self'].__dict__[i] = values[i]

class Example(object):
    def __init__(self, title='',backtitle='', height=20, width=50):
        save_args(vars())

My questions are as follows:

• Will this fail with certain classes or agruments
• Is it portable, will it work on Jython, etc.. It worked for me on python 2.7 and 3.2
• Is there a simpler alternative?
• Is there a python package out there that already does this?

Answer 1

It'll fail when your class uses __slots__. You could use setattr() instead:

from inspect import getargspec
def save_args(values):
    for i in getargspec(values['self'].__init__).args[1:]:
        setattr(values['self'], i, values[i])

provided the arguments keyword arguments to __init__ are all declared slots of course.

Otherwise this should work on any Python implementation.

You may be interested in a previous discussion of the topic, which sparked a Python-ideas list thread.

Answer 2

This requires you to write more code, silently ignores all erroneous arguments to the Example constructor, and doesn't support positional arguments to the Example constructor, but avoids use of inspect:

def save_args(obj, defaults, kwargs):
    for k,v in defaults.iteritems():
        if k in kwargs: v = kwargs[k]
        setattr(obj, k, v)

class Example(object):
    def __init__(self, **kwargs):
        defaults = { 'title': '',
                     'backtitle': '',
                     'height': 20,
                     'width': 50 }
        save_args(self, defaults, kwargs)