bpo-44456: Improve the syntax error when mixing keyword and positional patterns (GH-26793)

Grammar/python.gram

@@ -380,6 +380,7 @@ class_pattern[pattern_ty]:
                 CHECK(asdl_expr_seq*, _PyPegen_get_pattern_keys(p, keywords)))),
             CHECK(asdl_pattern_seq*, _PyPegen_get_patterns(p, keywords)),
             EXTRA) }
+    | invalid_class_pattern
 positional_patterns[asdl_pattern_seq*]:
     | args[asdl_pattern_seq*]=','.pattern+ { args }
 keyword_patterns[asdl_seq*]:
@@ -978,6 +979,13 @@ invalid_case_block:
 invalid_as_pattern:
     | or_pattern 'as' a="_" { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "cannot use '_' as a target") }
     | or_pattern 'as' !NAME a=expression { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "invalid pattern target") }
+invalid_class_pattern:
+    | name_or_attr '(' a=invalid_class_argument_pattern  { RAISE_SYNTAX_ERROR_KNOWN_RANGE(
+        PyPegen_first_item(a, pattern_ty),
+        PyPegen_last_item(a, pattern_ty),
+        "positional patterns follow keyword patterns") }
+invalid_class_argument_pattern[asdl_pattern_seq*]:
+    | [positional_patterns ','] keyword_patterns ',' a=positional_patterns { a }
 invalid_if_stmt:
     | 'if' named_expression NEWLINE { RAISE_SYNTAX_ERROR("expected ':'") }
     | a='if' a=named_expression ':' NEWLINE !INDENT {

Lib/test/test_syntax.py

@@ -1240,6 +1240,30 @@ Corner-cases that used to crash:
     ...     ...
     Traceback (most recent call last):
     SyntaxError: invalid pattern target
+
+    >>> match ...:
+    ...   case Foo(z=1, y=2, x):
+    ...     ...
+    Traceback (most recent call last):
+    SyntaxError: positional patterns follow keyword patterns
+
+    >>> match ...:
+    ...   case Foo(a, z=1, y=2, x):
+    ...     ...
+    Traceback (most recent call last):
+    SyntaxError: positional patterns follow keyword patterns
+
+    >>> match ...:
+    ...   case Foo(z=1, x, y=2):
+    ...     ...
+    Traceback (most recent call last):
+    SyntaxError: positional patterns follow keyword patterns
+
+    >>> match ...:
+    ...   case C(a=b, c, d=e, f, g=h, i, j=k, ...):
+    ...     ...
+    Traceback (most recent call last):
+    SyntaxError: positional patterns follow keyword patterns
 """
 
 import re

Misc/NEWS.d/next/Core and Builtins/2021-06-18-22-08-25.bpo-44456.L0Rhko.rst

@@ -0,0 +1,2 @@
+Improve the syntax error when mixing positional and keyword patterns. Patch
+by Pablo Galindo.

Parser/pegen.c

@@ -1557,6 +1557,13 @@ _PyPegen_seq_last_item(asdl_seq *seq)
     return asdl_seq_GET_UNTYPED(seq, len - 1);
 }
 
+void *
+_PyPegen_seq_first_item(asdl_seq *seq)
+{
+    return asdl_seq_GET_UNTYPED(seq, 0);
+}
+
+
 /* Creates a new name of the form <first_name>.<second_name> */
 expr_ty
 _PyPegen_join_names_with_dot(Parser *p, expr_ty first_name, expr_ty second_name)

Parser/pegen.h

@@ -149,6 +149,9 @@ void *_PyPegen_dummy_name(Parser *p, ...);
 void * _PyPegen_seq_last_item(asdl_seq *seq);
 #define PyPegen_last_item(seq, type) ((type)_PyPegen_seq_last_item((asdl_seq*)seq))
 
+void * _PyPegen_seq_first_item(asdl_seq *seq);
+#define PyPegen_first_item(seq, type) ((type)_PyPegen_seq_first_item((asdl_seq*)seq))
+
 #define CURRENT_POS (-5)
 
 Py_LOCAL_INLINE(void *)