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)
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)
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)
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)
**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')
myfunc()
*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')
Placing keyworded arguments ahead of positional arguments raises an exception:
myfunc(fruit='cherries',juice='orange','eggs','spam')
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!
thefuture 