[python] how decorator works

source: http://stackoverflow.com/questions/308999/what-does-functools-wraps-do

When you use a decorator, you're replacing one function with another. In other words, if you have a decorator

The left colume below is the exactly same as the right one.

def logged(func): def with_logging(*args, **kwargs): print func.__name__ + " was called" return func(*args, **kwargs) return with_logging @logged def f(x): """does some math""" return x + x * x

def logged(func): def with_logging(*args, **kwargs): print func.__name__ + " was called" return func(*args, **kwargs) return with_logging def f(x): """does some math""" return x + x * x f = logged(f)

In [73]: f??

Type:       function

String Form:<function with_logging at 0x7fa6d574faa0>

File:       /home/jinuine/<ipython-input-65-cf67ff536232>

Definition: f(*args, **kwargs)

Source:

        def with_logging(*args, **kwargs):

                print func.__name__ + " was called"

                return func(*args, **kwargs)

In [74]: f(3)
f was called
Out[74]: 12


------------------------------------------------------------------------------------------------------------------

how to use @decorator as @overrider from java.

source: http://stackoverflow.com/questions/1167617/in-python-how-do-i-indicate-im-overriding-a-method

def overrides(interface_class): def overrider(*args, **kwargs): print args, kwargs method = args[0] assert(method.__name__ in dir(interface_class)) return method return overrider class MySuperInterface(object): def my_method(self, name): print 'hello world! %s' % (name) class ConcreteImplementer(MySuperInterface): @overrides(MySuperInterface) def my_method(self): print 'hello kitty!, %s' % (name) 위의 클래스 선언시 아래가 찍힘. (<function my_method at 0x251c1b8>,) {}

def overrides(interface_class): def overrider(method): assert(method.__name__ in dir(interface_class)) return method return overrider class MySuperInterface(object): def my_method(self, name): print 'hello world! %s' % (name) class ConcreteImplementer(MySuperInterface): @overrides(MySuperInterface) def my_method(self): print 'hello kitty!, %s' % (name)

--------------------------------------------------------------------------------------------------------------------
source: http://stackoverflow.com/questions/739654/how-can-i-make-a-chain-of-function-decorators-in-python?rq=1

How can I make a chain of function decorators in Python?

def makebold(fn):
    def wrapped():
        return "<b>" + fn() + "</b>"
    return wrapped

def makeitalic(fn):
    def wrapped():
        return "<i>" + fn() + "</i>"
    return wrapped

@makebold
@makeitalic
def hello():
    return "hello world"

print hello() ## returns <b><i>hello world</i></b>

-----------------------------------------------------------------------------------------------------------------
source: http://stackoverflow.com/questions/739654/how-can-i-make-a-chain-of-function-decorators-in-python?rq=1

Alternatively, you could write a factory function which return a decorator which wraps the return value of the decorated function in a tag passed to the factory function. For example:

from functools import wraps

def wrap_in_tag(tag):
    def factory(func):
        @wraps(func)
        def decorator():
            return '<%(tag)s>%(rv)s</%(tag)s>' % (
                {'tag': tag, 'rv': func()})
        return decorator
    return factory

This enables you to write:

@wrap_in_tag('b')
@wrap_in_tag('i')
def say():
    return 'hello'

or

makebold = wrap_in_tag('b')
makeitalic = wrap_in_tag('i')

@makebold
@makeitalic
def say():
    return 'hello'

Personally I would have written the decorator somewhat differently:

from functools import wraps

def wrap_in_tag(tag):
    def factory(func):
        @wraps(func)
        def decorator(val):
            return func('<%(tag)s>%(val)s</%(tag)s>' %
                        {'tag': tag, 'val': val})
        return decorator
    return factory

which would yield:

@wrap_in_tag('b')
@wrap_in_tag('i')
def say(val):
    return val
say('hello')

Don't forget the construction for which decorator syntax is a shorthand:

say = wrap_in_tag('b')(wrap_in_tag('i')(say)))