[Python-ideas] Problems (and solutions?) in writing decorators

Sylvain MARIE sylvain.marie at se.com
Tue Mar 12 09:52:41 EDT 2019

David, Steven,

Thanks for your interest !

As you probably know, decorators and function wrappers are *completely different concepts*. A decorator can directly return the decorated function (or class), it does not have to return a wrapper. Even more, it can entirely replace the decorated item with something else (not even a function or class!). Try it: it is possible to write a decorator to replace a function with an integer, even though it is probably not quite useful :)

`decopatch` helps you write decorators, whatever they are. It "just" solves the annoying issue of having to handle the no-parenthesis and with-parenthesis calls. In addition as a 'goodie', it proposes two development styles: *nested* (you have to return a function) and *flat* (you directly write what will happen when the decorator is applied to something).
Now about creating signature-preserving function wrappers (in a decorator, or outside a decorator - again, that's not related). That use case is supposed to be covered by functools.wrapt. Unfortunately as explained here https://stackoverflow.com/questions/308999/what-does-functools-wraps-do/55102697#55102697 this is not the case because with functools.wrapt:
 - the wrapper code will execute even when the provided arguments are invalid.
 - the wrapper code cannot easily access an argument using its name, from the received *args, **kwargs. Indeed one would have to handle all cases (positional, keyword, default) and therefore to use something like Signature.bind().

For this reason I proposed a replacement in `makefun`: https://smarie.github.io/python-makefun/#signature-preserving-function-wrappers 
Now bridging the gap. Of course a very interesting use cases for decorators is to create decorators that create a signature-preserving wrapper. It is possible to combine decopatch and makefun for this: https://smarie.github.io/python-decopatch/#3-creating-function-wrappers .
Decopatch even proposes a "double-flat" development style where you directly write the wrapper body, as explained in the doc.

Did I answer your questions ?
Thanks again for the quick feedback !


-----Message d'origine-----
De : Python-ideas <python-ideas-bounces+sylvain.marie=se.com at python.org> De la part de Steven D'Aprano
Envoyé : mardi 12 mars 2019 12:30
À : python-ideas at python.org
Objet : Re: [Python-ideas] Problems (and solutions?) in writing decorators

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On Tue, Mar 12, 2019 at 09:36:41AM +0000, Sylvain MARIE via Python-ideas wrote:

> I therefore proposed
> https://emea01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fsma
> rie.github.io%2Fpython-makefun%2F&data=02%7C01%7Csylvain.marie%40s
> e.com%7C579232e7e10e475314c708d6a6de9d23%7C6e51e1adc54b4b39b5980ffe9ae
> 68fef%7C0%7C0%7C636879872385158085&sdata=nB9p9V%2BJ7gk%2Fsc%2BA5%2
> Fekk35bnYGvmEFJyCXaLDyLm9I%3D&reserved=0 . In particular it 
> provides an equivalent of `@functools.wraps` that is truly 
> signature-preserving

Tell us more about that please. I'm very interested in getting decorators preserve the original signature.

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