5.6. Adding new functions

So far, we have only been using the functions that come with Python, but it is also possible to add new functions. A function definition specifies the name of a new function and the sequence of statements that execute when the function is called. Once we define a function, we can reuse the function over and over throughout our program.

Here is an example:

def print_lyrics():
    print("I'm a lumberjack, and I'm okay.")
    print('I sleep all night and I work all day.')

def is a keyword that indicates that this is a function definition. The name of the function is print_lyrics. The rules for function names are the same as for variable names: letters, numbers and some punctuation marks are legal, but the first character can’t be a number. You can’t use a keyword as the name of a function, and you should avoid having a variable and a function with the same name.

The empty parentheses after the name indicate that this function doesn’t take any arguments. Later we will build functions that take arguments as their inputs.

Q-1: What does it means when there are empty parentheses after a function name?

• It lets you know whether a function is self-defined or included in a Python module.
• Incorrect! You can find all built-in functions with a quick search. Try again.
• It means that the function does not return anything.
• Incorrect! Return statements indicate what a function returns. Try again.
• It lets you know that nothing will print.
• Incorrect! A print statement indicates if a function prints something. Try again.
• It indicates that a function doesn't take any arguments.
• Correct! Empty parentheses indicate that the function doesn't take any arguments.

The first line of the function definition is called the header; the rest is called the body. The header has to end with a colon and the body has to be indented. By convention, the indentation is always four spaces. The body can contain any number of statements.

Q-2: What is the first line of a function definition called? What is every line after the first line called?

• body; header
• Incorrect! The order is off. Where does a header typically go? Try again.
• title; body
• Incorrect! Body is correct, but title is not. Try again.
• header; body
• Correct! The header is the first line of a function definition and the rest is the body.
• initialization; body
• Incorrect! We initialize variables, but functions are defined. Try again.

If you type a function definition in interactive mode, the interpreter prints ellipses (…) to let you know that the definition isn’t complete:

>>> def print_lyrics():
...     print("I'm a lumberjack, and I'm okay.")
...     print('I sleep all night and I work all day.')
...

To end the function, you have to enter an empty line (this is not necessary in a script).

Defining a function creates a variable with the same name.

def print_lyrics(): print("I'm a lumberjack, and I'm okay.") print('I sleep all night and I work all day.') print(print_lyrics) print(type(print_lyrics))

The value of print_lyrics is a function object, which has type “function”.

Q-4: Consider the code below. Which statement is true?

def printWeather():
    print("It is sunny!")

• The value of printWeather is a function object, which has type "function".
• Correct! The value of printWeather is a function object, meaning it has the type "function".
• The value of printWeather is a definition object, which has type "def".
• Incorrect! The codeblock shows a function definition, but that is not the object/type of printWeather. Try again.
• The value of printWeather is a def object, which has type "def".
• Incorrect! Function definitions use the ``def`` keyword, but that is not the object/type of printWeather. Try again.
• The value of printWeather is a function object, which has type "funct".
• Incorrect! The value of printWeather is a function object, but its type is not "funct". Try again.

The syntax for calling the new function is the same as for built-in functions:

def print_lyrics(): print("I'm a lumberjack, and I'm okay.") print('I sleep all night and I work all day.') print_lyrics()

Once you have defined a function, you can use it inside another function. For example, to repeat the previous refrain, we could write a function called repeat_lyrics and then call that function:

def print_lyrics(): print("I'm a lumberjack, and I'm okay.") print('I sleep all night and I work all day.') def repeat_lyrics(): print_lyrics() print_lyrics() repeat_lyrics()

But that’s not really how the song goes.

Construct a block of code that correctly creates a function called “printMenu” that prints the menu and prices, then call the function. Watch your indentation!

        def printMenu():
---
def printMenu() #paired
---
definition printMenu(): #paired
---
    print("Pizza, $5")
    print("Hamburger, $6")
    print("Soda, $1")
---
    print(Pizza, $5) #paired
    print(Hamburger, $6)
    print(Soda, $1)
---
printMenu()
---
printMenu(): #paired
        

The following code should define the function printPrice, that prints items and their prices, and define a second function printReceipt, that uses printPrice to print a receipt. Then, the code should call printReceipt. Watch your indentation!

        def printPrice():
---
def printPrice() #paired
---
definition printPrice(): #paired
---
    print("Pencils, $1")
    print("Pens, $2")
    print("Notebook, $1")
---
    print(Pencils, $1) #paired
    print(Pens, $2)
    print(Notebook, $1)
---
def printReceipt():
---
    print("Here is your receipt: ")
    printPrice()
    print("Thanks for shopping!")
---
    print(Here is your receipt: ) #paired
    printPrice():
    print(Thanks for shopping!)
---
printReceipt()
---
printReceipt(): #paired
        

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