Default and keyword arguments

Intermediate Python for Developers

George Boorman

Curriculum Manager, DataCamp

Average

# Create a custom function
def average(values):
    # Calculate the average
    average_value = sum(values) / len(values)

    # Round the results
    rounded_average = round(average_value, 2)

    # Return rounded_average as an output
    return rounded_average

values = Argument

Arguments

A couple arguing

Arguments are values provided to a function or method
Functions and methods have two types of arguments:
- Positional
- Keyword

Positional arguments

Provide arguments in order, separated by commas

# Round pi to 2 digits
round(3.1415926535, 2)

3.14

Keyword arguments

Provide arguments by assigning values to keywords
Useful for interpretation and tracking arguments
keyword = value

# Round pi to 2 digits 
round(number=3.1415926535

Keyword arguments

Provide arguments by assigning values to keywords
Useful for interpretation and tracking arguments
keyword = value

# Round pi to 2 digits 
round(number=3.1415926535, ndigits=2)

3.14

Identifying keyword arguments

# Get more information about the help function
help(round)

Help on built-in function round in module builtins:

round(number, ndigits=None)
    Round a number to a given precision in decimal digits.

    The return value is an integer if ndigits is omitted or None.  Otherwise,
    the return value has the same type as the number.  ndigits may be negative.

Keyword arguments

Output of calling help on the round function, annotated to show number is the first argument and ndigits is the second argument

Default arguments

Help on built-in function round in module builtins:

round(number, ndigits=None)
    Round a number to a given precision in decimal digits.

    The return value is an integer if ndigits is omitted or None.  Otherwise,
    the return value has the same type as the number.  ndigits may be negative.

None = no value / empty
Default argument: way of setting a default value for an argument
We overwrite None to 2
- Otherwise, the result is an int

Why have default arguments?

Helps us think about likely uses for our function
- Commonly used value - set it using a default argument

Potentially reduces code for users (if they stick with default values)

Maintains flexibility

Adding an argument

# Create a custom function
def average(values):
    average_value = sum(values) / len(values)
    rounded_average = round(average_value, 2)
    return rounded_average

Adding an argument

# Create a custom function
def average(values, rounded=False):

Adding an argument

# Create a custom function
def average(values, rounded=False):

    # Round average to two decimal places if rounded is True
    if rounded == True:

        average_value = sum(values) / len(values)
        rounded_average = round(average_value, 2)
        return rounded_average

Adding an argument

# Create a custom function
def average(values, rounded=False):

    # Round average to two decimal places if rounded is True
    if rounded == True:
        average_value = sum(values) / len(values)
        rounded_average = round(average_value, 2)
        return rounded_average

    # Otherwise, don't round
    else:
        average_value = sum(values) / len(values)
        return average_value

Using the modified average() function

sales = [125.97, 84.32, 99.78, 154.21, 78.50, 83.67, 111.13]

Using the modified average() function

# Get the average without rounding
average(sales, False)

105.36857142857141

# Get the average without rounding
average(sales)

105.36857142857141

Using the modified average() function

# Get the rounded average
average(values=sales, rounded=True)

105.37

Let's practice!

Intermediate Python for Developers