[Tutor] Writing decorators?

[Alan Gauld]

On 05/07/16 14:22, Alex Hall wrote:

> To simplify things, what might be an example of a decorator that, say,
> prints "decorated" before whatever string the decorated function prints?

> My attempt would be:
> 
> def prependDecorated(f):
>   def prepend():
>     return "decorated"+f()
>   #something should go at this level too?

Recall that a decorator is:

a function
that takes a function as its argument
and returns a function

Your code fails on the third item.

Lets take a trivial example first, a decorator that
does nothing. Define a function that takes a function
and returns the same function untouched:

>>> def donothing(f): return f

Now apply it to a square() function:

>>> @donothing
def square(x): return x*x

>>> square
<function square at 0x7f3633fcb0d0>
>>> square(4)
16

We could do the same without the @ shorthand by using

square2 = donothing(square)

But the @ syntax makes it more readable.

Now lets look at your task
We need a function that takes a function and
returns a function that prepends a string:

def prepend(f):
    def add_string(*args, **kwargs):  # in case f takes arguments
        return "decorated "+ str(f(*args,**kwargs))
    return add_string

Now we can apply that to a function

@prepend
def cube(n): return n*n*n

cube(3)

I'm biased because I was the tech editor but the book
Professional Python has a nice chapter on decorators.

HTH

-- 
Alan G
Author of the Learn to Program web site
http://www.alan-g.me.uk/
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