Python Basics 102- Python Functions.

In the previous post I introduced you to Python programming language, python installation and python fundamentals.
In this post we will go through Python functions

FUNCTIONS IN PYTHON

A function is a block of organized, reusable code that is used to perform a single, related action.
Functions minimize the repetition of code, hence they are reusable.
Below are the rules to define a function in Python.

• Function blocks begin with the keyword def followed by the function name and parentheses ( ( ) ).
• Any input parameters or arguments should be placed within these parentheses. You can also define parameters inside these parentheses.
• In the body start with a """docstring""", to explain what the function does.
• The code block within every function starts with a colon (:) and is indented. Check Pep 8
• The statement return [expression] exits a function, optionally passing back an expression to the caller. A return statement with no arguments is the same as return None.

Function syntax

def functionname( parameters ):
   "function_docstring"
   function_suite
   return [expression]

Function Example

# define a function that says Hello World.
def greet():
    print("Hello World")

#calling the function
greet()
#prints> Hello World

Calling a Function

When you want to use a function, you just need to call it. A function call instructs Python to execute the code inside the function.

Returning a value

A function can perform a task like the greet() function. Or it can return a value. The value that a function returns is called a return value.
To return a value from a function, you use the return statement inside the function body.

Python Parameters and Arguments

It’s important to distinguish between the parameters and arguments of a function.

• A parameter is a piece of information that a function needs. And you specify the parameter in the function definition.
• An argument is a piece of data that you pass into the function.

Here is an Example to help us differentiate a parameter and an argument.

def sum(a, b):
    return a + b


total = sum(10,20)
print(total)

In this example, the sum() function has two parameters a and b, and returns the sum of them.
In the following function call, a will be 10 and b will be 20 inside the function body. These values 10 and 20 are called arguments.

Python Decorator.

A decorator is a design pattern in Python that allows a user to add new functionality to an existing object without modifying its structure. Decorators are usually called before the definition of a function you want to decorate.

Syntax for a python decorator

from fastapi import FastAPI

app = FastAPI()

#Here is the decorator 
@app.get("/")
async def root():
    return {"message": "Hello World"}

Python Recursive Functions

A recursive function is a function that calls itself and always has condition that stops calling itself.

Python recursive function examples

1.Count Down to Zero
Suppose you need to develop a countdown function that counts down from a specified number to zero.
The following defines the count_down() function:

def count_down(start):
    """ Count down from a number  """
    print(start)

Example

def count_down(start):
    """ Count down from a number  """
    print(start)

    # call the count_down if the next
    # number is greater than 0
    next = start - 1
    if next > 0:
        count_down(next)


count_down(3)

Output:

3
2
1

1. Using a recursive function to calculate the sum of a sequence

Suppose that you need to calculate a sum of a sequence e.g., from 1 to 100. A simple way to do this is to use a for loop with the range() function:

def sum(n):
    total = 0
    for index in range(n+1):
        total += index

    return total


result = sum(100)
print(result)

Output:

5050

Python Lambda Expressions

Lambda Function, also referred to as 'Anonymous function' is same as a regular python function but can be defined without a name. While normal functions are defined using the def keyword, anonymous functions are defined using the lambda keyword.

lambda expression syntax:

lambda arguments: expression 

Lambda functions can have any number of arguments but only one expression. The expression is evaluated and returned. Lambda functions can be used wherever function objects are required.

Example:

double = lambda x: x * 2

print(double(5))

#sum of numbers
sum = lambda x: x+5
print(sum(5))
#> 5+5= 10

#multiple args
product = lambda x, y: x*y
print(product(5, 10))
#> 5*10 = 50

I hope this was helpful. Let's engage in a discussion on anything that I should add to make this article better.
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