decorator syntax: allow testlist instead of just dotted_name

[james]

BPO	19660
Nosy	@gvanrossum, @ncoghlan, @ericvsmith, @benjaminp, @merwok, @ericsnowcurrently, @berkerpeksag, @dirn, @jdemeyer, @gvanrossum, @Vgr255, @brandtbucher
Superseder	bpo-39702: PEP 614: Relaxing Grammar Restrictions On Decorators
Files	decorator-syntax.patch decorator-syntax.patch decorator-syntax.patch

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2020-02-20.16:20:55.152>
created_at = <Date 2013-11-20.01:27:05.121>
labels = ['interpreter-core', 'type-feature']
title = 'decorator syntax: allow testlist instead of just dotted_name'
updated_at = <Date 2020-02-20.16:20:55.151>
user = 'https://bugs.python.org/james'

bugs.python.org fields:

activity = <Date 2020-02-20.16:20:55.151>
actor = 'brandtbucher'
assignee = 'none'
closed = True
closed_date = <Date 2020-02-20.16:20:55.152>
closer = 'brandtbucher'
components = ['Interpreter Core']
creation = <Date 2013-11-20.01:27:05.121>
creator = 'james'
dependencies = []
files = ['32717', '32741', '32745']
hgrepos = []
issue_num = 19660
keywords = ['patch']
message_count = 20.0
messages = ['203456', '203531', '203532', '203538', '203539', '203554', '203555', '203569', '203728', '203743', '239697', '271744', '271750', '271755', '271758', '271762', '271764', '271767', '321307', '334141']
nosy_count = 15.0
nosy_names = ['gvanrossum', 'ncoghlan', 'eric.smith', 'benjamin.peterson', 'eric.araujo', 'infinity0', 'eric.snow', 'berker.peksag', 'dirn', 'jdemeyer', 'Guido.van.Rossum', 'james', 'peyton', 'abarry', 'brandtbucher']
pr_nums = []
priority = 'normal'
resolution = 'duplicate'
stage = 'resolved'
status = 'closed'
superseder = '39702'
type = 'enhancement'
url = 'https://bugs.python.org/issue19660'
versions = ['Python 3.6']

[james]

Decorator syntax currently allows only a dotted_name after the @. As far as I can tell, this was a gut-feeling decision made by Guido. [1]

I spoke with Nick Coghlan at PyTexas about this, and he suggested that if someone did the work, there might be interest in revisiting this restriction.

The attached patch allows any testlist to follow the @.

The following are now valid:

@(lambda x:x)
def f():
pass

@(spam if p else eggs)
def f():
    pass

@spam().ham().eggs()
def f():
    pass

[1] http://mail.python.org/pipermail/python-dev/2004-August/046711.html

[merwok]

Thanks for this! Tests should exercise the now-valid syntaxes, which also need documentation.

[merwok]

On second thought, as this patch allows one form that Guido doesn’t want (bar().foo()), maybe there should be a discussion on python-ideas.

[ncoghlan]

Nice! As a syntax change (albeit a minor one), I believe this will require a PEP for Python 3.5.

I know Guido indicated he was OK with relaxing the current restrictions, but I don't remember exactly where he said it, or how far he was willing to relax them.

[gvanrossum]

I don't feel very strongly, but I do think that most of the things the new syntax allows are not improvements -- they make the decorator harder to read. It was intentional to force you to compute a variable before you can use it as a decorator, e.g.

spamify = (spam if p else eggs)

@spamify
def f():
    pass

[ericsnowcurrently]

they make the decorator harder to read.

I agree.

[james]

I see this as removing a restriction and a special-case from the
decorator syntax (noting, of course, that these were introduced
deliberately.)

In terms of whether the new forms are improvements, my preference is to
leave this up to the judgement of the programmer, moderated of course by
their prevailing coding guide.

I would argue that this change does not force any additional complexity
on the programmer (who is free to take or leave it) or on the
interpreter (- the straightforwardness of the patch corroborates this.)

I would also argue that there are certainly cases where, in the midst of
some large codebase, the dotted_name restriction may seem a bit arbitrary.

This is likely true for:

class Foo:
    def bar(self, func):
        return func

    @staticmethod
    def baz(func):
        return func
	
    @staticmethod
    def quux():
        def dec(func):
            return func
        return dec

# invalid
@Foo().bar
def f(): pass

# valid
@Foo.baz
def f(): pass

# valid
@Foo.quux()
def f(): pass

For completeness' sake, I have attached a patch with an additional unit
test and amended documentation.

Should we proceed with writing a PEP for Python 3.5?

[ncoghlan]

Yes, a PEP for 3.5 on this will be valuable, whether it's accepted or not
(although I personally favour moving these restrictions out of the compiler
and into the PEP-8 style guide).

If I recall the past python-ideas threads correctly, the main objections to
the current syntax restrictions were:

• you can't look up decorators through a registry by default, since
  "@registry[name]" is disallowed
• it's not really a limitation anyway, since a pass through function still
  lets you write whatever you want:

    def deco(x): return x

@deco(registry[name])
def f(): ...

Now that the precedent of keeping decorator expressions simple has been
well and truly established, simplification of the grammar is the main
reason removing the special casing of decorator syntax from the compilation
toolchain appeals to me.

[benjaminp]

I think the complexity delta in the grammar is exactly 0.

[ericvsmith]

While I think that the dotted_name restriction should be relaxed and it should instead be a style guide issue, I have to agree with Benjamin here: the difference in grammar complexity is zero and shouldn't drive the decision.

[infinity0]

Yes, please get rid of this restriction. It's trivial to get around - you don't even need to define your own "pass-through", one already exists in the standard library:

>>> @(lambda: [lambda x: x][0])()
  File "<stdin>", line 1
    @(lambda: [lambda x: x][0])()
     ^
SyntaxError: invalid syntax

>>> from functools import partial as _
>>> @_( (lambda: [lambda x: x][0])() )
... def f(x): return x*x
... 
>>> f(3)
9

I don't know the rational behind disallowing bar().foo(), but it is the use-case I have in mind - something like:

@MainDecoratorFactory(params).tweakedDecorator(tweak_params)
def f(x):
  pass

or even

@MainDecoratorFactory(params).\
tweakedDecorator(tweak_params)
def f(x):
pass

It should be no more controversial than chaining decorators.

The alternative with the current restrictions would be tweakedDecorator(MainDecorator(params), tweak_params) which is more ugly and visually separates the "tweak" concepts.

It's not appropriate to merge MainDecoratorFactory and the tweaks together: there are several MainDecoratorFactories taking care of one main concern; they don't care about the tweaks. And vice versa; the tweaks shouldn't care about the main decorators.

[gvanrossum]

Nobody has posted a real use case. All the examples are toys. What are the
real use cases that are blocked by this? Readability counts!

[Vgr255]

TL;DR - Use case is dynamic decorators. Not all of the syntax would make sense, see below.

The main benefit of this feature would be for dynamic decorators (as was evidenced from others in this issue). In a project I contribute to, we use dynamic decorators to set a function as being a command, and we use the object (a wrapper around the function) directly, so we need a bit more boilerplate around the place.

Ultimately, we would definitely use such a feature (just the '@spam().eggs()' part; we'd have no use for the other ones) , but we probably won't notice its absence either; we've worked around it for years, after all.

As far as readability goes, I think allowing only the '@spam().eggs()' version would actually improve readability quite a bit, by reducing the need to separate the decorator assignment in two (or more) parts. I can see the desire to have a '@spam[eggs]' kind of syntax though, again for the dynamic decorators case.

I see no reason to allow creating lambdas or conditional expressions inside decorators expressions. If anything, that'll encourage anti-patterns, whereas e.g. '@spam(value=eggs)' would be more readable, and would let the decorator decide what it wants to do with the value.

And if your decorator can be expressed as a lambda, why don't you put that in/below the function? Surely it's less work than writing the lambda everytime ;)