Python Functions

A function is reusable block of code that performs a single task and can be used repeatedly in programs. In Python, a basic function can be declared as follows:

def addNumbers(a:int, b:int) -> int:
    return a+b

The above example, the following items should be noted:

• The def keyword declares the start of a function

• The name of this function is addNumbers

• The function accepts two arguments, a and b.

  • Each argument can be labelled to show the expected data type (i.e. a:int shows that the a argument is expecting an int).

  • The -> int declares what the function will return (see below).

• Similar to other Python constructs, the declaration line ends with a colon (:).

• In standard circumstances, a Python function will return a value

Functions are useful for repeated actions. A famous principle of software development is “Don’t Repeat Yourself” (aka DRY code). As an example, writing the same message to multiple users could be performed as follows:

user1 = "Carl"
user2 = "Jim"
user3 = "Fred"

print("Greetings " + user1 + ", welcome to this program.")
print("Greetings " + user2 + ", welcome to this program.")
print("Greetings " + user3 + ", welcome to this program.")

Clearly, the same lines are being repeated over and over. This could be re-written as a function:

def greet(user:str) -> str:
    return "Greetings " + user + ", welcome to this program."

user1 = "Carl"
user2 = "Jim"
user3 = "Fred"

print(greet(user1))
print(greet(user2))
print(greet(user3))

Both of these examples will have the same output, but using the function will require less effort for the programmer and be much more robust and maintainable.