Python Functions

This chapter is from the book

Python Distilled

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This chapter is from the book

This chapter is from the book 

Python Distilled

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5.4 Keyword Arguments

Function arguments can be supplied by explicitly naming each parameter and specifying a value. These are known as keyword arguments. Here is an example:

def func(w, x, y, z):
    statements

# Keyword argument invocation
func(x=3, y=22, w='hello', z=[1, 2])

With keyword arguments, the order of the arguments doesn’t matter as long as each required parameter gets a single value. If you omit any of the required arguments or if the name of a keyword doesn’t match any of the parameter names in the function definition, a TypeError exception is raised. Keyword arguments are evaluated in the same order as they are specified in the function call.

Positional arguments and keyword arguments can appear in the same function call, provided that all the positional arguments appear first, values are provided for all nonoptional arguments, and no argument receives more than one value. Here’s an example:

func('hello', 3, z=[1, 2], y=22)
func(3, 22, w='hello', z=[1, 2])    # TypeError. Multiple values for w

If desired, it is possible to force the use of keyword arguments. This is done by listing parameters after a * argument or just by including a single * in the definition. For example:

def read_data(filename, *, debug=False):
    ...

def product(first, *values, scale=1):
    result = first * scale
    for val in values:
        result = result * val
    return result

In this example, the debug argument to read_data() can only be specified by keyword. This restriction often improves code readability:

data = read_data('Data.csv', True)        # NO. TypeError
data = read_data('Data.csv', debug=True)  # Yes.

The product() function takes any number of positional arguments and an optional keyword-only argument. For example:

result = product(2,3,4)             # Result = 24
result = product(2,3,4, scale=10)   # Result = 240