What does a Python decorator do, and where is its code? [duplicate]

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Understanding Python decorators

What does a Python decorator do? And where can I see the codes that are running when I add a decorator to a method?

For example, when I add @login_required at the top of a method, does any code replace that line? How exactly does this line check the user session?

Answer 1

when I add @login_required at the top of a method, does any code replace that line?

Kind of. Adding @login_required before your view function has the same effect as doing this:

def your_view_function(request):
    # Function body

your_view_function = login_required(your_view_function)

For explanations of decorators in Python, see:

• http://www.python.org/dev/peps/pep-0318/
• http://wiki.python.org/moin/PythonDecorators#What_is_a_Decorator

So decorator functions accept an original function, and return a function that (probably) calls the original function, but does something else too.

In the case of login_required, I think it checks the request object passed to view functions to see if the user is authenticated.

Answer 2

A decorator is in effect a function wrapping another function or class. The function behind the decorator in your case is named login_required. Look in your imports to find it.

Answer 3

A decorator is a function which wraps another function. Say you have a function f(x) and you have a decorator h(x), the decorator function takes as argument your function f(x) and so in effect what you will have is a new function h(f(x)). It makes for cleaner code as for instance in your login_required, you do not have to put in the same code to test if a user is logged in rather you can wrap the function in a login_required function so that such a function is called only if the user is logged in. Study this snippet below

def login_required(restricted_func):
"""Decorator function for restricting access to restricted pages.
Redirects a user to login page if user is not authenticated.
Args:
    a function for returning a restricted page
Returns:
    a function 
"""
def permitted_helper(*args, **kwargs):
    """tests for authentication and then call restricted_func if
    authenticated"""
    if is_authenticated():
        return restricted_func(*args, **kwargs)
    else:
        bottle.redirect("/login")
return permitted_helper