Syntax quirk with variable annotations

[rhettinger]

BPO	35814
Nosy	@gvanrossum, @rhettinger, @ilevkivskyi, @pablogsal
PRs	bpo-35814: Allow same r.h.s. in annotated assignments as in normal ones #11667 bpo-35814: Allow same r.h.s. in annotated assignments as in normal ones #11667 bpo-35814: Allow same r.h.s. in annotated assignments as in normal ones #11667 bpo-35814: Update the annotated assignment docs #13757 bpo-35814: Allow unpacking in r.h.s of annotated assignment expressions #13760 bpo-35814: More updates to the annotated assignments docs #13794

^{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 2019-01-25.02:48:43.473>
created_at = <Date 2019-01-24.05:45:39.919>
labels = ['interpreter-core', 'type-bug', '3.8']
title = 'Syntax quirk with variable annotations'
updated_at = <Date 2019-06-04.00:44:20.979>
user = 'https://github.com/rhettinger'

bugs.python.org fields:

activity = <Date 2019-06-04.00:44:20.979>
actor = 'levkivskyi'
assignee = 'none'
closed = True
closed_date = <Date 2019-01-25.02:48:43.473>
closer = 'levkivskyi'
components = ['Interpreter Core']
creation = <Date 2019-01-24.05:45:39.919>
creator = 'rhettinger'
dependencies = []
files = []
hgrepos = []
issue_num = 35814
keywords = ['patch', 'patch', 'patch']
message_count = 8.0
messages = ['334280', '334299', '334335', '342355', '342356', '342869', '342908', '344388']
nosy_count = 4.0
nosy_names = ['gvanrossum', 'rhettinger', 'levkivskyi', 'pablogsal']
pr_nums = ['11667', '11667', '11667', '13757', '13760', '13794']
priority = 'low'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue35814'
versions = ['Python 3.8']

[rhettinger]

Am not sure how much we care about this, but parenthesis around tuples stops being optional when there is a variable annotation.

>>> from typing import Tuple
>>> t = 10, 'hello'                     # Parens not normally required
>>> t: Tuple[int, str] = (10, 'hello')  # Annotated allows parens
>>> t: Tuple[int, str] = 10, 'hello'    # Annotated w/o parens fails
SyntaxError: invalid syntax

[ilevkivskyi]

Good catch!

I think it was just overlooked rather than a conscious decision (unlike the left hand side, that generated lots of debates)

I will make a PR to allow everything that is allowed on r.h.s. of a normal assignment.

[ilevkivskyi]

New changeset 62c35a8 by Ivan Levkivskyi in branch 'master':
bpo-35814: Allow same r.h.s. in annotated assignments as in normal ones (GH-11667)
62c35a8

[gvanrossum]

I think PEP-526 does not clarify the intention here. Perhaps we could add a specific example? It currently shows:

t: Tuple[int, ...] = (1, 2, 3)

We could just add this there:

t: Tuple[int, ...] = 1, 2, 3

[gvanrossum]

(With a comment indicating that the second is only accepted in Python 3.8 and later.)

[ilevkivskyi]

Is PEP the best place for such updates? Maybe we can add a versionchanged note in https://docs.python.org/3/reference/simple_stmts.html#annotated-assignment-statements instead?

[gvanrossum]

Is PEP the best place for such updates? Maybe we can add a versionchanged note in https://docs.python.org/3/reference/simple_stmts.html#annotated-assignment-statements instead?

We should definitely update the docs, with versionchanged.

But we should also update the PEP, because the reason this was changed was an interpretation issue in the PEP. This is different from simply changing things in a later version because we've come up with a new idea or we've learned that something was a bad idea.

Now that the intention of the PEP has been clarified, the PEP itself ought to be updated with words expressing the clearer version; but because previously the intended syntax wasn't supported, it makes sense to also add a note there that this wasn't implemented correctly until 3.8.

[pablogsal]

New changeset 8565f6b by Pablo Galindo in branch 'master':
bpo-35814: Allow unpacking in r.h.s of annotated assignment expressions (GH-13760)
8565f6b