kwargs, args, **kwargs, and *args

They don’t exist

What? How can they not exist? Well, the Python only reserves a limited number of keywords. 33 to be exact. args and kwargs aren’t on the list. kwargs and args are variable names you choose.

If they don’t exist, why do I see them all over the place?

They have become conventional variable names for packed arguments and packed keyword arguments. You could use any other name you’d like that is allowable in Python. These are just conventions.

What is packing?

Packing is collecting a group of values into a single variable name. * collects values into a tuple while ** collects named values into a dictionary. The distinction is somewhat irrelevant for the purpose here, outside of the apparent bundling of name and value in the dictionary.

* and ** are actually used outside of args and kwargs. For instance, some functions return multiple values. They can be packed by calling them with a single variable, e.g. a = f(x) in place of a, b, c = f(x). Alternatively, if you only need the first value, you can instead call the function with a, *_ = f(x), which packs the rest of the returned values into _, which represents nowhere.

How does *args work in a function?

Consider a function into which we want to send a variable number of arguments.

def f(*args):
    print(args)

f('This', 'is', 'how', 'the', 'print', 'command', 'works.')

('This', 'is', 'how', 'the', 'print', 'command', 'works.')

Of course, this isn’t quite right. What this shows is that all of the arguments have been assembled into a tuple. They now are part of a list, and print as such. This isn’t likely what we meant to do. What we want to do now is unpack the tuple so that the print command perceives the tuple as a group of individual arguments.

def g(*args):    # packed into the tuple
    print(*args) # unpacked into individual arguments

g('This', 'is', 'how', 'the', 'print', 'command', 'works.')

This is how the print command works.

Now, the Python print command is much more sophisticated, but this starts to show the power of packing arguments this way.

What is an example where this could be helpful?

Say I want to write a function that returns the product of a number to the 3rd power, such as

However, I already have a more general function that returns

def power(a, b, c):
    return a*b**c

power(5, 2, 3)

40

What I can do is create a function that calls power, but packs and unpacks the arguments appropriately. We know that the last argument must be the number 3. We could simply explicitly write the other arguments:

def power3(a, b):
    return power(a, b, 3)

However, that’s not as lazy as we would like to be. We could have instead written:

def power3(*args):
    return power(*args, 3)

power3(5, 2)

40

Why would this be helpful? Well, there are numerous reasons. One might be error checking. It may be that power is from a module that is very powerful so you want to use that. However, your code may call it with arguments that are inappropriate for that library. Here I will use a string, but you can imagine that perhaps a complex number may not be allowed. So, we can write:

def power3(*args):
    if type(args[0]) is str:
        print('Oops. You sent me a string.')
        return
    else:
        return power(*args,3)

power3(5, 2)

40

power3('Hello')

Oops. You sent me a string.

Note that the string was packed into a tuple of length 1 so the code must access the 0th value.

The key here is the *, not the name args. Any variable name could have been used.

So what about **kwargs?

This is the same situation, but this time with named variables and dictionaries. This time consider writing a central finite differences operator such that

However, if we want this to work for any function returning a numerical value, but with an unknown set of unnamed and named arguments, we can abstract by using *args and **kwargs:

def diff(f, t, dt, *args, **kwargs):
    return (f(t+dt/2, *args, **kwargs)
            -f(t-dt/2, *args, **kwargs))/dt

f is the name of the function we will calculate the slope of, t is the variable we want the slope with respect to, dt is a step size for that derivative. All other arguments, named or not, at passed directly through to the function f.

Let’s define our power function again.

def power(t, coeff=5, exponent=3):
    return coeff*t**exponent

Next we obtain the slope when \(t=2\), with a coefficient of 2 and exponent of 3.

diff(power, 2, 0.001, coeff=2, exponent=3)

24.012001999995647

This can work with any other similarly formed function. This is a simple product of the three arguments, 2 unnamed, one named.

def product(x, y, z=3):
    return x*y*z

We obtain the slope at \(x=2\) with \(y=4\) and \(z=3\).

diff(product, 2, .01, 4, z=3)

11.999999999999744

The keys for using **kwargs is to remember: - kwargs is your variable name. You decide what it is. - kwargs is now a dictionary. If you want to use them to call another function, make sure to unpack it in the function call by using **. - You can access parts of kwargs just as you can any other dictionary.

Conclusion

Hopefully this at least clarifies why they are useful and how you can apply them. In my experience, they are most useful when a function is designed to call a future (as yet defined) that requires parameters that can not be anticipated. Regardless, please leave your comments. I hope this helps.