A decorator is a design pattern that allows you to dynamically alter the behavior of a function without modifying its source code. It typically takes a function as an argument and returns a new function that adds extra logic. Instead of calling the wrapper manually, Python's @ syntax applies it cleanly.

Here is a basic implementation that logs activity before and after a target function runs:

def monitor(target_func):
    def interceptor(value):
        print('--- Execution Start ---')
        target_func(value)
        print(f"Target function: {target_func.__name__}")
        print('--- Execution End ---')
    return interceptor

@monitor
def display_message(content):
    print(f"Message received: {content}")

display_message('Hello Decorators')

Output:

--- Execution Start ---
Message received: Hello Decorators
Target function: display_message
--- Execution End ---

Decorators with Configuration Arguments

When you need too pass specific settings to a decorator, the structure expands to three nested levels. The outermost layer captures the configuration, the middle layer captures the function, and the innermost layer executes the logic.

def configure_logger(prefix):
    def attach_decorator(method):
        def handler(*arguments, **keywords):
            print(f'Invoking: {method.__name__}')
            print(f'Input data: {arguments[0]}')
            print(f'Log prefix: {prefix}')
            return method(*arguments, **keywords)
        return handler
    return attach_decorator

@configure_logger("System Check")
def process_data(payload):
    print(f"Processing: {payload}")

process_data("Sample Payload")

If the @ syntax is confusing, you can deconstruct it into standard functon calls to see the data flow:

# 1. Pass the argument to the decorator factory
factory = configure_logger("System Check")

# 2. Pass the actual function to the factory result
wrapped_handler = factory(process_data)

# 3. Execute the final wrapped function
wrapped_handler("Sample Payload")

Output:

Invoking: process_data
Input data: Sample Payload
Log prefix: System Check
Processing: Sample Payload