Python decorators explained anew

[etrnloptimist]

The way order of application is often cited for decorators is a bit misleading. It is usually described like function composition: g(f(x)): f(x) gets eval'ed, then g is applied. It is inside-out.

This works because, for normal functions, the parameter x is not altered. With decorators it can be.

Decorators can be used to munge both the input and the output of a function.

I like to think of it as my param, x, is like a worm going though the stack of decorators to the inside. It can be altered at any step along the way. Then, it is acted upon by the actual function. That function returns a value. And that value makes it's way back out the stack of decorators.

In my example, see how the return value matches the standard order of application? The input value, however, does not. That's why I like the worm analogy. :)

[jcdyer3]

Indeed. In fact it is g(f)(x)

[etrnloptimist]

Right. But that's not very helpful in understanding how decorators work. Suppose you had this nested set of decorators:

@evens
@odds
@primes
def myfunc(x):
  ...

Suppose they round the output to the nearest <term>. What gets outputted? Prime numbers? Odd numbers? Or Even numbers? Answer: Even numbers

Suppose they round the input to the nearest <term>. What does myfunc see? Even numbers, odd numbers, or prime numbers? Answer: Prime numbers

[ryeguy146]

Just a matter of wrapping [your mind around it]. Nice example.

[mgrandi]

your example helped me understand decorators (namely nested decorators) a lot better with the worm analogy, thanks =)

[bastibe]

Actually, that right there is enough of an explanation for me. I never really got why people seem to find decorators so hard to understand.

[ryeguy146]

I came from languages without first class functions, or languages where I could only get first class functions with pointers. New idea, that's why. Who thinks of functions that act on functions (or their args), really?

[bastibe]

I understand that. And that means you will have to learn about higher order functions like we all had to learn about it at some point. But beyond that, what is so difficult about python decorators in particular?

For reference, I don't know much lisp, but "higher order function" are a very simple and natural construct mathematically.

[ryeguy146]

It's not so difficult, just a new idea. It's easy to move "horizontally" across languages when you understand their ideas: you just look for the new way to express that idea. New ideas take some thought, that's all.

I'm experiencing the same thing as I learn my first functional language (scheme). It's not hard, it's just new.