Decorators with parameters?

Question

I have a problem with the transfer of variable \'insurance_mode\' by the decorator. I would do it by the following decorator statement:

@execute_complete_rese         


        

Answer 1

The syntax for decorators with arguments is a bit different - the decorator with arguments should return a function that will take a function and return another function. So it should really return a normal decorator. A bit confusing, right? What I mean is:

def decorator_factory(argument):
    def decorator(function):
        def wrapper(*args, **kwargs):
            funny_stuff()
            something_with_argument(argument)
            result = function(*args, **kwargs)
            more_funny_stuff()
            return result
        return wrapper
    return decorator

Here you can read more on the subject - it's also possible to implement this using callable objects and that is also explained there.

Answer 2

I'd like to show an idea which is IMHO quite elegant. The solution proposed by t.dubrownik shows a pattern which is always the same: you need the three-layered wrapper regardless of what the decorator does.

So I thought this is a job for a meta-decorator, that is, a decorator for decorators. As a decorator is a function, it actually works as a regular decorator with arguments:

def parametrized(dec):
    def layer(*args, **kwargs):
        def repl(f):
            return dec(f, *args, **kwargs)
        return repl
    return layer

This can be applied to a regular decorator in order to add parameters. So for instance, say we have the decorator which doubles the result of a function:

def double(f):
    def aux(*xs, **kws):
        return 2 * f(*xs, **kws)
    return aux

@double
def function(a):
    return 10 + a

print function(3)    # Prints 26, namely 2 * (10 + 3)

With @parametrized we can build a generic @multiply decorator having a parameter

@parametrized
def multiply(f, n):
    def aux(*xs, **kws):
        return n * f(*xs, **kws)
    return aux

@multiply(2)
def function(a):
    return 10 + a

print function(3)    # Prints 26

@multiply(3)
def function_again(a):
    return 10 + a

print function(3)          # Keeps printing 26
print function_again(3)    # Prints 39, namely 3 * (10 + 3)

Conventionally the first parameter of a parametrized decorator is the function, while the remaining arguments will correspond to the parameter of the parametrized decorator.

An interesting usage example could be a type-safe assertive decorator:

import itertools as it

@parametrized
def types(f, *types):
    def rep(*args):
        for a, t, n in zip(args, types, it.count()):
            if type(a) is not t:
                raise TypeError('Value %d has not type %s. %s instead' %
                    (n, t, type(a))
                )
        return f(*args)
    return rep

@types(str, int)  # arg1 is str, arg2 is int
def string_multiply(text, times):
    return text * times

print(string_multiply('hello', 3))    # Prints hellohellohello
print(string_multiply(3, 3))          # Fails miserably with TypeError

A final note: here I'm not using functools.wraps for the wrapper functions, but I would recommend using it all the times.

Answer 3

In case both the function and the decorator have to take arguments you can follow the below approach.

For example there is a decorator named decorator1 which takes an argument

@decorator1(5)
def func1(arg1, arg2):
    print (arg1, arg2)

func1(1, 2)

Now if the decorator1 argument has to be dynamic, or passed while calling the function,

def func1(arg1, arg2):
    print (arg1, arg2)


a = 1
b = 2
seconds = 10

decorator1(seconds)(func1)(a, b)

In the above code

• seconds is the argument for decorator1
• a, b are the arguments of func1

Answer 4

In my instance, I decided to solve this via a one-line lambda to create a new decorator function:

def finished_message(function, message="Finished!"):

    def wrapper(*args, **kwargs):
        output = function(*args,**kwargs)
        print(message)
        return output

    return wrapper

@finished_message
def func():
    pass

my_finished_message = lambda f: finished_message(f, "All Done!")

@my_finished_message
def my_func():
    pass

if __name__ == '__main__':
    func()
    my_func()

When executed, this prints:

Finished!
All Done!

Perhaps not as extensible as other solutions, but worked for me.

Answer 5

I presume your problem is passing arguments to your decorator. This is a little tricky and not straightforward.

Here's an example of how to do this:

class MyDec(object):
    def __init__(self,flag):
        self.flag = flag
    def __call__(self, original_func):
        decorator_self = self
        def wrappee( *args, **kwargs):
            print 'in decorator before wrapee with flag ',decorator_self.flag
            original_func(*args,**kwargs)
            print 'in decorator after wrapee with flag ',decorator_self.flag
        return wrappee

@MyDec('foo de fa fa')
def bar(a,b,c):
    print 'in bar',a,b,c

bar('x','y','z')

Prints:

in decorator before wrapee with flag  foo de fa fa
in bar x y z
in decorator after wrapee with flag  foo de fa fa

See Bruce Eckel's article for more details.

Answer 6

Edit : for an in-depth understanding of the mental model of decorators, take a look at this awesome Pycon Talk. well worth the 30 minutes.

One way of thinking about decorators with arguments is

@decorator
def foo(*args, **kwargs):
    pass

translates to

foo = decorator(foo)

So if the decorator had arguments,

@decorator_with_args(arg)
def foo(*args, **kwargs):
    pass

translates to

foo = decorator_with_args(arg)(foo)

decorator_with_args is a function which accepts a custom argument and which returns the actual decorator (that will be applied to the decorated function).

I use a simple trick with partials to make my decorators easy

from functools import partial

def _pseudo_decor(fun, argument):
    def ret_fun(*args, **kwargs):
        #do stuff here, for eg.
        print ("decorator arg is %s" % str(argument))
        return fun(*args, **kwargs)
    return ret_fun

real_decorator = partial(_pseudo_decor, argument=arg)

@real_decorator
def foo(*args, **kwargs):
    pass

Update:

Above, foo becomes real_decorator(foo)

One effect of decorating a function is that the name foo is overridden upon decorator declaration. foo is "overridden" by whatever is returned by real_decorator. In this case, a new function object.

All of foo's metadata is overridden, notably docstring and function name.

>>> print(foo)
<function _pseudo_decor.<locals>.ret_fun at 0x10666a2f0>

functools.wraps gives us a convenient method to "lift" the docstring and name to the returned function.

from functools import partial, wraps

def _pseudo_decor(fun, argument):
    # magic sauce to lift the name and doc of the function
    @wraps(fun)
    def ret_fun(*args, **kwargs):
        #do stuff here, for eg.
        print ("decorator arg is %s" % str(argument))
        return fun(*args, **kwargs)
    return ret_fun

real_decorator = partial(_pseudo_decor, argument=arg)

@real_decorator
def bar(*args, **kwargs):
    pass

>>> print(bar)
<function __main__.bar(*args, **kwargs)>