Python decorator order

decorators.md

What order does Python execute decorators in? It seems like this changed sometime between when they were first introduced (2.4) and now (2.7).

Given the code:

@foo
@bar
@baz
def myfunc():
    pass

Is the decorator calling order baz -> bar -> foo, or is it foo -> bar -> baz? It used to be from the bottom to the top (the decorator closest to your function def runs first), but now it seems to be run from top to bottom.

Or rather, is the code equivalent to foo(bar(baz(myfunc))) (bottom to top) or baz(bar(foo(myfunc))) (top to bottom)

Links that support one vs. the other:

• Bottom to top (inside-out) order:
  • Multiple Decorators for python functions
  • PEP 318 - See "order of application" (bottom to top)
  • Python decorators finally demystified - @bread, @ingredients def sandwich => bread(ingredients(sandwich))
• Top to bottom (code) order:
  • How can I make a chain of function decorators in Python?
  • A guide to Python decorators @div_decorate, @p_decorate, @strong_decorate => <div><p><strong>...</strong></p></div>

Test script:

#!/usr/bin/env python

from functools import wraps

def login_required(f):
    @wraps(f)
    def decorated_function(*args, **kwargs):
        print "login_required called!"
        return f(*args, **kwargs)
    return decorated_function


def ajax_request(f):
    @wraps(f)
    def decorated_function(*args, **kwargs):
        print "ajax_request called!"
        return f(*args, **kwargs)
    return decorated_function


@login_required
@ajax_request
def test1():
    print "Test 1, login above ajax"


@ajax_request
@login_required
def test2():
    print "Test 2, ajax above login"


test1()
print "=" * 80
test2()

Output (Python 2.7.5):

** login_required called!
** ajax_request called!
Test 1, login above ajax
================================================================================
** ajax_request called!
** login_required called!
Test 2, ajax above login

http://bugs.python.org/issue1009444

This was back in Python 2.4. See this commit: python/cpython@ce220b0#diff-cb296cc5109f5640ff3f6d7198a6abeeL4135.

That issue implies that the decorators would execute from bottom to top, but my test script on Python 2.7.5 executed them from top to bottom.

@login_required
@ajax_request
def test1():
    print "Test 1, login above ajax"

** login_required called!
** ajax_request called!
Test 1, login above ajax

Unless there's something wrong with my test script.

If someone is still looking at it, the decorators are executed from top to bottom. You can read the docs or check it by trial.

Here is a code example that helps to remember

def decorator(extra):
    extra = f" {extra}"
    def inner(func):
        def wrapper(*args, **kwargs):
            print(f"before {func.__name__}{extra}")
            func(*args, **kwargs)
            print(f"after {func.__name__}{extra}")
        return wrapper
    return inner

@decorator('first')
@decorator('middle')
@decorator('last')
def hello():
    print('Hello')

hello()

This outputs

before wrapper first
before wrapper middle
before hello last
Hello
after hello last
after wrapper middle
after wrapper first

I feel like the before and after of the onion skinning is what mostly confuses people.
But also, notice the definition of hello is only passed to the first in the onion skin. What is passed to the second is the wrapped hello output of the @decorator('last'). Don't confuse execution order with definition order.

decorator("first")(decorator("second")(decorator("third")(hello)))()