Question

Suppose I have a class with a constructor (or other function) that takes a variable number of arguments and then sets them as class attributes conditionally.

I could set them manually, but it seems that variable parameters are common enough in python that there should be a common idiom for doing this. But I’m not sure how to do this dynamically.

I have an example using eval, but that’s hardly safe. I want to know the proper way to do this — maybe with lambda?

class Foo:
    def setAllManually(self, a=None, b=None, c=None):
        if a!=None: 
            self.a = a
        if b!=None:
            self.b = b
        if c!=None:
            self.c = c
    def setAllWithEval(self, **kwargs):
        for key in **kwargs:
            if kwargs[param] != None
                eval("self." + key + "=" + kwargs[param])

Answer 1

You could update the __dict__ attribute (which represents the instance attributes in the form of a dictionary) with the keyword arguments:

class Bar(object):
    def __init__(self, **kwargs):
        self.__dict__.update(kwargs)

then you can:

>>> bar = Bar(a=1, b=2)
>>> bar.a
1

and with something like:

allowed_keys = {'a', 'b', 'c'}
self.__dict__.update((k, v) for k, v in kwargs.items() if k in allowed_keys)

you could filter the keys beforehand (use iteritems instead of items if you’re still using Python 2.x).