How a Function is Declared in Python

Last Updated on April 27, 2023 by Prepbytes

Function declaration in Python is introduced to define a reusable piece of code that performs a specific task. It helps to modularize code, make it easier to read and maintain and reduce code duplication. Python function declaration also allows for better organization of code, making it easier to debug and test.

Introduction to Python Function Declaration

Python function declaration uses the keyword "def" followed by the function name and parentheses containing any necessary parameters. The function body is indented and contains the code to be executed when the function is called. The function may return a value using the "return" keyword.

Here is a basic syntax for defining a function in Python:

def function_name(parameter1, parameter2, ...):
    # function body
    # statements to be executed
    # return statement (optional)

Let’s learn about each of these components of a Function in detail.

• The Function Name
  When naming a function in Python, it is important to use a descriptive name that accurately conveys the purpose of the function. The name should be a valid Python identifier and follow the same naming conventions as variables. Specifically, function names should be written in lowercase letters, with words separated by underscores. This naming convention promotes readability and ensures that the function name is easily understandable by others who may review or modify the code.

• Function Parameters
  In Python, function parameters refer to the values that are provided to a function when it is called. They are specified within the parentheses following the function name and can consist of one or more values. If a function does not require any arguments, the parentheses should still be included. Function parameters can also have default values, which are used if the user does not explicitly provide a value for that parameter. This feature allows for more flexibility and usability in the function design.

  Here is an example:

  def calculate_area(length, width=10):
      # function body
      return length * width

  In this example, the “calculate_area” function takes two parameters: length and width. The width parameter has a default value of 10. If the user does not provide a value for width, the function will use the default value of 10.

• Function Body
  In Python, the function body refers to the set of instructions that are executed when the function is called. The body can consist of one or more statements, each of which is indented to the right of the function definition. Within the function body, developers can include variable declarations, conditional statements, loops, and even other function calls. These statements are written in Python code and are executed sequentially when the function is invoked.

• Return Statement
  Python has a return statement that sends a specified value back to the calling code from a function. If a return is not used, the function implicitly returns None as the default output.

  Here is an example:

  def calculate_area(length, width):
      # function body
      return length * width

  In this example, the calculate_area function returns the product of the length and width parameters.

• Calling a Function
  Functions in Python can be called from any part of the program using their name followed by arguments within parentheses. Upon execution, the function’s code block is run, returning any specified output to the calling code, thereby increasing program efficiency.

  Here is an example:

  def calculate_area(length, width):
     # function body
     return length * width
  area = calculate_area(5, 10)
  print(area)

  Output:

  50

  Explanation:
  In this example, the “calculate_area” function is called with the arguments 5 and 10, and the result is assigned to the area variable. The area variable is then printed to the console.

Function Arguments in Python

Function arguments in Python are the values that are passed to a function when it is called. Arguments can be used to customize the behavior of a function and perform specific tasks within the function body.

There are four types of arguments that can be used in Python functions.

1. Default arguments: It is an argument that takes a default value if no value is provided when the function is called. Default arguments are defined using the = operator in the function definition. Here’s an example:

   def greet(name='world'):
       print(f'Hello, {name}!')
   
   greet()   # Output: Hello, world!
   greet('Varun')   # Output: Hello, Varun!

2. Required arguments: There are arguments that must be passed to the function when it is called. If a required argument is not passed, the function will raise a TypeError. Here’s an example:

   def greet(name):
       print(f'Hello, {name}!')
   
   greet('Varun')   # Output: Hello, Varun!

3. Keyword arguments: It is an argument that is passed with a keyword, which is used to identify the argument. Keyword arguments can be used to pass arguments in any order, making it easier to understand the code. Here’s an example:

   def greet(name, age):
       print(f'Hello, {name}! You are {age} years old.')
   
   greet(age=23, name='Vaun')   # Output: Hello, Varun! You are 23 years old.

4. Variable number of arguments: A function can accept a variable number of arguments using the *args and *kwargs syntax. args is used to accept a variable number of non-keyword arguments, while **kwargs is used to accept a variable number of keyword arguments. Here’s an example:

   def add_numbers(*numbers):
       sum = 0
       for number in numbers:
           sum += number
       return sum
   
   result = add_numbers(5, 10, 15, 20)
   print(result)  # Output: 50

Examples of Python Function Declaration

Here are some examples of How to declare a function in Python:

Example 1: To print a message using a function without taking arguments. A simple function that takes no arguments and prints a message.

    def say_hello():
        print("Hello, world!")

Example 2: Return the value using a function with taking arguments. Another function in Python takes two arguments and returns the sum of the numbers.

    def add_numbers(x, y):
        return x + y

Example 3: A function with default argument values.

    def greet(name, greeting="Hello"):
        print(f"{greeting}, {name}!")

Example 4: A function with variable-length arguments.

    def print_arguments(*args):
        for arg in args:
            print(arg)

Example 5: A function with a docstring (a string that describes the function’s purpose)

    def square(x): 
        """
        Returns the square of a number.
        """
        return x**2

Conclusion
In conclusion, How to declare a function in Python is defined by using a block of code that can be executed repeatedly with different inputs. This block of code is given a name, which can be used to call the function with the desired inputs. The function can also have arguments that modify its behavior. The body of the function consists of a set of statements that are executed when the function is called. The return statement can be used to provide an output from the function.

Frequently Asked Questions (FAQs)

Q1. What is Python function declaration?
Ans: Python function declaration is a self-contained piece of code that serves a particular purpose. It accepts input arguments, executes a set of instructions on them, and produces a result. Functions are integral to programming and facilitate the partitioning of intricate tasks into simpler, more understandable components.

Q2. What are input arguments in a Python function declaration?
Ans: Input arguments are the values that are passed into a function when it is called. They allow you to provide the function with the information it needs to perform its task. Input arguments are defined in the function definition using parentheses, and they can be used within the function’s body.

Q3. Can you have multiple input arguments in a Python function declaration?
Ans: Yes, you can have multiple input arguments in a Python function declaration. You simply separate them with commas within the parentheses in the function definition. Here is an example:

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

In this example, we are defining a function called add_numbers that takes two input arguments called a and b. The function simply adds the two values together and returns the result.

Q4. What do you mean by return statement in a Python function declaration?
Ans: The return statement is used to specify the value that a function should return when it is called. It is optional, but it is required if you want to get a result back from a function. Here is an example:

    def square(number):
        return number * number

In this example, we are defining a function called square that takes one input argument called number. The function calculates the square of the number and returns the result using the return statement.

Q5. Can a Python function declaration return multiple values?
Ans: Yes, a Python function declaration can return multiple values. To do this, you simply separate the values with commas in the return statement. Here is an example:

def get_name_and_age():
    name = "John"
    age = 30
    return name, age

In this example, we are defining a function called get_name_and_age that returns two values: a name and an age. We simply separate the two values with a comma in the return statement.

Q6. What is the difference between a parameter and an argument in a Python function?
Ans: A parameter is a value that is defined within the function definition. An argument is the actual value that is passed into the function when it is called.

Q7. What is the syntax for calling a Python function?
Ans: To call a Python function, use the function name followed by parentheses containing any arguments required by the function.

Q8. Can we define a function within a function in Python?
Ans: Yes, we can define a function within a function in Python. This is called a nested function. The inner function has access to the variables in the outer function’s scope.