4.1. Function Calls

In the context of programming, a function is a named sequence of statements that performs a computation. When you define a function, you specify the name and the sequence of statements. Later, you can “call” the function by name. We have already seen many examples of function calls. Consider this one, for example:

>>> type(32)
<class 'int'>

The name of the function is type. The expression in parentheses is called the argument of the function. The argument is a value or variable that we are passing into the function for it to use in some way. The result, for the type function, is the type of the argument.

It is common to say that a function “takes” an argument and “returns” a result. The result is called the return value.

4.1.1. Using Return Values

When a function has a return value, we have a few common ways of using it. First, we can store the return value in a variable so we can use it somewhere else:

name = input("Please enter your name: ")

total = sum(values)

count = len(values)

wholenum = int(floatval)

Each of those is an example of calling a function and storing its return value in a variable. Let’s look at the first one in more detail:

name = input("Please enter your name: ")

If we break this down, we see that it is an example of a variable assignment. The rule for assignments says that variable on the left, name, is assigned the value of the expression on the right. On the right, we see input("Please enter your name: ") This is a function call, and "Please enter your name: " is its argument. That means that we are passing the prompt string into the function to be used by it. And then we know that the input() function produces a string containing whatever the user types in. That string is its return value. It is that return value that is actually assigned to the variable on the left.

It’s important to understand exactly how a function call is interpreted:

Syntax Pattern

Function calls have the form:

<function name>(<argument1>, <argument2>, ...)

There may be zero arguments (nothing written between the parentheses), one argument, or multiple arguments separated by commas. Each argument is a value or some other expression that evaluates to some value. The values of the arguments are passed into the function for it to use.

When the function finishes execution, the function call returns. If it has a return value (some do, some don’t), then the entire function call evaluates to that return value.

Applying that rule to the above example, the call to input(...) evaluates to the string containing whatever the user types in. Or in the case of total = sum(values), the call to sum(values) evaluates to the numerical summation of the numbers in values, and that result is then what is stored in the variable total, following the rule of the variable assignment statement. You have already seen and probably written a lot of code that uses return values like this (storing them in variables); now you can understand in some more detail exactly what is happening when you do.

Let’s look at common pattern we’ve been using for a while, and think about it in terms of what we have learned about functions:

age = int(input("Please enter your age: "))

This code has two function calls. It is calling the int() function and the input() function. We can see that the argument to input() is "Please enter your age: ". But what is the argument to int(), exactly?

The argument to int() is the return value of input(). The call to input() evaluates to whatever the user types in, and that string is then passed into int() as its argument. Function calls that return values can be used anywhere any other expression or value can be written.

Check your understanding


values = [1.1, 2.3, 5.8, 13.21]
total = sum(values)
count = len(values)
whole_total = int(sum(values))

How many function calls are in the above code?

How many function calls have values as an argument?

How many function calls have a single number as an argument?


name = 'ABCDEFGH'
something = last(firsthalf(name))

The last() function returns the last letter in its argument. The firsthalf() function returns the first half of its argument (e.g., the first 4 characters if its argument is an 8 character string).

What does the above code print?