Python Decorators

What Is A Decorator?

A decorator is a function that takes another function and returns a new function with extra behavior. Decorators are useful when you want to add the same behavior to many functions without copying code.

def simple_decorator(func):
    def wrapper():
        print("Before")
        func()
        print("After")
    return wrapper

The wrapper function controls what happens before and after the original function.

Using The @ Syntax

The @decorator_name syntax is a shortcut.

@simple_decorator
def greet():
    print("Hello")

This is similar to writing:

greet = simple_decorator(greet)

Now calling greet() actually calls the wrapper returned by the decorator.

Handling Arguments

Most real decorators need to accept arguments for the wrapped function.

def log_call(func):
    def wrapper(*args, **kwargs):
        print("Calling function")
        return func(*args, **kwargs)
    return wrapper

*args collects positional arguments, and **kwargs collects keyword arguments.

Preserving Function Metadata

Use functools.wraps so the wrapper keeps the original function’s name and documentation.

from functools import wraps

def log_call(func):
    @wraps(func)
    def wrapper(*args, **kwargs):
        print("Calling function")
        return func(*args, **kwargs)
    return wrapper

This helps debugging and documentation tools.

When Decorators Help

Decorators are common for logging, timing, access checks, caching, validation, and framework behavior. They are powerful, but they can also make code harder to trace if overused.

Use a decorator when the added behavior is reusable and separate from the main purpose of the function.

Common Mistakes

A common mistake is forgetting to return the wrapper from the decorator. Another is forgetting to return the original function’s result from inside the wrapper.

def wrapper(*args, **kwargs):
    func(*args, **kwargs)

If the wrapped function returns a value, this wrapper loses it. Use:

return func(*args, **kwargs)

Quick Summary

• A decorator wraps a function with extra behavior.
• @decorator is shorthand for reassigning the function.
• A wrapper usually calls the original function.
• Use *args and **kwargs to support flexible arguments.
• Use functools.wraps to preserve metadata.
• Return the wrapped function’s result when needed.

Practice Quiz

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