Clash Royale CLAN TAG#URR8PPP

Understanding decorators in Django

I'm currently trying to build my own decorator for function based views.

I understood the basic concept behind decorators but I'm still struggling to fully understand whats happening "behind the scenes".

I've this example decorator. Could someone explain me what happens here or how it works? And why do we need so many functions here?

def active_user_required():

def decorator(function):
def wrapper(request, *args, **kw):
if request.user.is_active:
return function(request, *args, **kw)
else:
raise PermissionDenied()
return wrapper
return decorator


@active_user_required

@active_user_required()

1 Answer
1

The key to understanding decorators in Python boils down to this: functions in Python are first-class objects. That means they can be passed as arguments and returned as values (among other things).

In the simplest form, a decorator takes a function as an argument and returns a new function that extends its functionality. For example, here is a decorator plusone that adds 1 to the original function's return value:

plusone

>>> def plusone(f):
... def wraps(*a):
... return f(*a) + 1
... return wraps
...
>>>
>>> @plusone
... def add(x, y):
... return x + y
...
>>>
>>> add(1, 2)
4


Reiterating, the important point about plusone isn't that it "adds 1", it's that it creates a new function (called wraps in this case) and returns that. On the last line, when add(1, 2) is called, it's actually the function wraps that is getting called (which, recall, calls your original add function). And you can prove it:

plusone

wraps

add(1, 2)

wraps

add

>>> add
<function wraps at 0x107b88578>


(Without the decorator, that would say <function add at 0x...>)

<function add at 0x...>

This can be extended to more complicated cases, such as decorators that take arguments of their own, but I don't think those details are to the root of your question. The key takeaway once more is that a decorator takes a function as an argument and then returns a function (and that returned function almost always extends the passed function in some way).

Hope that helps.

add

wraps

*a

**kw

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