The general form of a function definition:

---

def function_name(parameters):
    body

---

• def: a keyword indicating a function definition
• function_name: the function name
• parameters:
  • the parameter(s) of the function, 0 or more and are separated by a comma
  • a parameter is a variable whose value will be supplied when the function is called
• body:
  • 1 or more statements, often ending with a return statement

Example of a function definition:

def f(x):
    return x ** 2

return statement

The general form of a return statement:

---

return expression

---

The rules for executing a return statement:

1. Evaluate the expression. This produces a memory address.
2. Pass back that memory address to the caller. Exit the function.

Function Calls

Function calls are expressions and the result can be stored in a variable.

The general form of a function call:

---

function_name(arguments)

---

The rules for executing a function call:

1. Evaluate the arguments to produce memory addresses.
2. Store those memory addresses in the corresponding parameters.
3. Execute the body of the function.

---

Example of a function definition and function calls:

>>> def area(base, height):
    return base * height / 2
>>> area(3, 4)
6.0
>>> result = area(10, 7.45)
>>> result
37.25

Saving your programs to ".py" files

We usually save our Python programs in ".py" files. A file can contain multiple function definitions and other statements. Before calling a function from a ".py" file in the shell in IDLE, you need to first execute Run -> Run Module, or else the shell will not recognize the function call.