Functions as first class object in Python
In Python, functions are first class object (first class citizen too!). Programming language theorists defined some criteria for first class object of a programming language. A “first class object” is a program entity which can be :
- created at run time
- can be assigned to a variable
- can be passed as function argument
- can be returned from a function
Let’s examine these criteria for Python functions.
1. Creation in run time
Here, we have a recursive function named print_inversely which will print n down to 0. Number of calls of print_inversely function depends on value of n. Depending on value of n it calls itself. Before running the program it cannot decide about its behavior, number of calls etc. At the run time it calls itself recursively depending on the value of n. In other word, “here we are creating and calling the function at run time”
In Python, functions are object too.
This describes that, print_inversely function is an instance of function class.
2. Assigning to a variable
Here we are assigning the print_inversely function to our_first_class_function variable and calling the function through the variable!
3. Passing functions as argument
print_inversely_by_user_input function takes a function as argument and calls it by user input. Here, we are passing print_inversely to print_inversely_by_user_input as argument.
4. Returning function from function
print_one_or_two function returns a function depending of value of n. If we pass n=1 then it will return reference of print_one function otherwise it will return reference of print_two function. Here, a function returning another function!
Note: When a function takes a function as argument or returns another function then the function is called higher-order function.
This is my brief discussion on python functions as first class object (first class citizen!) :)
