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Fix SyntaxError messages for invalid assignment targets #127

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Fix SyntaxError messages for invalid assignment targets #127

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46 changes: 33 additions & 13 deletions Grammar/python.gram
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Original file line number Diff line number Diff line change
Expand Up @@ -60,7 +60,7 @@ simple_stmt[asdl_seq*]:
# will throw a SyntaxError.
small_stmt[stmt_ty] (memo):
| assignment
| e=star_expressions { _Py_Expr(e, EXTRA) }
| e=star_expressions !('=' | augassign) { _Py_Expr(e, EXTRA) }
| &'return' return_stmt
| &('import' | 'from') import_stmt
| &'raise' raise_stmt
Expand All @@ -72,6 +72,7 @@ small_stmt[stmt_ty] (memo):
| 'continue' { _Py_Continue(EXTRA) }
| &'global' global_stmt
| &'nonlocal' nonlocal_stmt
| targets_with_invalid
compound_stmt[stmt_ty]:
| &('def' | '@' | ASYNC) function_def
| &'if' if_stmt
Expand All @@ -96,7 +97,7 @@ assignment[stmt_ty]:
_Py_Assign(a, b, NEW_TYPE_COMMENT(p, tc), EXTRA) }
| a=target b=augassign c=(yield_expr | star_expressions) {
_Py_AugAssign(a, b->kind, c, EXTRA) }
| invalid_assignment
| invalid_ann_assignment

augassign[AugOperator*]:
| '+=' { _PyPegen_augoperator(p, Add) }
Expand Down Expand Up @@ -564,14 +565,15 @@ star_targets_seq[asdl_seq*]: a=','.star_target+ [','] { a }
star_target[expr_ty] (memo):
| '*' a=(!'*' star_target) {
_Py_Starred(CHECK(_PyPegen_set_expr_context(p, a, Store)), Store, EXTRA) }
| a=t_primary '.' b=NAME !t_lookahead { _Py_Attribute(a, b->v.Name.id, Store, EXTRA) }
| a=t_primary '[' b=slices ']' !t_lookahead { _Py_Subscript(a, b, Store, EXTRA) }
| a=t_primary '.' b=NAME &star_target_end { _Py_Attribute(a, b->v.Name.id, Store, EXTRA) }
| a=t_primary '[' b=slices ']' &star_target_end { _Py_Subscript(a, b, Store, EXTRA) }
| star_atom
star_atom[expr_ty]:
| a=NAME { _PyPegen_set_expr_context(p, a, Store) }
| a=NAME &star_target_end { _PyPegen_set_expr_context(p, a, Store) }
| '(' a=star_target ')' { _PyPegen_set_expr_context(p, a, Store) }
| '(' a=[star_targets_seq] ')' { _Py_Tuple(a, Store, EXTRA) }
| '[' a=[star_targets_seq] ']' { _Py_List(a, Store, EXTRA) }
star_target_end: ')' | ']' | ',' | '=' | 'in'
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@gvanrossum gvanrossum May 12, 2020

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I presume in is here because of for <target> in <expr>, but this would mean that you'd get a different error for

a in b = 42

than you'd get for

a < b = 42

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@lysnikolaou lysnikolaou May 12, 2020 •
edited
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Actually not, because when parsing the invalid target the whole expression gets parsed, which means that cannot assign to comparison is displayed for both.

You're right. What I said above was me talking about 1 in a = 42 for some reason, which would actually display the correct error message, since 1 is not a valid assignment target.

Another problem with this is that it isn't really usable in for-statements, because the in always gets parsed as the comparison operator in, which means that even something like for f() in a: pass displays the cannot assign to comparison error.


inside_paren_ann_assign_target[expr_ty]:
| ann_assign_subscript_attribute_target
Expand Down Expand Up @@ -599,8 +601,8 @@ del_target_end: ')' | ']' | ',' | ';' | NEWLINE

targets[asdl_seq*]: a=','.target+ [','] { a }
target[expr_ty] (memo):
| a=t_primary '.' b=NAME !t_lookahead { _Py_Attribute(a, b->v.Name.id, Store, EXTRA) }
| a=t_primary '[' b=slices ']' !t_lookahead { _Py_Subscript(a, b, Store, EXTRA) }
| a=t_primary '.' b=NAME &target_end { _Py_Attribute(a, b->v.Name.id, Store, EXTRA) }
| a=t_primary '[' b=slices ']' &target_end { _Py_Subscript(a, b, Store, EXTRA) }
| t_atom
t_primary[expr_ty]:
| a=t_primary '.' b=NAME &t_lookahead { _Py_Attribute(a, b->v.Name.id, Load, EXTRA) }
Expand All @@ -614,11 +616,11 @@ t_primary[expr_ty]:
| a=atom &t_lookahead { a }
t_lookahead: '(' | '[' | '.'
t_atom[expr_ty]:
| a=NAME { _PyPegen_set_expr_context(p, a, Store) }
| a=NAME &target_end { _PyPegen_set_expr_context(p, a, Store) }
| '(' a=target ')' { _PyPegen_set_expr_context(p, a, Store) }
| '(' b=[targets] ')' { _Py_Tuple(b, Store, EXTRA) }
| '[' b=[targets] ']' { _Py_List(b, Store, EXTRA) }

target_end: ')' | ']' | ',' | augassign | ':'

# From here on, there are rules for invalid syntax with specialised error messages
incorrect_arguments:
Expand All @@ -627,17 +629,16 @@ incorrect_arguments:
RAISE_SYNTAX_ERROR("Generator expression must be parenthesized") }
| a=args ',' args { _PyPegen_arguments_parsing_error(p, a) }
invalid_kwarg:
| expression '=' { RAISE_SYNTAX_ERROR("expression cannot contain assignment, perhaps you meant \"==\"?") }
| expression '=' {
RAISE_SYNTAX_ERROR_NO_COL_OFFSET("expression cannot contain assignment, perhaps you meant \"==\"?") }
invalid_named_expression:
| a=expression ':=' expression {
RAISE_SYNTAX_ERROR("cannot use assignment expressions with %s", _PyPegen_get_expr_name(a)) }
invalid_assignment:
invalid_ann_assignment:
| list ':' { RAISE_SYNTAX_ERROR("only single target (not list) can be annotated") }
| tuple ':' { RAISE_SYNTAX_ERROR("only single target (not tuple) can be annotated") }
| expression ':' expression ['=' annotated_rhs] {
RAISE_SYNTAX_ERROR("illegal target for annotation") }
| a=expression ('=' | augassign) (yield_expr | star_expressions) {
RAISE_SYNTAX_ERROR_NO_COL_OFFSET("cannot assign to %s", _PyPegen_get_expr_name(a)) }
invalid_block:
| NEWLINE !INDENT { RAISE_INDENTATION_ERROR("expected an indented block") }
invalid_comprehension:
Expand All @@ -656,3 +657,22 @@ invalid_double_type_comments:
invalid_del_target:
| a=star_expression &del_target_end {
RAISE_SYNTAX_ERROR("cannot delete %s", _PyPegen_get_expr_name(a)) }

targets_with_invalid:
| target_with_invalid !','
| target_with_invalid (',' target_with_invalid)* [',']
target_with_invalid:
| '*' a=(!'*' target_with_invalid)
| a=t_primary '.' b=NAME &(star_target_end | target_end)
| a=t_primary '[' b=slices ']' &(star_target_end | target_end)
| target_atom_with_invalid
| invalid_target
target_atom_with_invalid:
| a=NAME &(star_target_end | target_end) { _PyPegen_set_expr_context(p, a, Store) }
| '(' a=target_with_invalid ')' { _PyPegen_set_expr_context(p, a, Store) }
| '(' a=[targets_with_invalid] ')' { _Py_Tuple(a, Store, EXTRA) }
| '[' a=[targets_with_invalid] ']' { _Py_List(a, Store, EXTRA) }
| invalid_target
invalid_target:
| a=star_expression &(star_target_end | target_end) {
RAISE_SYNTAX_ERROR_NO_COL_OFFSET("cannot assign to %s", _PyPegen_get_expr_name(a)) }
2 changes: 1 addition & 1 deletion Lib/test/test_peg_parser.py
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Original file line number Diff line number Diff line change
Expand Up @@ -724,7 +724,7 @@ def test_incorrect_ast_generation_with_specialized_errors(self) -> None:
peg_parser.parse_string(source)
self.assertTrue(
error_text in se.exception.msg,
f"Actual error message does not match expexted for {source}"
f"Actual error message does not match expected for {source}"
)

@unittest.expectedFailure
Expand Down
58 changes: 27 additions & 31 deletions Lib/test/test_syntax.py
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Original file line number Diff line number Diff line change
Expand Up @@ -100,30 +100,29 @@
This test just checks a couple of cases rather than enumerating all of
them.

# All of the following also produce different error messages with pegen
# >>> (a, "b", c) = (1, 2, 3)
# Traceback (most recent call last):
# SyntaxError: cannot assign to literal
>>> (a, "b", c) = (1, 2, 3)
Traceback (most recent call last):
SyntaxError: cannot assign to literal

# >>> (a, True, c) = (1, 2, 3)
# Traceback (most recent call last):
# SyntaxError: cannot assign to True
>>> (a, True, c) = (1, 2, 3)
Traceback (most recent call last):
SyntaxError: cannot assign to True

>>> (a, __debug__, c) = (1, 2, 3)
Traceback (most recent call last):
SyntaxError: cannot assign to __debug__

# >>> (a, *True, c) = (1, 2, 3)
# Traceback (most recent call last):
# SyntaxError: cannot assign to True
>>> (a, *True, c) = (1, 2, 3)
Traceback (most recent call last):
SyntaxError: cannot assign to True

>>> (a, *__debug__, c) = (1, 2, 3)
Traceback (most recent call last):
SyntaxError: cannot assign to __debug__

# >>> [a, b, c + 1] = [1, 2, 3]
# Traceback (most recent call last):
# SyntaxError: cannot assign to operator
>>> [a, b, c + 1] = [1, 2, 3]
Traceback (most recent call last):
SyntaxError: cannot assign to operator

>>> a if 1 else b = 1
Traceback (most recent call last):
Expand Down Expand Up @@ -300,28 +299,25 @@
... 290, 291, 292, 293, 294, 295, 296, 297, 298, 299) # doctest: +ELLIPSIS
(0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, ..., 297, 298, 299)

# >>> f(lambda x: x[0] = 3)
# Traceback (most recent call last):
# SyntaxError: expression cannot contain assignment, perhaps you meant "=="?
>>> f(lambda x: x[0] = 3)
Traceback (most recent call last):
SyntaxError: expression cannot contain assignment, perhaps you meant "=="?

The grammar accepts any test (basically, any expression) in the
keyword slot of a call site. Test a few different options.

# >>> f(x()=2)
# Traceback (most recent call last):
# SyntaxError: expression cannot contain assignment, perhaps you meant "=="?
# >>> f(a or b=1)
# Traceback (most recent call last):
# SyntaxError: expression cannot contain assignment, perhaps you meant "=="?
# >>> f(x.y=1)
# Traceback (most recent call last):
# SyntaxError: expression cannot contain assignment, perhaps you meant "=="?
# >>> f((x)=2)
# Traceback (most recent call last):
# SyntaxError: expression cannot contain assignment, perhaps you meant "=="?
# >>> f(True=2)
# Traceback (most recent call last):
# SyntaxError: cannot assign to True
>>> f(x()=2)
Traceback (most recent call last):
SyntaxError: expression cannot contain assignment, perhaps you meant "=="?
>>> f(a or b=1)
Traceback (most recent call last):
SyntaxError: expression cannot contain assignment, perhaps you meant "=="?
>>> f(x.y=1)
Traceback (most recent call last):
SyntaxError: expression cannot contain assignment, perhaps you meant "=="?
>>> f((x)=2)
Traceback (most recent call last):
SyntaxError: expression cannot contain assignment, perhaps you meant "=="?
>>> f(__debug__=1)
Traceback (most recent call last):
SyntaxError: cannot assign to __debug__
Expand Down
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