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GitHub Repository: pierian-data/complete-python-3-bootcamp
 Path: blob/master/03-Methods and Functions/07-args and kwargs.ipynb
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Kernel: Python 3

Content Copyright by Pierian Data

*args and **kwargs

Work with Python long enough, and eventually you will encounter *args and **kwargs. These strange terms show up as parameters in function definitions. What do they do? Let's review a simple function:

def myfunc(a,b):
    return sum((a,b))*.05

myfunc(40,60)
5.0

This function returns 5% of the sum of a and b. In this example, a and b are positional arguments; that is, 40 is assigned to a because it is the first argument, and 60 to b. Notice also that to work with multiple positional arguments in the sum() function we had to pass them in as a tuple.

What if we want to work with more than two numbers? One way would be to assign a lot of parameters, and give each one a default value.

def myfunc(a=0,b=0,c=0,d=0,e=0):
    return sum((a,b,c,d,e))*.05

myfunc(40,60,20)
6.0

Obviously this is not a very efficient solution, and that's where *args comes in.

*args

When a function parameter starts with an asterisk, it allows for an arbitrary number of arguments, and the function takes them in as a tuple of values. Rewriting the above function:

def myfunc(*args):
    return sum(args)*.05

myfunc(40,60,20)
6.0

Notice how passing the keyword "args" into the sum() function did the same thing as a tuple of arguments.

It is worth noting that the word "args" is itself arbitrary - any word will do so long as it's preceded by an asterisk. To demonstrate this:

def myfunc(*spam):
    return sum(spam)*.05

myfunc(40,60,20)
6.0

**kwargs

Similarly, Python offers a way to handle arbitrary numbers of keyworded arguments. Instead of creating a tuple of values, **kwargs builds a dictionary of key/value pairs. For example:

def myfunc(**kwargs):
    if 'fruit' in kwargs:
        print(f"My favorite fruit is {kwargs['fruit']}")  # review String Formatting and f-strings if this syntax is unfamiliar
    else:
        print("I don't like fruit")
        
myfunc(fruit='pineapple')
My favorite fruit is pineapple 
myfunc()
I don't like fruit

*args and **kwargs combined

You can pass *args and **kwargs into the same function, but *args have to appear before **kwargs

def myfunc(*args, **kwargs):
    if 'fruit' and 'juice' in kwargs:
        print(f"I like {' and '.join(args)} and my favorite fruit is {kwargs['fruit']}")
        print(f"May I have some {kwargs['juice']} juice?")
    else:
        pass
        
myfunc('eggs','spam',fruit='cherries',juice='orange')
I like eggs and spam and my favorite fruit is cherries May I have some orange juice?

Placing keyworded arguments ahead of positional arguments raises an exception:

myfunc(fruit='cherries',juice='orange','eggs','spam')
File "<ipython-input-8-fc6ff65addcc>", line 1 myfunc(fruit='cherries',juice='orange','eggs','spam') ^ SyntaxError: positional argument follows keyword argument

As with "args", you can use any name you'd like for keyworded arguments - "kwargs" is just a popular convention.

That's it! Now you should understand how *args and **kwargs provide the flexibilty to work with arbitrary numbers of arguments!