10.4. Function Input

Earlier in this chapter, we used the image of the function machine, noting that the machine takes input and provides output. Let’s take a closer look at what we send in and what we get out of a function.

10.4.1. Arguments and Parameters

Over the past few sections, we defined two terms that are very similar and often confused: arguments and parameters. Both deal with the input we send to a function. (Remember, input consists of the values that control how the function does its job).

Seeing the difference between the two terms is tricky, and the easiest way to clear up the issue is with an example.


The function say_hello takes a single value, which we expect to be a person’s name, and prints a greeting.

def say_hello(name):
   print("Hello, {0}!".format(name))


Console Output

Hello, Lamar!
Hello, rutabaga!

In this example, name is a parameter. It is part of the function definition, and it behaves like a variable within the function. A parameter receives an input value sent to the function.

The value "Lamar" used in the function call on line 4 is an argument. It is a specific value sent by the function call. Similarly, the value 'rutabaga' in line 5 is a different argument sent to the say_hello function.

When we call the say_hello function in line 4, Python sends the value "Lamar" to the function. Before running the function body, however, Python assigns that value to the parameter name. In the background, Python completes the statement name = "Lamar".

When we call the say_hello function in line 5, Python sends a new argument to the function and assigns that value to name before running the function code (name = 'rutabaga').

The difference between a parameter and an argument is the same as that between a variable and a value. A variable refers to a specific value, just like a parameter refers to a specific argument when a function is called. Like a value, an argument is a concrete piece of data.


Arguments: Specific data values sent by the function call to a named function.

Parameters: Variables defined in a function that receive data. Python assigns data to these variables.

10.4.2. Mismatched Arguments and Parameters

Let’s say we define a function with a single parameter. When we call that function, what happens if we provide two arguments? What if we call the function but give no arguments?

Try It!

The say_hello function defines one parameter, name, which we expect to be a string value.

  1. On line 4, call the function WITHOUT using any arguments: say_hello().

  2. Next, try calling the function with two or more arguments: say_hello('Soda', 'Pop').

What error messages do you see?

In this example, we receive a TypeError message each time we call the say_hello function with the wrong number of arguments. As defined in line 1, the function has one parameter, so it expects to receive one data value. When we send the wrong number of arguments, the program crashes with a runtime error, since we did not tell the program how to deal with missing or extra data.

Read the error messages again, and note that we are given information about why the program crashed. How useful! The messages tell us how many missing or extra arguments we used in the function call.

10.4.3. Default Parameter Values

One way to deal with missing arguments is to provide a default value for a parameter. Since each parameter is just a variable, we can tell Python to assign it the default value IF its argument is missing from the function call.

The general syntax is:

def function_name(parameter_name = default_value):

Applied to the say_hello function from the previous example, this looks like:


def say_hello(name = 'World'):
   print("Hello, {0}!".format(name))


Console Output

Hello, RBG!
Hello, World!

Now, if we provide an argument in the function call (line 4), Python assigns that value to name. If we do not provide an argument (line 5), Python assigns the value 'World' to name.

10.4.4. Check Your Understanding


What does the following code output?

def string_repeater(a_string):
   repeated = a_string + a_string

  1. "BobBob"
  2. Nothing (no output)
  3. repeated
  4. The value of a_string