Allowing arbitrary expressions in the @expression syntax

[maggyero]

BPO	37196
Nosy	@mwhudson, @rhettinger, @etrepum, @tiran, @jimfulton, @maggyero, @brandtbucher
Superseder	bpo-39702: PEP 614: Relaxing Grammar Restrictions On Decorators

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

Show more details

GitHub fields:

assignee = None
closed_at = <Date 2020-02-20.16:20:23.015>
created_at = <Date 2019-06-07.14:05:15.545>
labels = ['interpreter-core', 'type-feature', '3.7']
title = 'Allowing arbitrary expressions in the @expression syntax'
updated_at = <Date 2020-02-20.16:20:23.014>
user = 'https://github.com/maggyero'

bugs.python.org fields:

activity = <Date 2020-02-20.16:20:23.014>
actor = 'brandtbucher'
assignee = 'none'
closed = True
closed_date = <Date 2020-02-20.16:20:23.015>
closer = 'brandtbucher'
components = ['Interpreter Core']
creation = <Date 2019-06-07.14:05:15.545>
creator = 'maggyero'
dependencies = []
files = []
hgrepos = []
issue_num = 37196
keywords = []
message_count = 5.0
messages = ['344939', '344940', '344944', '344946', '344952']
nosy_count = 8.0
nosy_names = ['mwh', 'rhettinger', 'bob.ippolito', 'exarkun', 'christian.heimes', 'j1m', 'maggyero', 'brandtbucher']
pr_nums = []
priority = 'normal'
resolution = 'duplicate'
stage = 'resolved'
status = 'closed'
superseder = '39702'
type = 'enhancement'
url = 'https://bugs.python.org/issue37196'
versions = ['Python 3.7']

[maggyero]

Could we allow arbitrary expressions in the @expression syntax for applying decorators to functions/classes? The current grammar restriction to:

decorator: '@' dotted_name [ '(' [arglist] ')' ] NEWLINE

is very surprising and I don't understand the real motivation.

I find it weird that you are not able to do that:

    def f():
        def g():
            def h(x):
                pass
            return h
        return g
    
    @f()()
    def i():
        pass

since you get:

    @f()()
        ^
SyntaxError: invalid syntax

but the following is perfectly valid:

    def f():
        def g():
            def h(x):
                pass
            return h
        return g
    
    def g(x):
        def h(x):
            pass
        return g
    
    @g(f()())
    def h():
        pass

See this post for more details: https://stackoverflow.com/questions/56490579

[tiran]

The syntax is deliberately limited. The reasons are explained at:

https://www.python.org/dev/peps/pep-0318/#current-syntax
https://mail.python.org/pipermail/python-dev/2004-August/046711.html

[maggyero]

@christian Heimes

The reasons are explained at:

Yes I read that. But I am wondering if after 15 years Guido still has this "gut feeling". Because my gut feeling as a Python *user* who has discovered decorators and stumbled on this restriction was: "what?!".

[gvanrossum]

What you want to write is as unreadable as it ever was.

[maggyero]

@guido van Rossum

What you want to write is as unreadable as it ever was.

How is @g(f()()) more readable than @f()()? Yet the former is allowed.