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

[David Mertz]

The wrapt module I linked to (not funtools.wraps) provides all the
capabilities you mention since 2013. It allows mixed use of decorators as
decorator factories. It has a flat style.

There are some minor API difference between your libraries and wrapt, but
the concept is very similar. Since yours is something new, I imagine you
perceive some win over what wrapt does.

On Tue, Mar 12, 2019, 9:52 AM Sylvain MARIE <sylvain.marie at se.com> wrote:

> 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 !
> Best,
>
> Sylvain
>
> -----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
>
> [External email: Use caution with links and attachments]
>
> ________________________________
>
>
>
> 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.
>
>
> --
> Steven
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