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?

Community · Accepted Answer · 2017-05-23 12:11:06Z

6

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.

edited May 23, 2017 at 12:11

CommunityBot

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answered Mar 18, 2013 at 18:27

Martijn Pieters

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7 Comments

mgilson Over a year ago

FGITW -- That's what I get for trying to address each of OP's questions individually when really only the first one was important :)

abarnert Over a year ago

There was a discussion about this on python-ideas somewhere in the past 6 months, where (after everyone decided nothing had to be added to the language) people proposed and argued about 300 different implementations. It's worth searching the archives if you really care about this. (IIRC, if you don't care about portability, there's a solution using frame hackery that's handles some cases that getargspec can't… but I don't remember what cases.)

mgilson Over a year ago

This might also be neatly handled in a meta-class which wraps the existing __init__, although I haven't really fully formed how that would work in my head (mostly because I'm pretty inexperienced with metaclasses)

Martijn Pieters Over a year ago

@mgilson: a decorator on a __init__ method with just a pass body would do too.

mgilson Over a year ago

Yeah, that's true too. I do have some experience with decorators :)

|

zwol · Accepted Answer · 2013-03-18 18:35:31Z

1

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)

answered Mar 18, 2013 at 18:35

zwol

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2 Comments

mgilson Over a year ago

What's wrong with inspect? As long as the ugly is sufficiently hidden, I don't see any problem with it :)

zwol Over a year ago

Nothing's wrong with inspect, I just wanted to demonstrate that it could be done without.

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