[Python-Dev] order of decorator application? (original) (raw)

Phillip J. Eby pje at telecommunity.com
Tue Mar 23 11:42:29 EST 2004

At 10:24 AM 3/23/04 -0600, Skip Montanaro wrote:

(I'm working on PEP 318...)

Is there a concensus on the order of application for decorators? Should def func(a,b) [d1,d2]: pass be equivalent to def func(a,b): pass func = d1(d2(func))

No.

or

def func(a,b): pass func = d2(d1(func))

Almost. :) Try this phrasing, which provides its own justification for the interpretation:

def func(a,b): pass

for decorator in [d1,d2]: func = decorator(func)

This is still "almost", because 'func' should only be bound once, at the conclusion of the operation. IOW, more like:

def _temp(a,b): pass

_temp.name = "func"

for decorator in [d1,d2]: _temp = decorator(_temp)

func = _temp

In practice, the compiler can and should unroll the loop, keep the intermediate values on the stack, and have the right function name assigned from the start. But the specification should require that 'func' not be rebound until the operation is concluded. This allows multi-function objects to be accumulated, such as for properties, generic functions, and so on, without losing the "previous" definition. For example, I should be able to do something like:

def some_attr(self) [getter]:
    ...

def some_attr(self,value) [setter]:
    ...

And have the first 'some_attr' value in effect when 'setter()' is called on the second 'some_attr'. This is also important for creating generic functions, or doing signature overloading for interfacing with languages like C++ or Java.

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