[FIXED] When to use *args in Python?

Issue

When writing Python 3.x functions, I've often stumbled upon the problem of writing code that accepts both a single value and a number of values. Take this trivial function:

def capitalize(word):
    return word.capitalize()

This function works well with a single string as input, but will fail if provided with multiple strings, or a list of strings.

We could turn the function to accept a list of strings, that way it would work if provided both with one value, or multiple. Even if I passed a single value, however, it would need to be wrapped in a list, and the return value would be a list as well, even when containing one element.

def capitalize(words):
    return [word.capitalize() for word in words]

I recently read up on *args, and thought i could get around the awkwardness of having to pass a single string wrapped in a list as argument to the above function by rewriting it to be like the following:

def capitalize(*words):
    return [word.capitalize() for word in words]

It seems to me that this is a legittimate use of *args. However, I found a pyCon 2015 talk in which the speaker claims "variable positional arguments are good for removing noise and make your code readable, not to turn a single call api into an N api"

https://www.youtube.com/watch?v=WjJUPxKB164 at the 5:30 mark.

Is this a reasonable use of *args? If this is not how *args is supposed to be used, is there an established way to handle single vs multi-parameter functions?

Solution

Yes it's fine to use *args here.

You should use *args when it's easier to read than a list of argument (def foo(a,b,c,d,e...)) and does not make harder to know which argument is used for what.

Your example is the perfect example of a good usage of *args, all elements within args will be processed in the exacte same way.

You should avoid it when you need to perform different compute for each argument, like:

def foo(*args):
     do_one_stuff(args[1], args[2])
     if args[3]:
         do_another_stuff(args[3], args[4])
     if args[6] != args[7]:
         return args[0]
     else:
         return None

This example should make you feel what a function could be if you make a bad usage of *args.

Answered By - Arount