Python def Keyword

In Python, the def keyword is used to define functions. A function is a block of reusable code that performs a specific task. Using functions helps keep code organized, reduces redundancy, and makes programs easier to read and maintain.

Syntax

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def function_name(parameters):
    """Optional docstring"""
    # Function body
    return value  # (Optional) Return statement

Components of a Function

Component	Description
`def`	Keyword used to define a function.
`function_name`	Name of the function (should be meaningful and follow Python naming conventions).
`parameters`	Optional values passed to the function for processing.
`docstring`	Optional multi-line string that describes what the function does.
`function body`	Contains the statements that define the function’s behavior.
`return`	Optional statement that sends a value back to the caller.

Return Value

A function may or may not return a value. If a function has a return statement, it sends the computed result back to the caller. Otherwise, it returns None by default.

Examples

1. Defining and Calling a Simple Function

In this example, we define a function that prints a message and then call it.

We use the def keyword to define a function called greet. The function does not take any parameters and simply prints “Hello, Python!”. To execute the function, we call it by writing its name followed by parentheses: greet().

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def greet():
    print("Hello, World!")

# Calling the function
greet()

Output:

Hello, World!

2. Function with Parameters

We can pass values (parameters) to functions to customize their behavior.

Here, we define a function greet_user that takes a parameter name. When the function is called, it prints a greeting message using the provided name.

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def greet_user(name):
    print("Hello,", name, "!")

# Calling the function with an argument
greet_user("Arjun")

Output:

Hello, Arjun!

3. Function with Return Value

Functions can return values using the return statement.

Here, we define a function square that takes a number as input and returns its square. The return statement sends the computed value back to the caller.

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def square(num):
    return num * num

# Calling the function and storing the returned value
result = square(4)
print("Square of 4:", result)

Output:

Square of 4: 16

4. Handling Errors: Missing Arguments

If a function expects an argument but is called without one, Python raises a TypeError.

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def greet_user(name):
    print("Hello,", name, "!")

# Calling the function without an argument (this will raise an error)
try:
    greet_user()
except TypeError as e:
    print("Error:", e)

Output:

Error: greet_user() missing 1 required positional argument: 'name'

To fix this, we should always provide an argument when calling the function or define a default parameter value.

5. Function with Default Parameter

We can provide default values for parameters so that the function works even if an argument is not provided.

Here, the greet_user function has a default value of "Guest" for the name parameter. If no argument is provided, it uses the default value.

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def greet_user(name="Guest"):
    print("Hello,", name, "!")

# Calling function with and without argument
greet_user("Arjun")
greet_user()  # Uses default value

Output:

Hello, Arjun!
Hello, Guest!

Using default values makes functions more flexible and prevents errors when arguments are missing.

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Python def Keyword