Optional Parameters (Keyword Arguments)

4.10. Optional Parameters (Keyword Arguments)#

Python allows you to have parameters that are optional. These are called keyword arguments. Keyword arguments must come after all of our mandatory parameters. We must also set a default value. Like with mandatory parameters, we can have multiple keyword arguments. The syntax for this is as follows:

def function_name(param_1, param_2, ..., k_param_1=default, k_param_2=default):

Example 1: Numeric keyword argument

The increase function will by default, increase x by 1. But if you specify a different increment amount it will increase x by the specified amount.

def increase(x, increment=1):
    return x + increment

print(increase(5))
print(increase(5, increment=10))

6
15

Example 2: Boolean keyword argument

The difference function will calculate the difference between x and y. If the keyword positive is True it will return a positive value, otherwise it will return a negative value.

def difference(x, y, positive=True):
    diff = x - y

    if diff > 0 and positive:
        return diff
    else:
        return -diff

print(difference(5, 2))
print(difference(5, 2, positive=False))

3
-3

Keyword arguments must come after all of our mandatory parameters. If you try to switch the order you will get a SyntaxError with the error parameter without a default follows parameter with a default, which essentially is telling you that a mandatory parameter is following a keyword argument.

def difference(x, positive=True, y):
    diff = x - y

    if diff > 0 and positive:
        return diff
    else:
        return -diff

File "/home/main.py", line 1
    def difference(x, positive=True, y):
                                    ^
SyntaxError: parameter without a default follows parameter with a default

4.10.1. Parameters vs Arguments#

Often the terms ‘parameter’ and ‘argument’ are used interchangeably, but there is a subtle distinction between the two terms. Parameters are the names of the variables as specified in the function definition, i.e. the variable names used when creating the function, whereas arguments are the actual values that are passed to the function when the function is called.

Example

In this add function x and y are the parameters. When the function is called we pass it the arguments 5 and 3.

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

print(add(5, 3))

If you give a function the wrong number of arguments it will result in a TypeError and notify you that you’re missing an argument.

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


print(add(5))

Traceback (most recent call last):
    File "/home/main.py", line 4, in <module>
        print(add(5))
            ^^^^^^
    TypeError: add() missing 1 required positional argument: 'y'

Optional Parameters (Keyword Arguments)

Contents

4.10. Optional Parameters (Keyword Arguments)#

4.10.1. Parameters vs Arguments#