This material is optional. You are not expected to know this for a test and there are no labs or projects using this material.

Keyword arguments

Normally, we will call the print() function like this:

print('hello')

or

number = 5
print(number)

You can even give print() multiple things to print:

name = 'Sarah'
course = 'CS110'
grade = 'A'

print(name, course, grade)

This will print the values of all arguments, separated by a space:

Sarah CS110 A

Positional arguments

The examples above all use positional arguments, meaning the position matters. For example, we might define a function:

def compute(a, b, c):
    return (a + b) * c

The position of these arguments matters! If I call this function:

result = compute(3, 4, 5)

then result will be (3 + 4) * 5 = 35. However, if we instead call it like this:

result = compute(5, 4, 3)

then result will be (5 + 4) * 3 = 27.

Keyword arguments

Some functions take keyword arguments, meaning their value is defined by a keyword instead of their position. One function that works this way is print:

name = 'Sarah'
course = 'CS110'
grade = 'A'

print(name, course, grade, sep=',')

Here, we give sep as a keyword argument, where sep stands for separator — any characters you want printed between the positional arguments. The default, when you don’t provide the sep argument, is a single space.

However, if you use sep='a' then print() will use a as a separator. This is how you use a keyword argument — you specify its keyword, use an =, and then give its value.

In the code above, we tell print() to use a comma as the separator, so this code will print:

Sarah,CS110,A

Using a comma as a separator when printing data is nice — you can create CSV files that programs like Excel can read. Python also has a variety of libraries that can read CSV files and create plots.

The print() function also takes end as a set of characters you want to end each line with. By default each line is ended with a newline (n). But we could change that:

name = 'Sarah'
course = 'CS110'
grade = 'A'

print(name, course, grade, end='**')

This will print:

Sarah CS110 A**

Multiple keyword arguments

You can use as many keyword arguments as the function supports. Since print() allows both sep and end, we can list both of them, in any order we prefer, as long as they come after any positional arguments.

name = 'Sarah'
course = 'CS110'
grade = 'A'

print(name, course, grade, sep=', ', end='**')

This will print:

Sarah, CS110, A**

Keyword arguments in functions you write

Now we are going to show you how to write functions that take keyword arguments.

Here is a function that prints bullets at the start of each line:

def print_with_bullets(lines, bullet='*'):
    for line in lines:
        print(f'{bullet} {line}')

The first argument is a list of lines that you want to print. This is a positional argument. The second argument is the character you want to use for the bullets. This is a keyword argument because it has an = after the name of the argument, and then a default value.

If you call this function without the bullets argument:

if __name__ == '__main__':
    lines = [
        'Hello.',
        'Cosmo is the best mascot.',
        'Python is pretty cool.'
    ]

    print_with_bullets(lines)

then it will print a * in front of every line:

* Hello.
* Cosmo is the best mascot.
* Python is pretty cool.

However, if you provide the bullets argument, then you can print whatever you want at the start of each line:

print_with_bullets(lines, bullet='>')

This will print:

> Hello.
> Cosmo is the best mascot.
> Python is pretty cool.