Decorators with parameters?

It is well known that the following two pieces of code are nearly equivalent:

@dec
def foo():
    pass    foo = dec(foo)

############################################
foo = dec(foo)

A common mistake is to think that @ simply hides the leftmost argument.

@dec(1, 2, 3)
def foo():
    pass    
###########################################
foo = dec(foo, 1, 2, 3)

It would be much easier to write decorators if the above is how @ worked. Unfortunately, that’s not the way things are done.

---

Consider a decorator Waitwhich haults program execution for a few seconds. If you don't pass in a Wait-time then the default value is 1 seconds. Use-cases are shown below.

##################################################
@Wait
def print_something(something):
    print(something)

##################################################
@Wait(3)
def print_something_else(something_else):
    print(something_else)

##################################################
@Wait(delay=3)
def print_something_else(something_else):
    print(something_else)

When Wait has an argument, such as @Wait(3), then the call Wait(3) is executed before anything else happens.

That is, the following two pieces of code are equivalent

@Wait(3)
def print_something_else(something_else):
    print(something_else)

###############################################
return_value = Wait(3)
@return_value
def print_something_else(something_else):
    print(something_else)

This is a problem.

if `Wait` has no arguments:
    `Wait` is the decorator.
else: # `Wait` receives arguments
    `Wait` is not the decorator itself.
    Instead, `Wait` ***returns*** the decorator

---

One solution is shown below:

Let us begin by creating the following class, DelayedDecorator:

class DelayedDecorator:
    def __init__(i, cls, *args, **kwargs):
        print("Delayed Decorator __init__", cls, args, kwargs)
        i._cls = cls
        i._args = args
        i._kwargs = kwargs
    def __call__(i, func):
        print("Delayed Decorator __call__", func)
        if not (callable(func)):
            import io
            with io.StringIO() as ss:
                print(
                    "If only one input, input must be callable",
                    "Instead, received:",
                    repr(func),
                    sep="\n",
                    file=ss
                )
                msg = ss.getvalue()
            raise TypeError(msg)
        return i._cls(func, *i._args, **i._kwargs)

Now we can write things like:

 dec = DelayedDecorator(Wait, delay=4)
 @dec
 def delayed_print(something):
    print(something)

Note that:

• dec does not not accept multiple arguments.

• dec only accepts the function to be wrapped.

  import inspect class PolyArgDecoratorMeta(type): def call(Wait, *args, **kwargs): try: arg_count = len(args) if (arg_count == 1): if callable(args[0]): SuperClass = inspect.getmro(PolyArgDecoratorMeta)[1] r = SuperClass.call(Wait, args[0]) else: r = DelayedDecorator(Wait, *args, **kwargs) else: r = DelayedDecorator(Wait, *args, **kwargs) finally: pass return r

  import time class Wait(metaclass=PolyArgDecoratorMeta): def init(i, func, delay = 2): i._func = func i._delay = delay

  def __call__(i, *args, **kwargs):
      time.sleep(i._delay)
      r = i._func(*args, **kwargs)
      return r 
  

The following two pieces of code are equivalent:

@Wait
def print_something(something):
     print (something)

##################################################

def print_something(something):
    print(something)
print_something = Wait(print_something)

We can print "something" to the console very slowly, as follows:

print_something("something")

#################################################
@Wait(delay=1)
def print_something_else(something_else):
    print(something_else)

##################################################
def print_something_else(something_else):
    print(something_else)

dd = DelayedDecorator(Wait, delay=1)
print_something_else = dd(print_something_else)

##################################################

print_something_else("something")

---

Final Notes

It may look like a lot of code, but you don't have to write the classes DelayedDecorator and PolyArgDecoratorMeta every-time. The only code you have to personally write something like as follows, which is fairly short:

from PolyArgDecoratorMeta import PolyArgDecoratorMeta
import time
class Wait(metaclass=PolyArgDecoratorMeta):
 def __init__(i, func, delay = 2):
     i._func = func
     i._delay = delay

 def __call__(i, *args, **kwargs):
     time.sleep(i._delay)
     r = i._func(*args, **kwargs)
     return r