functions

Functions

Need For Functions

• Code reuse is a very important part of programming in any language.
• Increasing code size makes it harder to maintain.
• For a large programming project to be successful, it is essential to abide by the Don't Repeat Yourself, or DRY, principle.

FACT : Bad, repetitive code is said to abide by the WET principle, which stands for Write Everything Twice, or We Enjoy Typing.

What is a Function?

• A function is a block of code which only runs when it is called.
• You can pass data, known as parameters, into a function.
• A function can return data as a result.
• Any statement that consists of a word followed by information in parentheses is a function call.
  Here are some examples that you've already seen:

print("Hello world!")
range(2, 20)
str(12)
range(10, 20, 3)

NOTE : The words in front of the parentheses are function names, and the comma-separated values inside the parentheses are function arguments.

Functions in Python

• In addition to using pre-defined functions, you can create your own functions by using the def statement.
• Here is an example of a function named my_func.
  It takes no arguments, and prints "spam" three times. It is defined, and then called.
  The statements in the function are executed only when the function is called.
  

CODE

def my_func():
   print("spam")
   print("spam")
   print("spam")

my_func()

OUTPUT

>>>
spam
spam
spam
>>>

NOTE : The code block within every function starts with a colon (:) and is indented.

• You must define functions before they are called, in the same way that you must assign variables before using them.
  

CODE

hello()

def hello():
    print("Hello world!")

OUTPUT

>>>
NameError: name 'hello' is not defined
>>>

Arguments

• All the function definitions we've looked at so far have been functions of zero arguments, which are called with empty parentheses.
  
• However, most functions take arguments.
  
• The argument is defined inside the parentheses.
  
• The example below defines a function that takes one argument:
  

CODE

def print_with_exclamation(word):
   print(word + "!")
    
print_with_exclamation("spam")
print_with_exclamation("eggs")
print_with_exclamation("python")

OUTPUT

>>>
spam!
eggs!
python!
>>>

• You can also define functions with more than one argument; separate them with commas.
  

CODE

def print_sum_twice(x, y):
   print(x + y)
   print(x + y)

print_sum_twice(5, 8)

OUTPUT

>>>
13
13
>>>

• Function arguments can be used as variables inside the function definition.
• However, they cannot be referenced outside of the function's definition.
• This also applies to other variables created inside a function.
  

CODE

def function(variable):
   variable += 1
   print(variable)

function(7)
print(variable)

OUTPUT

>>>
8

NameError: name 'variable' is not defined
>>>

NOTE : Technically, parameters are the variables in a function definition, and arguments are the values put into parameters when functions are called.

Functions as Objects

• Although they are created differently from normal variables, functions are just like any other kind of value.
• They can be assigned and reassigned to variables, and later referenced by those names.
  

CODE

def multiply(x, y):
   return x * y

a = 4
b = 7
operation = multiply
print(operation(a, b))

OUTPUT

>>>
28
>>>

• Functions can also be used as arguments of other functions.
  

CODE

def add(x, y):
  return x + y

def do_twice(func, x, y):
  return func(func(x, y), func(x, y))

a = 5
b = 10

print(do_twice(add, a, b))

OUTPUT

>>>
30
>>>

HINT : As you can see, the function do_twice takes a function as its argument and calls it in its body.

Links and References

https://www.w3schools.com/python/python_functions.asp
https://www.tutorialspoint.com/python/python_functions.htm
https://www.programiz.com/python-programming/function