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Disallow expressions like (a) = 42 #79388
Description
| BPO | 35207 |
|---|---|
| Nosy | @gvanrossum, @mdickinson, @benjaminp, @skrah, @serhiy-storchaka, @pablogsal |
| PRs |
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 = 'https://github.com/pablogsal'
closed_at = <Date 2018-11-12.21:35:48.684>
created_at = <Date 2018-11-10.04:13:18.571>
labels = ['interpreter-core', 'invalid', '3.8']
title = 'Disallow expressions like (a) = 42'
updated_at = <Date 2018-11-12.21:35:48.683>
user = 'https://github.com/pablogsal'bugs.python.org fields:
activity = <Date 2018-11-12.21:35:48.683>
actor = 'pablogsal'
assignee = 'pablogsal'
closed = True
closed_date = <Date 2018-11-12.21:35:48.684>
closer = 'pablogsal'
components = ['Interpreter Core']
creation = <Date 2018-11-10.04:13:18.571>
creator = 'pablogsal'
dependencies = []
files = []
hgrepos = []
issue_num = 35207
keywords = ['patch']
message_count = 7.0
messages = ['329590', '329591', '329593', '329596', '329644', '329731', '329740']
nosy_count = 6.0
nosy_names = ['gvanrossum', 'mark.dickinson', 'benjamin.peterson', 'skrah', 'serhiy.storchaka', 'pablogsal']
pr_nums = ['10444']
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue35207'
versions = ['Python 3.8']
Activity
pablogsal commentedon Nov 10, 2018
As commented on bpo-33878, the fact that expressions like (x) = 42 are accepted is, in reality, an implementation detail and they should be disallowed.
pablogsal commentedon Nov 10, 2018
I am not sure if this is the cleanest/simplest implementation...if you think there is a better way to do this I am happy to change the Pull Request :)
gvanrossum commentedon Nov 10, 2018
I could have sworn Benjamin showed a fix for this during the core-dev sprint in Seattle...
serhiy-storchaka commentedon Nov 10, 2018
Actually,
(a) = 42doesn't contradict the grammar.assignment_stmt ::= (target_list "=")+ (starred_expression | yield_expression)
target_list ::= target ("," target)* [","]
target ::= identifier
| "(" [target_list] ")"
| "[" [target_list] "]"
| attributeref
| subscription
| slicing
| "*" target
So that if we decide to disallow such things, we should change the definition in the language reference.
I thought the code should be similar to the code added in bpo-34641. But maybe there is a way to simplify both code.
gvanrossum commentedon Nov 10, 2018
Hm, I was thinking of #9212, but
that's about keyword arg (where the interpreter incorrectly allowed
f((kw)=expr). But on thinking about it more I believe we should allow (a) =
(b), so there is no code change required. Sorry for the false alarm.
skrah commentedon Nov 12, 2018
I just want to add one more voice for allowing the status quo:
C, OCaml, SML, Haskell allow the assignment, Ruby disallows it.
The ML family must allow it, since "let (x) = 10" is pattern matching
under the hood, and (10) = 10.
In C (gcc, clang at least, I didn't check the standard) it may be convenience.
Python's assignment unpacking is similar to pattern matching, so
while the construct is surprising at first, I think it's the right
thing to do.
gvanrossum commentedon Nov 12, 2018
Let's close this already. It won't happen. Sorry for the noise.