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

David Mertz mertz at gnosis.cx
Tue Mar 12 10:29:57 EDT 2019

The documentation for wrapt mentions:

Decorators With Optional Arguments

Although opinion can be mixed about whether the pattern is a good one, if
the decorator arguments all have default values, it is also possible to
implement decorators which have optional arguments.
As Graham hints in his docs, I think repurposing decorator factories as
decorators is an antipattern. Explicit is better than implicit.

While I *do* understands that what decotools and makefun do are technically
independent, I'm not sure I ever want them independently in practice. I did
write the book _Functional Programming in Python_, so I'm not entirely
unfamiliar with function wrappers.

On Tue, Mar 12, 2019, 10:18 AM David Mertz <mertz at gnosis.cx> wrote:

> 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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