Higher-Order Functions — Python

A programming language is said to support First Class Functions if it treats functions as first-class objects. By definition, a “first-class object” in a program entity is an object that can be passed around just like other objects. It has the following characteristics:

• can have properties and methods.
• can be assigned to a variable.
• can be passed as an argument to a function.
• can be returned as a result of another function.

Let’s try to understand each property through code.

1. Properties and Methods

Each Method/Function you create in Python has a set of default properties and methods, which can inspect using the dir() method. In the below example, I have defined a hello_world function that prints the string Hello World!.

def hello_world():
   print("Hello World!")

When we call the dir() function on the hello_world method, we could see all the default methods that are part of it.

print(dir(hello_world))
['__annotations__', '__call__', '__class__', '__closure__', '__code__', '__defaults__', '__delattr__', '__dict__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__get__', '__getattribute__', '__globals__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__kwdefaults__', '__le__', '__lt__', '__module__', '__name__', '__ne__', '__new__', '__qualname__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__']

You can call any of these methods tied with the function. For example:

hello_world.__name__
# 'hello_world'

You can also use type to understand that the function we create is an instance of the function class.

print(type(hello_world))
# <class 'function'>

2. Assigning Function to Variables

We can also assign functions to variables.

def hello_world_function(name):
   print("Hello " + name + "!")
hello_world_variable = hello_world_function

Here we are assigning hello_world_function function to the variable hello_world_variable. Now hello_world_variable is a functions object which means, we can call it just like the hello_world_function.

hello_world_variable("Tharun")
# Hello Tharun!

This assignment does not call the function instead it takes the function object referenced by hello_world_function and creates a second name pointing to it.

hello_world_function
# <function hello at 0x0000020127C982F0>
hello_world_variable
# <function hello at 0x0000020127C982F0>

3. Function as an Argument

Since Function is an object, you can pass it as an argument much similar to a variable.

Let’s consider iterating over a list of items and printing them sequentially. We can easily build an iterate function.

def iterate(items):
   for item in items:
      print(item)

This is usual stuff. What if we want to do something different from printing the items? That’s where Higher-Order Functions come in. We can create a function iterate_custom that takes in both the item list and the function that needs to be applied to each item.

def iterate_custom(items, function):
   for item in items:
      function(item)

By doing this, we have created a function that can do anything with the list that involves sequential iteration. This is a higher level of abstraction and this also makes our code reusable.

4. Return Function from Function

This is usually done to have a wrapper function that decides the control flow or to decide which function should be called. For example:

def square(num):
   return num * num
def cube(num):
   return square(num) * num
def power_of_num(power):
   if power == 2:
      return square
   elif power == 3:
      return cube
num_powers = power_of_num(2)
# num_powers is assigned with the square method
num_powers(5)
# 25

We have defined methods square and cube which are pretty usual. The third method power_of_num is a wrapper function that returns either of the first two methods based on the variable value. In this case, power_of_num is called with the variable value 2. Square method would be returned now and assigned in the variable num_powers. Now, if we call the num_powers variable, it would act as the square method.

num_powers = power_of_num(3)
# num_powers is assigned with the cube method
num_powers(5)
# 125

Got questions? Feel free to comment here.

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