Capturing one of many args in a python init

Question

If I have a class like this:

class foo(object):
    def __init__(self, a, b=None, c=None, d=None):
    print a, b, c, d

and a derived class like this:

class bar(foo):
    def __init__(self, *args, **kwargs):
        if "c" in kwargs:
            kwargs['c'] = 'else'  # CHANGE C IFF IT IS PRESENT
        super(bar, self).__init__(*args, **kwargs)

when someone calls this constructor, they could do it like this:

bar('a','b','c','d')

or they could call it like this:

bar('a', c='something')

In the second case, my constructor works as planned, but in the case of the first call c sneaks thru in the args array. This looks like I would have to watch the length of the args array as well as kwargs, and that seems brittle to the point of unusable. Is there anything you can do to make this situation better, other than just enumerate the arguments from foo in bar? (A somewhat brittle practice itself, but easier to recognize).

falsetru · Accepted Answer · 2014-01-08 02:59:07Z

2

How about populating kwargs with args?

class bar(foo):
    def __init__(self, *args, **kwargs):
        for name, value in zip(['a', 'b', 'c', 'd'], args): # <---
            kwargs[name] = value                            # <---
        args = ()                                           # <---
        if "c" in kwargs:
            kwargs['c'] = 'else'
        super(bar, self).__init__(*args, **kwargs)

UPDATE

Alternative that use inspect.getcallargs:

import inspect

class bar(foo):
    def __init__(self, *args, **kwargs):
        kwargs = inspect.getcallargs(foo.__init__, self, *args, **kwargs)
        kwargs.pop('self')
        if 'c' in kwargs:
            kwargs['c'] = 'else'
        super(bar, self).__init__(**kwargs)

edited Jan 8, 2014 at 2:59

answered Jan 7, 2014 at 15:14

falsetru

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4 Comments

boatcoder Over a year ago

Part of the problem with this is I'm still required to know the names and order of the parameters, and if that base class isn't under your control, it could change. I could as easily enumerate the named parameters in __init__ to match those of the base class and then I don't need zip, but if that order changes, (that's the brittleness I'm speaking of).

falsetru Over a year ago

@Mark0978, You can use inspect.getargspec(foo.__init__).args[1:] in place of ['a', 'b', 'c', 'd'].

falsetru Over a year ago

@Mark0978, As JAB answered, you can also use inspect.getcallargs. See ideone.com/EjD1r8

boatcoder Over a year ago

Yea, that's why I put the comment on this answer instead of JAB's.

JAB · Accepted Answer · 2014-01-07 17:55:18Z

This is more brittle than you think. Have you considered the situation where someone passes a keyword argument not present in foo.__init__'s argument list (e.g. bar('a', f='something'))? My recommendation would be requiring bar to take keyword arguments only and then filter the keys not present in foo.__init__'s argument list (which you can determine via introspection using inspect.getargspec [or related functions for newer versions of Python, particularly the Signature and Parameter objects starting in 3.3] if the arguments may change). That in itself has its own form of brittleness, of course, as a programmer using bar would need to know the relevant argument names for foo's constructor, but depending on what bar is being used for they may need to know what arguments foo takes anyway, and when someone knows that they usually know what the names of the arguments are as well.

Now that I'm looking at inspect again, I realize that there's another method you could use: inspect.getcallargs, which might be more useful for you. With this, you could do, say, inspect.getcallargs(super(bar, self).__init__, *[self, 1], **{'c':3, 'd':4}) and obtain the following dict: {'a': 1, 'self': <bar object>, 'b': None, 'c': 3, 'd': 4}. Then you can modify that dictionary and supply it as super(bar, self).__init__(**fixed_dict) or something of that sort. You'd still have the issue with keyword arguments not present in foo.__init__'s argument list, though (getcallargs raises the same errors foo.__init__ will when passed invalid arguments).

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