Restrict syntax for tuple literals with one element

[danijar]

BPO	36637
Nosy	@brettcannon, @benjaminp, @1st1, @tirkarthi, @danijar

^{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 2019-04-15.18:29:48.901>
created_at = <Date 2019-04-15.18:27:31.413>
labels = ['interpreter-core']
title = 'Restrict syntax for tuple literals with one element'
updated_at = <Date 2019-04-15.18:29:48.899>
user = 'https://github.com/danijar'

bugs.python.org fields:

activity = <Date 2019-04-15.18:29:48.899>
actor = 'brett.cannon'
assignee = 'none'
closed = True
closed_date = <Date 2019-04-15.18:29:48.901>
closer = 'brett.cannon'
components = ['Interpreter Core']
creation = <Date 2019-04-15.18:27:31.413>
creator = 'danijar'
dependencies = []
files = []
hgrepos = []
issue_num = 36637
keywords = []
message_count = 2.0
messages = ['340298', '340300']
nosy_count = 5.0
nosy_names = ['brett.cannon', 'benjamin.peterson', 'yselivanov', 'xtreak', 'danijar']
pr_nums = []
priority = 'normal'
resolution = 'rejected'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue36637'
versions = []

[danijar]

A tuple can be created with or without parentheses:

a = (1, 2, 3)

a = 1, 2, 3

While both are intuitive in this example, omitting the parentheses can lead to hard to find errors when there is only one element:

a = (1,)

a = 1,

The first is clear but the second can easily occur as a typo when the programmer actually just wanted to assign an integer (comma is next to enter on many keyboards).

I think ideally, omitting parentheses in the single element case would throw a SyntaxError.

On the other hand, I assume that it could be difficult to separate the behavior or tuple creating with an without parentheses, since the parentheses are probably not actually part of the tuple literal.

[brettcannon]

Thanks for the idea, but this would break way too much code to warrant changing it. Plus making this act differently just in the single-item case is too much special-casing/surprise.