A First Function Definition

A First Function Definition#

Note

Source: Adapted from the C# edition (functions/firstfunc.rst) and the SE4ML Python chapter (chapter_python.rst, lines 485–562). The birthday example mirrors the C# original; def syntax details follow the SE4ML presentation.

Programs often need to repeat the same sequence of steps in more than one place. Rather than copy and paste, we define a function — a named block of code we can call by name.

The Birthday Problem#

Suppose you want to print a birthday song for a friend named Emily:

print("Happy Birthday to you!")
print("Happy Birthday to you!")
print("Happy Birthday, dear Emily.")
print("Happy Birthday to you!")

Now suppose you also want to print the song for a friend named Andre. You could copy the four lines and change the name — but what if you also decide to add a fifth line? You would have to make the change in two places. Functions solve this problem.

Defining a Function#

A function definition uses the def keyword:

def happy_birthday():
    print("Happy Birthday to you!")
    print("Happy Birthday to you!")
    print("Happy Birthday, dear Emily.")
    print("Happy Birthday to you!")

Key things to notice:

  • The def line ends with a colon :.

  • The body of the function is indented — the standard is 4 spaces.

  • The function name follows the same rules as variable names: lowercase with underscores (snake_case).

  • The parentheses () are required even when there are no parameters.

Calling a Function#

To run the code in a function, you call it by writing its name followed by parentheses:

def happy_birthday():
    print("Happy Birthday to you!")
    print("Happy Birthday to you!")
    print("Happy Birthday, dear Emily.")
    print("Happy Birthday to you!")

happy_birthday()

Output:

Happy Birthday to you!
Happy Birthday to you!
Happy Birthday, dear Emily.
Happy Birthday to you!

The function definition does not produce any output on its own. Only when you call it does the code run.

Calling Multiple Times#

The advantage becomes clear when you call the function more than once:

happy_birthday()
happy_birthday()

Output:

Happy Birthday to you!
Happy Birthday to you!
Happy Birthday, dear Emily.
Happy Birthday to you!
Happy Birthday to you!
Happy Birthday to you!
Happy Birthday, dear Emily.
Happy Birthday to you!

One definition, used wherever needed.