Improve syntax error for invalid comparisons

[pablogsal]

BPO	43797
Nosy	@aroberge, @serhiy-storchaka, @lysnikolaou, @pablogsal
PRs	bpo-43797: Improve syntax error for invalid comparisons #25317 bpo-43797: Ensure that early = are not matched by the parser as invalid comparisons #25375 bpo-43797: Handle correctly invalid assignments inside function calls and generators #25390

^{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 2021-04-12.15:59:58.988>
created_at = <Date 2021-04-09.19:11:31.900>
labels = []
title = 'Improve syntax error for invalid comparisons'
updated_at = <Date 2021-04-13.16:51:41.828>
user = 'https://github.com/pablogsal'

bugs.python.org fields:

activity = <Date 2021-04-13.16:51:41.828>
actor = 'pablogsal'
assignee = 'none'
closed = True
closed_date = <Date 2021-04-12.15:59:58.988>
closer = 'pablogsal'
components = []
creation = <Date 2021-04-09.19:11:31.900>
creator = 'pablogsal'
dependencies = []
files = []
hgrepos = []
issue_num = 43797
keywords = ['patch']
message_count = 8.0
messages = ['390659', '390700', '390703', '390728', '390729', '390730', '390862', '390977']
nosy_count = 4.0
nosy_names = ['aroberge', 'serhiy.storchaka', 'lys.nikolaou', 'pablogsal']
pr_nums = ['25317', '25375', '25390']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue43797'
versions = []

[pablogsal]

Improve syntax error for invalid comparisons such as:

>>> if x = 3:
  File "<stdin>", line 1
    if x = 3:
         ^
SyntaxError: invalid syntax

to print:

>>> if x = 3:
  File "<stdin>", line 1
    if x = 3:
         ^
SyntaxError: cannot assign to name. Maybe you meant '==' or ':=' instead of '='?

instead

[serhiy-storchaka]

"cannot assign to name" looks wrong. Usually we can assign to name, unless it is a keyword. And the problem is not that we cannot assign to name (we can), but that is is an invalid syntax, assignment is a statement, not an expression.

Seems it handles only simplest cases with "=" in "if". It does not handle "while x = 3" and "if y and x = 3". Could it be possible to make it more general.

[serhiy-storchaka]

I tried to add

| a=NAME '=' {
    RAISE_SYNTAX_ERROR_KNOWN_LOCATION(
        a, "invalid syntax. Maybe you meant '==' or ':=' instead of '='?") }
| a=bitwise_or '=' {
    RAISE_SYNTAX_ERROR_KNOWN_LOCATION(
        a, "invalid syntax. Maybe you meant '==' instead of '='?") }

in invalid_named_expression, but it does not have any effect.

[pablogsal]

These are good points, let me investigate a bit

[pablogsal]

in invalid_named_expression, but it does not have any effect.

That is because the rule for named_expressions are:

named_expression[expr_ty]:
| a=NAME ':=' ~ b=expression
| expression !':='
| invalid_named_expression

and the second alternative (| expression !':=') succeeds with an assignment, not letting the invalid_named_expression run

[pablogsal]

I have updated the PR with:

• Works for while statements (we can make it work for other kinds of statements but is a bit tricky because those have different grammar paths. We could do this in other PRs.

• I changed the error message for when the LHS is a name. In other cases we still want the special error message. For example:

>>> f() = 1
  File "<stdin>", line 1
    f() = 1
        ^
SyntaxError: cannot assign to function call. Maybe you meant '==' instead of '='?

If we still want something different we could say:

"cannot assign to ... here". The "here" makes it clear that in other places could be possible to do an assignment (for instance with names or attributes).

[pablogsal]

New changeset b86ed8e by Pablo Galindo in branch 'master':
bpo-43797: Improve syntax error for invalid comparisons (bpo-25317)
b86ed8e

[pablogsal]

New changeset 30ed93b by Pablo Galindo in branch 'master':
bpo-43797: Handle correctly invalid assignments inside function calls and generators (GH-25390)
30ed93b