Python Arbitrary Arguments – *args and **kwargs

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*args – allow you to pass multiple non-key arguments

**kwargs – allow you to pass multiple keyword arguments

The * is called the “unpacking operator”.

When you invoke a function that has args or kwargs as parameters, you will pack all of those arguments into a tuple if its args, or a dictionary if the parameter is kwargs.

args

def add(x, y):
    return x + y

print(add(1, 2))

The output will be 3, but this is very limiting – what if I need to add 3 numbers? If we pass 3 arguments, the function will fail.  This add function only takes two positional arguments.

If we replace the parameters with *args, the arguments we pass into the function will be packed into a tuple.

def add(*args):
    total = 0
    for arg in args:
        total += arg
    return total

print(add(1, 2, 3, 4))

In the above example, there is no limit to the number of arguments that may be passed to the function.

Another Example

def display_name(*args):
   print(f"Hello", end=" ")
   for arg in args:
       print(arg, end=" ")

display_name("Dr.", "Spongebob", "Harold", "Squarepants","II")

kwargs

Kwargs use two unpacking operators followed by the word kwargs,**kwargs

Kwargs will be packed into a dictionary.

def print_address(**kwargs):
    for value in kwargs.values():
        print(value, end=" ")

print_address(street="9 Byron Street",
              city="Ringwood",
              state="VIC",
              postcode="3134")

Outputs

9 Byron Street Ringwood VIC 3134

We could use the key, value pair feature of a dictionary to alter the layout to include the key.

def print_address(**kwargs):
    for key, value in kwargs.items():
        print(f"{key}: {value}")

print_address(street="9 Byron Street",
              city="Ringwood",
              state="VIC",
              postcode="3134")

Outputs

street: 9 Byron Street
city: Ringwood
state: VIC
postcode: 3134

Test Yourself with this exercise

Print a Shipping Label that uses *args and **kwargs to display a name and address.  The function should check whether there is a PO Box or an Apartment number and only display these values if they exist.

Dr. Spongebob Squarepants 
1/9 Byron Street
Ringwood
VIC, 3134

Solution

def shipping_label(*args, **kwargs):
    for arg in args:
        print(arg, end=" ")
    print()

    if "apt" in kwargs:
        print(f"{kwargs.get('apt')}/{kwargs.get('street')}")
    elif "pobox" in kwargs:
        print(f"{kwargs.get('street')}")
        print(f"{kwargs.get('pobox')}")
    else:
        print(f"{kwargs.get('street')}")

    print(f"{kwargs.get('city')}")
    print(f"{kwargs.get('state')} {kwargs.get('postcode')}")

shipping_label("Dr.", "Spongebob", "Squarepants",
               apt="1",
               street="9 Byron Street",
               city="Ringwood",
               state="VIC",
               postcode="3134")