Creating a Function

• Functions in Python begin with the def keyword followed by a function name and brackets (“()”)
• The code within the function then starts with after the colon (“:”) at the end of the brackets, and is indented once

def function_name():
    print("Hello 4061CEM")

Using a Function

• Functions can be called using their function name, followed by a set of brackets
  • this is often known as the function caller

def function_name():
    print("Hello 4061CEM")

function_name()

Returning a Value from a Function

• Functions can also return data from inside it using the return statement
• Useful if you have performed some operations inside a function and need to use the output

def my_sum():
    x = 2
    return 5 + x

my_sum()

• You may want to store the returned value from a function in a variable, or print it to the terminal

summed = my_sum()
print(my_sum())

Empty Functions (1)

• The purpose of a function is to have some re-usable code, therefore they cannot be empty
  • if you insist on having a function with no code, then you can use the pass statement

def my_function():
    pass

Empty Functions (2)

• Empty functions offers to purpose or use; unless it is a placeholder for future code
  • in this instance, you would want it to raise a warning, such as: NotImplemented

def my_function():
    return NotImplemented

Parameter and Argument Example

• Parameters/arguments are specified after the declaration of the function name and inside the brackets
  • you are able to add as many parameters/arguments as you want, separating them with a comma (,)

def hello_person(name):
    print("Hello " + name + " and welcome to 4061CEM")

hello_person("Ian")
hello_person("Terry")
hello_person("Daniel")

Passing Arguments to Functions (1)

• There are two ways of passing arguments to a function: pass by value or pass by reference

Pass by Value

• The function creates a copy of the variable passed to it as an argument
  • the actual variable itself is not affected

x = 10
def change_int(x):
    x = 20

change_int(x)

Passing Arguments to Functions (2)

Pass by Reference

• The actual variable is passed to the called function
  • changes made to the variable inside the function will affect the original value

x = [4, 0, 6, 1]
def change_value(_list):
    _list[1] = -9

change_value(x)

Number of Arguments

• When calling a function, it must be called with the correct number of arguments
  • if you have a function with three arguments then you have to call the function with three arguments

def hello_person(name, code):
    print(f"Hello {name} and welcome to {code}!")

hello_person("Ian", "4061CEM")
hello_person("Terry", "4059CEM")

Default Parameter Values

• A function can be called without an argument if a default value has been assigned to the parameter
• The default value will only be evaluated once and makes a difference when the default value is a mutable object
  • i.e. a list, dictionary or an instance of most classes

def hello_person(name="Ian", code="4061CEM"):
    print(f"Hello {name} and welcome to {code}!")

hello_person()
hello_person(name="Terry", code="4059CEM")
hello_person("Daniel")

Keyword Arguments

• Keyword arguments are related to the function calls
• When they are used in a function call, the caller identifies the arguments by its parameter name
  • the notation of using this method is: parameter = value
  • when used in a function call, the order of arguments do not matter

def hello_person(code, name):
    print("Hello " + name + " and welcome to " + code + "!")

hello_person(name="Ian", code="4061CEM")
hello_person(code="4059CEM", name="Terry")

Arbitrary Arguments

• When you do not know the number of arguments that will be passed into a function, add an asterisk (*) before the parameter name
• The function will then receive a tuple of arguments and access the items accordingly
  • the tuple will remain empty if no arguments are passed through

def hello_person(*details):
    print("Hello " + details[0] + " and welcome to " + details[1] + "!")

hello_person("Ian", "4061CEM")
hello_person("Terry", "4059CEM")

Arbitrary Keyword Arguments

• When you do not know the number of keyword arguments that will be passed into a function, add a double asterisk (**) before the parameter name
• The function will receive a dictionary of arguments and access the items accordingly

def hello_person(**details):
    print("Hello " + details['name'] + " and welcome to " + details['code'] + "!")

hello_person(name="Ian", code="4061CEM")
hello_person(code="4059CEM", name="Terry")

Function Annotations

• Function annotations are optional metadata information about the various data types used by user-defined functions
  • these annotations are stored in the __annotations__ attribute of a function
• Annotations for a parameter are defined with a colon (:) after the name of the parameter
• Annotations for a return are defined by a -> followed by the data type
  • this is placed between the list of parameters and the colon denoting the end of the def statement

def my_sum(x: int) -> int:
    return 5 + x

print(my_sum.__annotations__)