Misleading error for ((a) := 1)

[serhiy-storchaka]

Bug report

>>> ((a) := 1)
  File "<python-input-0>", line 1
    ((a) := 1)
      ^
SyntaxError: cannot use assignment expressions with name

Of course, it can. You only need to remove parentheses around the name.

>>> (a := 1)
1

Linked PRs

• gh-138294: Fix Misleading error for ((a) := 1) #138627

[StanFromIreland]

I think we can just add a rule like, e.g.:

| '(' a=NAME ')' ':=' expression { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "cannot parenthesize a named assignment expression") }

to:

cpython/Grammar/python.gram

Lines 1263 to 1271 in 7bca924

	invalid_named_expression(memo):
	| a=expression ':=' expression {
	RAISE_SYNTAX_ERROR_KNOWN_LOCATION(
	a, "cannot use assignment expressions with %s", _PyPegen_get_expr_name(a)) }
	| a=NAME '=' b=bitwise_or !('='|':=') {
	RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "invalid syntax. Maybe you meant '==' or ':=' instead of '='?") }
	| !(list|tuple|genexp|'True'|'None'|'False') a=bitwise_or b='=' bitwise_or !('='|':=') {
	RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "cannot assign to %s here. Maybe you meant '==' instead of '='?",
	_PyPegen_get_expr_name(a)) }

[terryjreedy]

For assignment expressions, outer parentheses are irrelevant in that the content (a):=1 raises with (as above) or without them.*

"assignment expression name" seems better to me than "named assignment expression". All assignment expressions have a target name, and are 'named' in that sense. Even better, "cannot parenthesize target name in assignment expression"

• assignment statements do care about outer parentheses, which signal expression statement, but not the inner.

>>> (x)=1
>>> ((x)=1)
SyntaxError: cannot assign to name here. Maybe you meant '==' instead of '='?