Deal with undefined arguments more elegantly

Question

The accepted paradigm to deal with mutable default arguments is:

def func(self, a = None):
    if a is None:
        a = <some_initialisation>
    self.a = a

As I might have to do this for several arguments, I would need to write very similar 3 lines over and over again. I find this un-pythonically a lot of text to read for a very very standard thing to do when initialising class objects or functions.

Isn't there an elegant one-liner to replace those 3 lines dealing with the potentially undefined argument and the standard required copying to the class instance variables?

Answer 1

If a "falsy" value (0, empty string, list, dict, etc.) is not a valid value for a, then you can cut down the initialization to one line:

a = a or <initialize_object>

Answer 2

Another way of doing the same thing is as follows:

def func(self,**kwargs):
    self.a=kwargs.get('a',<a_initialization>)
    ...

This has the added bonus that the value of a passed to the function could be None and the initialization won't overwrite it. The disadvantage is that a user using the builtin help function won't be able to tell what keywords your function is looking for unless you spell it out explicitly in the docstring.

EDIT

One other comment. The user could call the above function with keywords which are not pulled out of the kwargs dictionary. In some cases, this is good (if you want to pass the keywords to another function for instance). In other cases, this is not what you want. If you want to raise an error if the user provides an unknown keyword, you can do the following:

def func(self,**kwargs):
    self.a=kwargs.pop('a',"Default_a")
    self.b=kwargs.pop('b',"Default_b")
    if(kwargs):  
        raise  ... #some appropriate exception...possibly using kwargs.keys() to say which keywords were not appropriate for this function.