# Decorators in Python

Decorator is a function, which modifies another function and returns it. The new function is often referred to as ==Decorated function==.

> Why to use it ?

Suppose we are passing our name as an argument to a function and we want a greeting before it, let's say Hello followed by the name, then we can simply modify the function and add a hello before it, that can be the easiest way. But in case when we are working on a big project, with many functions and we want a similar modification in all of it, then changing individual function would be a lot of task and it will also effect the readability. In such cases, using decorators is advised.

> Before getting into the syntax of Decorators, it is important to know about how a function can be used in many ways.

1. Functions can be assigned to variable


2. Functions can be passed as an argument


3. Functions can be returned from another function


### Syntax

Now as discussed above Decorators are used to modify an function. Lets get into the synatx of decorators with some coding examples. Suppose we want to add a greeting and a thank you test to an function :-

def greet(func):
    print("Hello")
    func()
    print("Thank You")
@greet
def hello():
    print("viewer")

>This is same as writing the below code :-

def greet(func):
    print("Hello")
    func()
    print("Thank You")
def hello():
    print("viewer")
greet(hello)

>in both the cases, the output will be the same.

**output**
Hello
viewer
Thank you

>In a more detailed manner, we can also modify the code to return a function that is the modified function.

def greet(func):
    def mfunc():
        print("Hello")
        func()
        print("Thank You")
    return mfunc
@greet
def hello():
    print("viewer")
hello()

>In case we are passing arguments, we need to pass them in the following manner :-

def greet(func):
    def mfunc(*args, **kwargs):
        print("Hello")
        func(*args, **kwargs)
        print("Thank You")
    return mfunc
@greet
def hello(a, b):
    print(a+b)
hello(5,10)

**Output**
Hello
15
Thank You

### Chaining Decorators

It simply means decorating a function with multiple decorators.

def func1(func): 
    def inner(): 
        x = func() 
        return x * x 
    return inner 

def func2(func): 
    def inner(): 
        x = func() 
        return 2 * x 
    return inner 

@func1
@func2
def num1(): 
    return 10

@func2
@func1
def num2():
    return 10
  
print(num1()) 
print(num2())

**Output**
400
200

The above code can be seen as :- print(func1(func2(num))) and print(func2(func1(num))).

### Practical Uses

One common use of Decorators is to add logging to a function. For example, you can use a decorator to the arguments and return value of a function each time it is called.

### MCQ Questions

1. What symbol is used to decorate a function
- !
- &
- /
- @

2. Decorators modify a function
- Temporarily
- permanent
- can't be determined
- none of the above

3. Which of the following is true
- Functions can be assigned to a variable
- Functions can be passed as an argument
- Functions can be returned from another functions
- all of the above

4. What type of arguments are used to create general purpose decorators?
- Positional
- Arbitrary
- keyword
- default

5. Identify the decorator function in the below code:

    def greet(func):
        print("Hello")
        func()
        print("Thank You")
    @greet
    def hello():
        print("viewer")

- greet()
- hello()

6. Find the output of the below code below code:

    def greet(func):
        print("Hello")
        func()
        print("Thank You")
    @greet
    def hello():
        print("viewer")

- Hello
- Hello
Thank You
- Hello
viewer
Thank You
- error

7. which of the following statements are true:
statement-1: Nested functions are possible
statement-2: Multiple decorators for a function can exist

- Both statement -1 and 2 are true
- statement -1 is true, statement -2 is false
- statement -1 is falsr, statement -2 is true
- Both the statemnts-1 and 2 are false

8. What’s the primary purpose of decorators in Python?

- To change the return type of a function
- To optimize the performance of a function
- To modify the behavior of a function
- To create new functions dynamically

9. Which statements about the the variables referenced and evaluated are true?

   def reference():
    def inner():
         return 42
         return inner

   def evaluation():
    def inner():
        return 42
        return inner()
   referenced = reference()
   evaluated = evaluation()

- There’s no difference between the two.
- The variable evaluated will be the integer 42.
- The variable referenced will be a reference to a function object.
- Neither referenced nor evaluated will hold the value 42.

10. What’s the purpose of the @ symbol when using decorators?

- It unlocks a library of built-in decorators.
- It increases code complexity.
- It simplifies applying a decorator to a function.

11. You’ve decorated a function that accepts an argument, but when running it, you get an error:
TypeError: wrapper() takes 0 positional arguments but 1 was given
What might be a good approach to try to fix this error?

- Using *args and **kwargs in the inner wrapper function
- Creating separate decorators for functions with and without arguments
- Removing the arguments from the function

12. Is it possible to define a decorator without an inner function?

- Yes
- No

13. Which of the following statements are true in regard to applying multiple decorators to a function?

- The order of decorators doesn’t matter.
- You can’t apply more than one decorator to a function.
- You can apply several decorators by stacking them on top of each other.
- The order of decorators matters.

14. How many functions should you define to create a decorator with arguments?

- One
- Two
- Four
- Three

15. Idenify the line with error:

    def greet(func):
        print("Hello")
        func()
        print("Thank You")
    !greet
    def hello():
        print("viewer")

- Line 1
- Line 3
- Line 5
- Line 6

16. Idenify the line with error:

    def greet(func):
        print("Hello")
        func()
        print("Thank You")
    @greet
    def hello():
        print("viewer")

- Line 1
- Line 3
- Line 5
- No error

17. Find the output of the following code

    def func1(func): 
        def inner(): 
            x = func() 
            return 2 * x 
        return inner 

    def func2(func): 
        def inner(): 
            x = func() 
            return x * x 
        return inner 

    @func1
    @func2
    def num1(): 
        return 10
    
    @func2
    @func1
    def num2():
        return 10
  
    print(num1()) 
    print(num2())

- 200
400
- Error
- 400
200

18. Which of the statements correctly interprets the below code

     def func1(func): 
        def inner(): 
            x = func() 
            return 2 * x 
        return inner 

    def func2(func): 
        def inner(): 
            x = func() 
            return x * x 
        return inner 

    @func1
    @func2
    def num1(): 
        return 10
    print(num1())

- print(func2(func1(num1)))
- print(func2(num))
- print(func1(num))
- print(func1(func2(num1)))

19. The new function obtained after being modified by a decorator is termed as

- Decorator function
- Modified Function
- Decorated Function
- New Function

20. What will be the output of the following code?

    def create_adder(x): 
    def adder(y): 
        return x+y 

    return adder 
    add_15 = create_adder(15) 
    print(add_15(10))

- 15
- 10
- Error
- 25