SyntaxError misidentified in 3.10.0b1 when = used instead of : in dict literal

[aroberge]

BPO	44180
Nosy	@aroberge, @lysnikolaou, @pablogsal, @miss-islington, @shreyanavigyan
PRs	bpo-44180: Report generic syntax errors in the furthest position reached in the first parser pass #26253 [3.10] bpo-44180: Report generic syntax errors in the furthest position reached in the first parser pass (GH-26253) #26281 bpo-44180: Fix edge cases in invalid assigment rules in the parser #26283 [3.10] bpo-44180: Fix edge cases in invalid assigment rules in the parser (GH-26283) #26292

^{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 = None
created_at = <Date 2021-05-19.18:38:05.183>
labels = ['interpreter-core', '3.10']
title = 'SyntaxError misidentified in 3.10.0b1 when = used instead of : in dict literal'
updated_at = <Date 2021-05-21.18:20:51.072>
user = 'https://github.com/aroberge'

bugs.python.org fields:

activity = <Date 2021-05-21.18:20:51.072>
actor = 'miss-islington'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Parser']
creation = <Date 2021-05-19.18:38:05.183>
creator = 'aroberge'
dependencies = []
files = []
hgrepos = []
issue_num = 44180
keywords = ['patch']
message_count = 10.0
messages = ['393964', '393965', '393966', '393967', '393968', '393969', '393970', '393971', '394119', '394139']
nosy_count = 5.0
nosy_names = ['aroberge', 'lys.nikolaou', 'pablogsal', 'miss-islington', 'shreyanavigyan']
pr_nums = ['26253', '26281', '26283', '26292']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue44180'
versions = ['Python 3.10']

[aroberge]

When an equal sign is used instead of a colon in creating a dict literal, depending on the context, either the "bad token" is misidentified OR the would-be helpful error message is incorrect in this particular case.

1. Example of bad token.
   Previously, the = sign was identified correctly:

>>> ages = {'Alice'=22, 'Bob'=23}
  File "<stdin>", line 1
    ages = {'Alice'=22, 'Bob'=23}
                   ^
SyntaxError: invalid syntax

With Python 3.10.0b1, the comma is identified as the bad token:

>>> ages = {'Alice'=22, 'Bob'=23}
  File "<stdin>", line 1
    ages = {'Alice'=22, 'Bob'=23}
                      ^
SyntaxError: invalid syntax

2. Example of incorrect error message

Previously, we got the traditional and unhelpful "invalid syntax" but with the bad token identified correctly:

>>> ages = {'Alice'=22}
  File "<stdin>", line 1
    ages = {'Alice'=22}
                   ^
SyntaxError: invalid syntax

With Python 3.10.0b1, we get the following:

>>> ages = {'Alice'=22}
  File "<stdin>", line 1
    ages = {'Alice'=22}
            ^^^^^^^
SyntaxError: cannot assign to literal here. Maybe you meant '==' instead of '='?

I suspect that the ratio (== suggestion correct/ : suggestion correct) would be vanishingly small. ;-)

[shreyanavigyan]

The bad token example needs a fix but the second is actually ok because we have another type we write within curly braces "{" and there the error is actually perfect. Like if we follow the error message, we use

ages = {'Alice'==22}

And we actually get a "set" with the first value as False. Sets behaves syntactically like list but have different behaviors of mathematical sets.

[pablogsal]

I have an idea of what's causing the first one, but the second I am not sure we can detect easily we are in that case. Do you have a suggestion for what to place in the second case?

[shreyanavigyan]

I'm don't the second is a problem at all. What the error message is suggesting is perfect since it doesn't know if we're trying to use a set or dict.

[shreyanavigyan]

Sorry for the typos.

[pablogsal]

I don't know, it think is more likely that people are triying to use the "=" as a ":" and constructing a literal. Is not a bad error, but I think is not the best one

[shreyanavigyan]

Hmmm... Then is it possible to add an exception where the error message will result in,

SyntaxError: cannot assign to literal here. Maybe you meant ':', "==" or ":=" instead of '='?

only if we're dealing with curly braces?

[aroberge]

In the second case, I understand very well that it could have been a set literal. In my (limited) experience, I have never seen a set literal containing a single element obtained from an == comparison.

Since dict can be built by using keyword arguments, I tend to assume that using = in an literal that starts with { is meant to be a dict.

In
>>> ages = {'Alice' = 22}

replacing the equal sign by either ==, :, or a comma would generate no SyntaxError. Clearly (in my mind anyway, and in previous Python versions), the "bad token" is the equal sign, and not the string Alice.

Here's what I show with friendly:

======

>> ages = {'Alice'=22}

Traceback (most recent call last):
  File "<friendly-console:1>", line 1
    ages = {'Alice'=22}
                   ^
SyntaxError: invalid syntax
>>> why()

It is possible that you used an equal sign = instead of a colon : to assign values to keys in a dict before or at the
position indicated by ^.

=====
Admitedly, this suggestion could also be wrong - but the focus on this case (imo) should be on the "bad token" shown, which should be the equal sign.

[pablogsal]

New changeset 07dba47 by Miss Islington (bot) in branch '3.10':
bpo-44180: Report generic syntax errors in the furthest position reached in the first parser pass (GH-26253) (GH-26281)
07dba47

[miss-islington]

New changeset ae1732d by Miss Islington (bot) in branch '3.10':
bpo-44180: Fix edge cases in invalid assigment rules in the parser (GH-26283)
ae1732d