bpo-43822: Improve syntax errors for missing commas (GH-25377)

Doc/library/token-list.inc

@@ -211,6 +211,8 @@
 
 .. data:: TYPE_COMMENT
 
+.. data:: SOFT_KEYWORD
+
 .. data:: ERRORTOKEN
 
 .. data:: N_TOKENS

Grammar/Tokens

@@ -59,6 +59,7 @@ AWAIT
 ASYNC
 TYPE_IGNORE
 TYPE_COMMENT
+SOFT_KEYWORD
 ERRORTOKEN
 
 # These aren't used by the C tokenizer but are needed for tokenize.py

Grammar/python.gram

@@ -7,6 +7,7 @@ _PyPegen_parse(Parser *p)
     // Initialize keywords
     p->keywords = reserved_keywords;
     p->n_keyword_lists = n_keyword_lists;
+    p->soft_keywords = soft_keywords;
 
     // Run parser
     void *result = NULL;
@@ -459,6 +460,7 @@ expressions[expr_ty]:
     | a=expression ',' { _PyAST_Tuple(CHECK(asdl_expr_seq*, _PyPegen_singleton_seq(p, a)), Load, EXTRA) }
     | expression
 expression[expr_ty] (memo):
+    | invalid_expression
     | a=disjunction 'if' b=disjunction 'else' c=expression { _PyAST_IfExp(b, a, c, EXTRA) }
     | disjunction
     | lambdef
@@ -778,6 +780,13 @@ invalid_kwarg:
     | expression a='=' {
         RAISE_SYNTAX_ERROR_KNOWN_LOCATION(
             a, "expression cannot contain assignment, perhaps you meant \"==\"?") }
+
+invalid_expression:
+    # !(NAME STRING) is not matched so we don't show this error with some invalid string prefixes like: kf"dsfsdf"
+    # Soft keywords need to also be ignored because they can be parsed as NAME NAME
+    | !(NAME STRING | SOFT_KEYWORD) a=disjunction expression {
+        RAISE_ERROR_KNOWN_LOCATION(p, PyExc_SyntaxError, a->lineno, a->end_col_offset - 1, "invalid syntax. Perhaps you forgot a comma?") }
+
 invalid_named_expression:
     | a=expression ':=' expression {
         RAISE_SYNTAX_ERROR_KNOWN_LOCATION(

Include/token.h

@@ -69,8 +69,9 @@ extern "C" {
 #define ASYNC           56
 #define TYPE_IGNORE     57
 #define TYPE_COMMENT    58
-#define ERRORTOKEN      59
-#define N_TOKENS        63
+#define SOFT_KEYWORD    59
+#define ERRORTOKEN      60
+#define N_TOKENS        64
 #define NT_OFFSET       256
 
 /* Special definitions for cooperation with parser */

Lib/test/test_genexps.py

@@ -103,7 +103,7 @@ Verify that parenthesis are required when used as a keyword argument value
     >>> dict(a = i for i in range(10))
     Traceback (most recent call last):
        ...
-    SyntaxError: invalid syntax
+    SyntaxError: invalid syntax. Maybe you meant '==' or ':=' instead of '='?
 
 Verify that parenthesis are required when used as a keyword argument value
 

Lib/test/test_syntax.py

@@ -248,22 +248,36 @@ SyntaxError: did you forget parentheses around the comprehension target?
 
 # Missing commas in literals collections should not
 # produce special error messages regarding missing
-# parentheses
+# parentheses, but about missing commas instead
 
 >>> [1, 2 3]
 Traceback (most recent call last):
-SyntaxError: invalid syntax
+SyntaxError: invalid syntax. Perhaps you forgot a comma?
 
 >>> {1, 2 3}
 Traceback (most recent call last):
-SyntaxError: invalid syntax
+SyntaxError: invalid syntax. Perhaps you forgot a comma?
 
 >>> {1:2, 2:5 3:12}
 Traceback (most recent call last):
-SyntaxError: invalid syntax
+SyntaxError: invalid syntax. Perhaps you forgot a comma?
 
 >>> (1, 2 3)
 Traceback (most recent call last):
+SyntaxError: invalid syntax. Perhaps you forgot a comma?
+
+# Make sure soft keywords constructs don't raise specialized
+# errors regarding missing commas
+
+>>> match x:
+...     y = 3
+Traceback (most recent call last):
+SyntaxError: invalid syntax
+
+>>> match x:
+...     case y:
+...        3 $ 3
+Traceback (most recent call last):
 SyntaxError: invalid syntax
 
 From compiler_complex_args():
@@ -864,7 +878,7 @@ leading to spurious errors.
    SyntaxError: cannot assign to attribute here. Maybe you meant '==' instead of '='?
 
 Ensure that early = are not matched by the parser as invalid comparisons
-   >>> f(2, 4, x=34); {1,2 a}
+   >>> f(2, 4, x=34); 1 $ 2
    Traceback (most recent call last):
    SyntaxError: invalid syntax
 

Lib/token.py

@@ -62,12 +62,13 @@ AWAIT = 55
 ASYNC = 56
 TYPE_IGNORE = 57
 TYPE_COMMENT = 58
+SOFT_KEYWORD = 59
 # These aren't used by the C tokenizer but are needed for tokenize.py
-ERRORTOKEN = 59
-COMMENT = 60
-NL = 61
-ENCODING = 62
-N_TOKENS = 63
+ERRORTOKEN = 60
+COMMENT = 61
+NL = 62
+ENCODING = 63
+N_TOKENS = 64
 # Special definitions for cooperation with parser
 NT_OFFSET = 256
 

Misc/NEWS.d/next/Core and Builtins/2021-04-13-02-32-18.bpo-43822.lej0OO.rst

@@ -0,0 +1,2 @@
+Improve syntax errors in the parser for missing commas between expressions.
+Patch by Pablo Galindo.

Parser/pegen.c

@@ -943,6 +943,23 @@ _PyPegen_string_token(Parser *p)
     return _PyPegen_expect_token(p, STRING);
 }
 
+
+expr_ty _PyPegen_soft_keyword_token(Parser *p) {
+    Token *t = _PyPegen_expect_token(p, NAME);
+    if (t == NULL) {
+        return NULL;
+    }
+    char *the_token;
+    Py_ssize_t size;
+    PyBytes_AsStringAndSize(t->bytes, &the_token, &size);
+    for (char **keyword = p->soft_keywords; *keyword != NULL; keyword++) {
+        if (strncmp(*keyword, the_token, size) == 0) {
+            return _PyPegen_name_token(p);
+        }
+    }
+    return NULL;
+}
+
 static PyObject *
 parsenumber_raw(const char *s)
 {
@@ -1151,6 +1168,7 @@ _PyPegen_Parser_New(struct tok_state *tok, int start_rule, int flags,
     p->tok = tok;
     p->keywords = NULL;
     p->n_keyword_lists = -1;
+    p->soft_keywords = NULL;
     p->tokens = PyMem_Malloc(sizeof(Token *));
     if (!p->tokens) {
         PyMem_Free(p);

Parser/pegen.h

@@ -59,6 +59,7 @@ typedef struct {
     int fill, size;
     PyArena *arena;
     KeywordToken **keywords;
+    char **soft_keywords;
     int n_keyword_lists;
     int start_rule;
     int *errcode;
@@ -125,6 +126,7 @@ int _PyPegen_lookahead(int, void *(func)(Parser *), Parser *);
 Token *_PyPegen_expect_token(Parser *p, int type);
 Token *_PyPegen_expect_forced_token(Parser *p, int type, const char* expected);
 expr_ty _PyPegen_expect_soft_keyword(Parser *p, const char *keyword);
+expr_ty _PyPegen_soft_keyword_token(Parser *p);
 Token *_PyPegen_get_last_nonnwhitespace_token(Parser *);
 int _PyPegen_fill_token(Parser *p);
 expr_ty _PyPegen_name_token(Parser *p);

Parser/token.c

@@ -65,6 +65,7 @@ const char * const _PyParser_TokenNames[] = {
     "ASYNC",
     "TYPE_IGNORE",
     "TYPE_COMMENT",
+    "SOFT_KEYWORD",
     "<ERRORTOKEN>",
     "<COMMENT>",
     "<NL>",

Tools/peg_generator/pegen/c_generator.py

@@ -46,6 +46,7 @@ _PyPegen_parse(Parser *p)
     // Initialize keywords
     p->keywords = reserved_keywords;
     p->n_keyword_lists = n_keyword_lists;
+    p->soft_keywords = soft_keywords;
 
     return start_rule(p);
 }
@@ -66,6 +67,7 @@ BASE_NODETYPES = {
     "NAME": NodeTypes.NAME_TOKEN,
     "NUMBER": NodeTypes.NUMBER_TOKEN,
     "STRING": NodeTypes.STRING_TOKEN,
+    "SOFT_KEYWORD": NodeTypes.SOFT_KEYWORD,
 }
 
 
@@ -411,6 +413,7 @@ class CParserGenerator(ParserGenerator, GrammarVisitor):
         if subheader:
             self.print(subheader)
         self._setup_keywords()
+        self._setup_soft_keywords()
         for i, (rulename, rule) in enumerate(self.todo.items(), 1000):
             comment = "  // Left-recursive" if rule.left_recursive else ""
             self.print(f"#define {rulename}_type {i}{comment}")
@@ -474,6 +477,15 @@ class CParserGenerator(ParserGenerator, GrammarVisitor):
                     self.print("},")
         self.print("};")
 
+    def _setup_soft_keywords(self) -> None:
+        soft_keywords = sorted(self.callmakervisitor.soft_keywords)
+        self.print("static char *soft_keywords[] = {")
+        with self.indent():
+            for keyword in soft_keywords:
+                self.print(f'"{keyword}",')
+            self.print("NULL,")
+        self.print("};")
+
     def _set_up_token_start_metadata_extraction(self) -> None:
         self.print("if (p->mark == p->fill && _PyPegen_fill_token(p) < 0) {")
         with self.indent():