bpo-46838: Syntax error improvements for function definitions (GH-31590)

Grammar/python.gram

@@ -306,14 +306,16 @@ slash_with_default[SlashWithDefault*]:
     | a=param_no_default* b=param_with_default+ '/' &')' { _PyPegen_slash_with_default(p, (asdl_arg_seq *)a, b) }
 
 star_etc[StarEtc*]:
+    | invalid_star_etc
     | '*' a=param_no_default b=param_maybe_default* c=[kwds] {
         _PyPegen_star_etc(p, a, b, c) }
     | '*' ',' b=param_maybe_default+ c=[kwds] {
         _PyPegen_star_etc(p, NULL, b, c) }
     | a=kwds { _PyPegen_star_etc(p, NULL, NULL, a) }
-    | invalid_star_etc
 
-kwds[arg_ty]: '**' a=param_no_default { a }
+kwds[arg_ty]:
+    | invalid_kwds
+    | '**' a=param_no_default { a }
 
 # One parameter.  This *includes* a following comma and type comment.
 #
@@ -339,7 +341,7 @@ param_maybe_default[NameDefaultPair*]:
     | a=param c=default? tc=TYPE_COMMENT? &')' { _PyPegen_name_default_pair(p, a, c, tc) }
 param[arg_ty]: a=NAME b=annotation? { _PyAST_arg(a->v.Name.id, b, NULL, EXTRA) }
 annotation[expr_ty]: ':' a=expression { a }
-default[expr_ty]: '=' a=expression { a }
+default[expr_ty]: '=' a=expression { a } | invalid_default
 
 # If statement
 # ------------
@@ -836,14 +838,16 @@ lambda_slash_with_default[SlashWithDefault*]:
     | a=lambda_param_no_default* b=lambda_param_with_default+ '/' &':' { _PyPegen_slash_with_default(p, (asdl_arg_seq *)a, b) }
 
 lambda_star_etc[StarEtc*]:
+    | invalid_lambda_star_etc
     | '*' a=lambda_param_no_default b=lambda_param_maybe_default* c=[lambda_kwds] {
         _PyPegen_star_etc(p, a, b, c) }
     | '*' ',' b=lambda_param_maybe_default+ c=[lambda_kwds] {
         _PyPegen_star_etc(p, NULL, b, c) }
     | a=lambda_kwds { _PyPegen_star_etc(p, NULL, NULL, a) }
-    | invalid_lambda_star_etc
 
-lambda_kwds[arg_ty]: '**' a=lambda_param_no_default { a }
+lambda_kwds[arg_ty]:
+    | invalid_lambda_kwds
+    | '**' a=lambda_param_no_default { a }
 
 lambda_param_no_default[arg_ty]:
     | a=lambda_param ',' { a }
@@ -1151,6 +1155,26 @@ invalid_parameters:
         RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "non-default argument follows default argument") }
     | param_no_default* a='(' param_no_default+ ','? b=')' {
         RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "Function parameters cannot be parenthesized") }
+    | a="/" ',' {
+        RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "at least one argument must precede /") }
+    | (slash_no_default | slash_with_default) param_maybe_default* a='/' {
+        RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "/ may appear only once") }
+    | (slash_no_default | slash_with_default)? param_maybe_default* '*' (',' | param_no_default) param_maybe_default* a='/' {
+        RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "/ must be ahead of *") }
+    | param_maybe_default+ '/' a='*' {
+        RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "expected comma between / and *") }
+invalid_default:
+    | a='=' &(')'|',') { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "expected default value expression") }
+invalid_star_etc:
+    | a='*' (')' | ',' (')' | '**')) { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "named arguments must follow bare *") }
+    | '*' ',' TYPE_COMMENT { RAISE_SYNTAX_ERROR("bare * has associated type comment") }
+    | '*' param a='=' { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "var-positional argument cannot have default value") }
+    | '*' (param_no_default | ',') param_maybe_default* a='*' (param_no_default | ',') {
+        RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "* argument may appear only once") }
+invalid_kwds:
+    | '**' param a='=' { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "var-keyword argument cannot have default value") }
+    | '**' param ',' a=param { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "arguments cannot follow var-keyword argument") }
+    | '**' param ',' a[Token*]=('*'|'**'|'/') { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "arguments cannot follow var-keyword argument") }
 invalid_parameters_helper: # This is only there to avoid type errors
     | a=slash_with_default { _PyPegen_singleton_seq(p, a) }
     | param_with_default+
@@ -1159,14 +1183,26 @@ invalid_lambda_parameters:
         RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "non-default argument follows default argument") }
     | lambda_param_no_default* a='(' ','.lambda_param+ ','? b=')' {
         RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "Lambda expression parameters cannot be parenthesized") }
+    | a="/" ',' {
+        RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "at least one argument must precede /") }
+    | (lambda_slash_no_default | lambda_slash_with_default) lambda_param_maybe_default* a='/' {
+        RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "/ may appear only once") }
+    | (lambda_slash_no_default | lambda_slash_with_default)? lambda_param_maybe_default* '*' (',' | lambda_param_no_default) lambda_param_maybe_default* a='/' {
+        RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "/ must be ahead of *") }
+    | lambda_param_maybe_default+ '/' a='*' {
+        RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "expected comma between / and *") }
 invalid_lambda_parameters_helper:
     | a=lambda_slash_with_default { _PyPegen_singleton_seq(p, a) }
     | lambda_param_with_default+
-invalid_star_etc:
-    | a='*' (')' | ',' (')' | '**')) { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "named arguments must follow bare *") }
-    | '*' ',' TYPE_COMMENT { RAISE_SYNTAX_ERROR("bare * has associated type comment") }
 invalid_lambda_star_etc:
     | '*' (':' | ',' (':' | '**')) { RAISE_SYNTAX_ERROR("named arguments must follow bare *") }
+    | '*' lambda_param a='=' { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "var-positional argument cannot have default value") }
+    | '*' (lambda_param_no_default | ',') lambda_param_maybe_default* a='*' (lambda_param_no_default | ',') {
+        RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "* argument may appear only once") }
+invalid_lambda_kwds:
+    | '**' lambda_param a='=' { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "var-keyword argument cannot have default value") }
+    | '**' lambda_param ',' a=lambda_param { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "arguments cannot follow var-keyword argument") }
+    | '**' lambda_param ',' a[Token*]=('*'|'**'|'/') { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "arguments cannot follow var-keyword argument") }
 invalid_double_type_comments:
     | TYPE_COMMENT NEWLINE TYPE_COMMENT NEWLINE INDENT {
         RAISE_SYNTAX_ERROR("Cannot have two type comments on def") }
@@ -1269,4 +1305,4 @@ invalid_kvpair:
     | a=expression !(':') {
         RAISE_ERROR_KNOWN_LOCATION(p, PyExc_SyntaxError, a->lineno, a->end_col_offset - 1, a->end_lineno, -1, "':' expected after dictionary key") }
     | expression ':' a='*' bitwise_or { RAISE_SYNTAX_ERROR_STARTING_FROM(a, "cannot use a starred expression in a dictionary value") }
-    | expression a=':' {RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "expression expected after dictionary key and ':'") }
+    | expression a=':' {RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "expression expected after dictionary key and ':'") }

Lib/test/test_syntax.py

@@ -351,6 +351,210 @@
 Traceback (most recent call last):
 SyntaxError: invalid syntax
 
+>>> def foo(/,a,b=,c):
+...    pass
+Traceback (most recent call last):
+SyntaxError: at least one argument must precede /
+
+>>> def foo(a,/,/,b,c):
+...    pass
+Traceback (most recent call last):
+SyntaxError: / may appear only once
+
+>>> def foo(a,/,a1,/,b,c):
+...    pass
+Traceback (most recent call last):
+SyntaxError: / may appear only once
+
+>>> def foo(a=1,/,/,*b,/,c):
+...    pass
+Traceback (most recent call last):
+SyntaxError: / may appear only once
+
+>>> def foo(a,/,a1=1,/,b,c):
+...    pass
+Traceback (most recent call last):
+SyntaxError: / may appear only once
+
+>>> def foo(a,*b,c,/,d,e):
+...    pass
+Traceback (most recent call last):
+SyntaxError: / must be ahead of *
+
+>>> def foo(a=1,*b,c=3,/,d,e):
+...    pass
+Traceback (most recent call last):
+SyntaxError: / must be ahead of *
+
+>>> def foo(a,*b=3,c):
+...    pass
+Traceback (most recent call last):
+SyntaxError: var-positional argument cannot have default value
+
+>>> def foo(a,*b: int=,c):
+...    pass
+Traceback (most recent call last):
+SyntaxError: var-positional argument cannot have default value
+
+>>> def foo(a,**b=3):
+...    pass
+Traceback (most recent call last):
+SyntaxError: var-keyword argument cannot have default value
+
+>>> def foo(a,**b: int=3):
+...    pass
+Traceback (most recent call last):
+SyntaxError: var-keyword argument cannot have default value
+
+>>> def foo(a,*a, b, **c, d):
+...    pass
+Traceback (most recent call last):
+SyntaxError: arguments cannot follow var-keyword argument
+
+>>> def foo(a,*a, b, **c, d=4):
+...    pass
+Traceback (most recent call last):
+SyntaxError: arguments cannot follow var-keyword argument
+
+>>> def foo(a,*a, b, **c, *d):
+...    pass
+Traceback (most recent call last):
+SyntaxError: arguments cannot follow var-keyword argument
+
+>>> def foo(a,*a, b, **c, **d):
+...    pass
+Traceback (most recent call last):
+SyntaxError: arguments cannot follow var-keyword argument
+
+>>> def foo(a=1,/,**b,/,c):
+...    pass
+Traceback (most recent call last):
+SyntaxError: arguments cannot follow var-keyword argument
+
+>>> def foo(*b,*d):
+...    pass
+Traceback (most recent call last):
+SyntaxError: * argument may appear only once
+
+>>> def foo(a,*b,c,*d,*e,c):
+...    pass
+Traceback (most recent call last):
+SyntaxError: * argument may appear only once
+
+>>> def foo(a,b,/,c,*b,c,*d,*e,c):
+...    pass
+Traceback (most recent call last):
+SyntaxError: * argument may appear only once
+
+>>> def foo(a,b,/,c,*b,c,*d,**e):
+...    pass
+Traceback (most recent call last):
+SyntaxError: * argument may appear only once
+
+>>> def foo(a=1,/*,b,c):
+...    pass
+Traceback (most recent call last):
+SyntaxError: expected comma between / and *
+
+>>> def foo(a=1,d=,c):
+...    pass
+Traceback (most recent call last):
+SyntaxError: expected default value expression
+
+>>> def foo(a,d=,c):
+...    pass
+Traceback (most recent call last):
+SyntaxError: expected default value expression
+
+>>> def foo(a,d: int=,c):
+...    pass
+Traceback (most recent call last):
+SyntaxError: expected default value expression
+
+>>> lambda /,a,b,c: None
+Traceback (most recent call last):
+SyntaxError: at least one argument must precede /
+
+>>> lambda a,/,/,b,c: None
+Traceback (most recent call last):
+SyntaxError: / may appear only once
+
+>>> lambda a,/,a1,/,b,c: None
+Traceback (most recent call last):
+SyntaxError: / may appear only once
+
+>>> lambda a=1,/,/,*b,/,c: None
+Traceback (most recent call last):
+SyntaxError: / may appear only once
+
+>>> lambda a,/,a1=1,/,b,c: None
+Traceback (most recent call last):
+SyntaxError: / may appear only once
+
+>>> lambda a,*b,c,/,d,e: None
+Traceback (most recent call last):
+SyntaxError: / must be ahead of *
+
+>>> lambda a=1,*b,c=3,/,d,e: None
+Traceback (most recent call last):
+SyntaxError: / must be ahead of *
+
+>>> lambda a=1,/*,b,c: None
+Traceback (most recent call last):
+SyntaxError: expected comma between / and *
+
+>>> lambda a,*b=3,c: None
+Traceback (most recent call last):
+SyntaxError: var-positional argument cannot have default value
+
+>>> lambda a,**b=3: None
+Traceback (most recent call last):
+SyntaxError: var-keyword argument cannot have default value
+
+>>> lambda a, *a, b, **c, d: None
+Traceback (most recent call last):
+SyntaxError: arguments cannot follow var-keyword argument
+
+>>> lambda a,*a, b, **c, d=4: None
+Traceback (most recent call last):
+SyntaxError: arguments cannot follow var-keyword argument
+
+>>> lambda a,*a, b, **c, *d: None
+Traceback (most recent call last):
+SyntaxError: arguments cannot follow var-keyword argument
+
+>>> lambda a,*a, b, **c, **d: None
+Traceback (most recent call last):
+SyntaxError: arguments cannot follow var-keyword argument
+
+>>> lambda a=1,/,**b,/,c: None
+Traceback (most recent call last):
+SyntaxError: arguments cannot follow var-keyword argument
+
+>>> lambda *b,*d: None
+Traceback (most recent call last):
+SyntaxError: * argument may appear only once
+
+>>> lambda a,*b,c,*d,*e,c: None
+Traceback (most recent call last):
+SyntaxError: * argument may appear only once
+
+>>> lambda a,b,/,c,*b,c,*d,*e,c: None
+Traceback (most recent call last):
+SyntaxError: * argument may appear only once
+
+>>> lambda a,b,/,c,*b,c,*d,**e: None
+Traceback (most recent call last):
+SyntaxError: * argument may appear only once
+
+>>> lambda a=1,d=,c: None
+Traceback (most recent call last):
+SyntaxError: expected default value expression
+
+>>> lambda a,d=,c: None
+Traceback (most recent call last):
+SyntaxError: expected default value expression
+
 >>> import ast; ast.parse('''
 ... def f(
 ...     *, # type: int

Misc/NEWS.d/next/Core and Builtins/2022-02-25-22-42-30.bpo-46838.RB6kEy.rst

@@ -0,0 +1,2 @@
+Improve syntax errors for incorrect function definitions. Patch by Pablo
+Galindo