Defining and calling functions

Imagine a bakery's till program that prints the same three-line header at the top of every receipt, every daily report and every price list. If the header is copied into five places and the phone number changes, you have to find and fix all five copies, and you will probably miss one.

A function solves this. It is a block of code with a name. You write the code once, and then call the function by name every time you want that code to run. Change the function and every place that calls it gets the change. You have already used functions that Python provides, such as print(), len() and input(). In this track you write your own.

def print_header():
    print("SAFA BAKERY")
    print("Olaya Street, Riyadh")
    print("-" * 20)


print_header()
print("Receipt #1041")
print_header()
print("Daily report")

The parts of a definition

def print_header():
    print("SAFA BAKERY")

• def is the keyword that starts a function definition.
• print_header is the function's name. The same naming rules as variables apply.
• () holds the function's parameters (inputs). This one has none; the next lesson adds them.
• The colon : ends the header line, as with if and for.
• The indented lines below are the body: the code that runs when the function is called.

Defining a function does not run its body. Python only remembers "there is a function called print_header, and this is its code". The body runs when you call the function by writing its name followed by parentheses: print_header().

print("Opening the till")

def greet_customer():
    print("Welcome to Safa Bakery")

print("Ready for customers")
greet_customer()
greet_customer()

Calling needs parentheses

Writing the name without parentheses refers to the function but does not run it. greet_customer is the function itself; greet_customer() is a call. Forgetting the parentheses is a silent mistake: Python raises no error, the body never runs.

A function must also be defined before the line that calls it runs. Python reads the file from top to bottom, so calling greet_customer() above its def raises a NameError.

show_menu()

def show_menu():
    print("Bread, Croissant, Date cake")

Functions can call other functions

The body of a function can contain any code, including calls to other functions you wrote. This lets you build bigger tasks from smaller named steps:

def print_divider():
    print("=" * 24)

def print_closing_message():
    print_divider()
    print("Thank you for shopping!")
    print_divider()

Naming functions

A function does something, so its name usually starts with a verb: print_header, show_menu, calculate_total, send_reminder. Use lowercase words joined by underscores (called snake_case). Avoid vague names such as do_it or function1, and never reuse a built-in name such as print, list or sum for your own function: you would hide the built-in.

def print_divider():
    print("=" * 24)


def print_closing_message():
    print_divider()
    print("Thank you for shopping!")
    print("Open daily 6:00 - 23:00")
    print_divider()


print("Total: 37.50 SAR")
print_closing_message()

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