Function decorator having arguments is complicated

Steven D'Aprano steve+comp.lang.python at pearwood.info
Mon Apr 27 09:36:15 CEST 2015


On Monday 27 April 2015 12:37, Makoto Kuwata wrote:

> I want to ask Python experts about function decorator which has arguments.
> 
> I feel that function decorator having arguments is complicated,
> because three 'def' are nested:
> 
>   def multiply(n):
>     def deco(func):
>       def newfunc(*args, **kwargs):
>         return n * func(*args, **kwargs)
>       return newfunc
>     return deco

Yes. But not that complicated -- it is only three levels, and the pattern is 
pretty simple:

- outer function takes arguments and returns a decorator;
- middle function is the decorator to be returned;
- inner function is the new function returned by the decorator.

So we start with a regular decorator:

def deco(func):
    def newfunc(*args, **kwargs):
        return n * func(*args, **kwargs)
    return newfunc

indent it one more level, and wrap it in a function:

def multiply(n):
    # as above, indented one extra level


What's really tricky is a decorator which can *optionally* take arguments or 
not:

@decorator(x, y)
def function(): ...


# not the same as @decorate()
@decorate
def function(): ...


> If function decorator notation could take arguments,
> decorator definition would be more simple:

Yes, but then we have to learn more syntax:

@decorate
@decorate n
@decorate(n)
@decorate()  # uses the default for multiply

could all be different.

Although I agree with you that writing a decorator factory is a bit 
complicated, it is simple to break it up into individual sub-steps:

* I know how to write a function that adapts or wraps another 
  function.

* I know how to write a decorator: write a function, indent it 
  one extra level, and wrap it in an outer function;

* I know how to write a decorator factory: write a decorator, 
  indent it one extra level, and wrap it in an outer function;

which could even be extended:

* I know how to write a decorator-factory factory: write a 
  decorator factory, indent it one extra level, and wrap it 
  in an outer function.


Saving one layer is not important enough to justify specialised syntax.



-- 
Steve




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