What is first class function in Python

Question

I am still confused about what first-class functions are. If I understand correctly, first-class functions should use one function as an object. Is

Answer 1

"First-Class Functions" (FCF) are functions which are treated as so called "First-Class Citizens" (FCC). FCC's in a programming language are objects (using the term "objects" very freely here) which:

• Can be used as parameters
• Can be used as a return value
• Can be assigned to variables
• Can be stored in data structures such as hash tables, lists, ...

Actually, very roughly and simply put, FCF's are variables of the type 'function' (or variables which point to a function). You can do with them everything you can do with a 'normal' variable.

Knowing this, both this_is_another_example(myarg) and this_is_example(myarg1) are First-Class Functions, since all functions are First-Class in certain programming languages.

Answer 2

First Class functions in Python

First class objects in a language are handled uniformly throughout. They may be stored in data structures, passed as arguments, or used in control structures. A programming language is said to support first-class functions if it treats functions as first-class objects. Python supports the concept of First Class functions.

Properties of first class functions:
A function is an instance of the Object type.
- You can store the function in a variable. - You can pass the function as a parameter to another function. - You can return the function from a function. - You can store them in data structures such as hash tables, lists, …

Examples illustrating First Class functions in Python
1. Functions are objects: Python functions are first class objects. In the example below, we are assigning function to a variable. This assignment doesn’t call the function. It takes the function object referenced by shout and creates a second name pointing to it, yell.

    # Python program to illustrate functions
    # can be treated as objects
    def shout(text):  
        return text.upper()      
    print shout('Hello') 
    yell = shout 
    print yell('Hello')

Output:
HELLO
HELLO

2. Functions can be passed as arguments to other functions: Because functions are objects we can pass them as arguments to other functions. Functions that can accept other functions as arguments are also called higher-order functions. In the example below, we have created a function greet which takes a function as an argument.

    # Python program to illustrate functions
    # can be passed as arguments to other functions
    def shout(text):
        return text.upper()
    def whisper(text):
        return text.lower()
    def greet(func):
        #storing the function in a variable
        greeting = func("Hi, I am created by a function passed as an argument.")
        print greeting     
    greet(shout)
    greet(whisper)

Output:
HI, I AM CREATED BY A FUNCTION PASSED AS AN ARGUMENT.
hi, i am created by a function passed as an argument.

3. Functions can return another function: Because functions are objects we can return a function from another function. In the below example, the create_adder function returns adder function.

    #Python program to illustrate functions
    #Functions can return another function    
    def create_adder(x):  
        def adder(y):  
            return x+y  
        return adder  
    add_15 = create_adder(15)
    print add_15(10)

Output:
25