Use function arguments in Python

• 3 minutes

Now that you know how to create a function with no inputs, the next step is to create functions that require an argument. Using arguments makes functions more flexible, because they can do more and conditionalize what they do.

Requiring an argument

If you're piloting a rocket ship, a function without required inputs is like a computer with a button to tell you the time. If you press the button, a computerized voice tells you the time. But a required input can be a destination to calculate travel distance. Required inputs are called arguments to the function.

To require an argument, put it within the parentheses:

def distance_from_earth(destination):
    if destination == "Moon":
        return "238,855"
    else:
        return "Unable to compute to that destination"

Try calling the distance_from_earth() function without any arguments:

distance_from_earth()

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: distance_from_earth() missing 1 required positional argument: 'destination'

Python raises TypeError with an error message that says the function requires an argument named destination. If the rocket ship's computer is asked to compute the travel distance with a destination, it should prompt that a destination is a requirement. The example code has two paths for a response, one for the Moon and the other one for anything else. Use the Moon as input to get an answer:

distance_from_earth("Moon")

238,855

Because there's a catch-all condition, try using any other string as the destination to check that behavior:

distance_from_earth("Saturn")

Unable to compute to that destination

Multiple required arguments

To use multiple arguments, you must separate them by using a comma. Let's create a function that can calculate how many days it takes to reach a destination, given distance and a constant speed:

def days_to_complete(distance, speed):
    hours = distance/speed
    return hours/24

Now use the distance from Earth to the Moon to calculate how many days it would take to get to the Moon at a speed limit of 75 miles per hour:

days_to_complete(238855, 75)

132.69722222222222

Functions as arguments

You can use the value of the days_to_complete() function and assign it to a variable, and then pass it to round() (a built-in function that rounds to the closest whole number) to get a whole number:

total_days = days_to_complete(238855, 75)
round(total_days)

133

However, a useful pattern is to pass functions to other functions instead of assigning the returned value:

round(days_to_complete(238855, 75))

133