bpo-42997: Improve error message for missing : before suites (GH-24292)

* Add to the peg generator a new directive ('&&') that allows to expect
  a token and hard fail the parsing if the token is not found. This
  allows to quickly emmit syntax errors for missing tokens.

* Use the new grammar element to hard-fail if the ':' is missing before
  suites.

Doc/tools/extensions/peg_highlight.py

@@ -27,6 +27,12 @@ class PEGLexer(RegexLexer):
     tokens = {
         "ws": [(r"\n", Text), (r"\s+", Text), (r"#.*$", Comment.Singleline),],
         "lookaheads": [
+            # Forced tokens
+            (r"(&&)(?=\w+\s?)", bygroups(None)),
+            (r"(&&)(?='.+'\s?)", bygroups(None)),
+            (r'(&&)(?=".+"\s?)', bygroups(None)),
+            (r"(&&)(?=\(.+\)\s?)", bygroups(None)),
+
             (r"(?<=\|\s)(&\w+\s?)", bygroups(None)),
             (r"(?<=\|\s)(&'.+'\s?)", bygroups(None)),
             (r'(?<=\|\s)(&".+"\s?)', bygroups(None)),

Grammar/python.gram

@@ -162,22 +162,22 @@ dotted_name[expr_ty]:
     | NAME
 
 if_stmt[stmt_ty]:
-    | 'if' a=named_expression ':' b=block c=elif_stmt {
+    | 'if' a=named_expression &&':' b=block c=elif_stmt {
         _Py_If(a, b, CHECK(asdl_stmt_seq*, _PyPegen_singleton_seq(p, c)), EXTRA) }
-    | 'if' a=named_expression ':' b=block c=[else_block] { _Py_If(a, b, c, EXTRA) }
+    | 'if' a=named_expression &&':' b=block c=[else_block] { _Py_If(a, b, c, EXTRA) }
 elif_stmt[stmt_ty]:
-    | 'elif' a=named_expression ':' b=block c=elif_stmt {
+    | 'elif' a=named_expression &&':' b=block c=elif_stmt {
         _Py_If(a, b, CHECK(asdl_stmt_seq*, _PyPegen_singleton_seq(p, c)), EXTRA) }
-    | 'elif' a=named_expression ':' b=block c=[else_block] { _Py_If(a, b, c, EXTRA) }
-else_block[asdl_stmt_seq*]: 'else' ':' b=block { b }
+    | 'elif' a=named_expression &&':' b=block c=[else_block] { _Py_If(a, b, c, EXTRA) }
+else_block[asdl_stmt_seq*]: 'else' &&':' b=block { b }
 
 while_stmt[stmt_ty]:
-    | 'while' a=named_expression ':' b=block c=[else_block] { _Py_While(a, b, c, EXTRA) }
+    | 'while' a=named_expression &&':' b=block c=[else_block] { _Py_While(a, b, c, EXTRA) }
 
 for_stmt[stmt_ty]:
-    | 'for' t=star_targets 'in' ~ ex=star_expressions ':' tc=[TYPE_COMMENT] b=block el=[else_block] {
+    | 'for' t=star_targets 'in' ~ ex=star_expressions &&':' tc=[TYPE_COMMENT] b=block el=[else_block] {
         _Py_For(t, ex, b, el, NEW_TYPE_COMMENT(p, tc), EXTRA) }
-    | ASYNC 'for' t=star_targets 'in' ~ ex=star_expressions ':' tc=[TYPE_COMMENT] b=block el=[else_block] {
+    | ASYNC 'for' t=star_targets 'in' ~ ex=star_expressions &&':' tc=[TYPE_COMMENT] b=block el=[else_block] {
         CHECK_VERSION(stmt_ty, 5, "Async for loops are", _Py_AsyncFor(t, ex, b, el, NEW_TYPE_COMMENT(p, tc), EXTRA)) }
     | invalid_for_target
 
@@ -190,18 +190,20 @@ with_stmt[stmt_ty]:
        CHECK_VERSION(stmt_ty, 5, "Async with statements are", _Py_AsyncWith(a, b, NULL, EXTRA)) }
     | ASYNC 'with' a[asdl_withitem_seq*]=','.with_item+ ':' tc=[TYPE_COMMENT] b=block {
        CHECK_VERSION(stmt_ty, 5, "Async with statements are", _Py_AsyncWith(a, b, NEW_TYPE_COMMENT(p, tc), EXTRA)) }
+    | invalid_with_stmt
+
 with_item[withitem_ty]:
     | e=expression 'as' t=star_target &(',' | ')' | ':') { _Py_withitem(e, t, p->arena) }
     | invalid_with_item
     | e=expression { _Py_withitem(e, NULL, p->arena) }
 
 try_stmt[stmt_ty]:
-    | 'try' ':' b=block f=finally_block { _Py_Try(b, NULL, NULL, f, EXTRA) }
-    | 'try' ':' b=block ex[asdl_excepthandler_seq*]=except_block+ el=[else_block] f=[finally_block] { _Py_Try(b, ex, el, f, EXTRA) }
+    | 'try' &&':' b=block f=finally_block { _Py_Try(b, NULL, NULL, f, EXTRA) }
+    | 'try' &&':' b=block ex[asdl_excepthandler_seq*]=except_block+ el=[else_block] f=[finally_block] { _Py_Try(b, ex, el, f, EXTRA) }
 except_block[excepthandler_ty]:
-    | 'except' e=expression t=['as' z=NAME { z }] ':' b=block {
+    | 'except' e=expression t=['as' z=NAME { z }] &&':' b=block {
         _Py_ExceptHandler(e, (t) ? ((expr_ty) t)->v.Name.id : NULL, b, EXTRA) }
-    | 'except' ':' b=block { _Py_ExceptHandler(NULL, NULL, b, EXTRA) }
+    | 'except' &&':' b=block { _Py_ExceptHandler(NULL, NULL, b, EXTRA) }
 finally_block[asdl_stmt_seq*]: 'finally' ':' a=block { a }
 
 return_stmt[stmt_ty]:
@@ -216,11 +218,11 @@ function_def[stmt_ty]:
     | function_def_raw
 
 function_def_raw[stmt_ty]:
-    | 'def' n=NAME '(' params=[params] ')' a=['->' z=expression { z }] ':' tc=[func_type_comment] b=block {
+    | 'def' n=NAME '(' params=[params] ')' a=['->' z=expression { z }] &&':' tc=[func_type_comment] b=block {
         _Py_FunctionDef(n->v.Name.id,
                         (params) ? params : CHECK(arguments_ty, _PyPegen_empty_arguments(p)),
                         b, NULL, a, NEW_TYPE_COMMENT(p, tc), EXTRA) }
-    | ASYNC 'def' n=NAME '(' params=[params] ')' a=['->' z=expression { z }] ':' tc=[func_type_comment] b=block {
+    | ASYNC 'def' n=NAME '(' params=[params] ')' a=['->' z=expression { z }] &&':' tc=[func_type_comment] b=block {
         CHECK_VERSION(
             stmt_ty,
             5,
@@ -300,7 +302,7 @@ class_def[stmt_ty]:
     | a=decorators b=class_def_raw { _PyPegen_class_def_decorators(p, a, b) }
     | class_def_raw
 class_def_raw[stmt_ty]:
-    | 'class' a=NAME b=['(' z=[arguments] ')' { z }] ':' c=block {
+    | 'class' a=NAME b=['(' z=[arguments] ')' { z }] &&':' c=block {
         _Py_ClassDef(a->v.Name.id,
                      (b) ? ((expr_ty) b)->v.Call.args : NULL,
                      (b) ? ((expr_ty) b)->v.Call.keywords : NULL,
@@ -718,7 +720,7 @@ invalid_double_type_comments:
     | TYPE_COMMENT NEWLINE TYPE_COMMENT NEWLINE INDENT {
         RAISE_SYNTAX_ERROR("Cannot have two type comments on def") }
 invalid_with_item:
-    | expression 'as' a=expression {
+    | expression 'as' a=expression &(',' | ')' | ':') {
         RAISE_SYNTAX_ERROR_INVALID_TARGET(STAR_TARGETS, a) }
 
 invalid_for_target:
@@ -731,3 +733,7 @@ invalid_group:
 invalid_import_from_targets:
     | import_from_as_names ',' {
         RAISE_SYNTAX_ERROR("trailing comma not allowed without surrounding parentheses") }
+
+invalid_with_stmt:
+    | [ASYNC] 'with' ','.(expression ['as' star_target])+ &&':'
+    | [ASYNC] 'with' '(' ','.(expressions ['as' star_target])+ ','? ')' &&':'

Lib/test/test_syntax.py

@@ -229,7 +229,7 @@ SyntaxError: cannot assign to function call
 
 >>> with a as b
 Traceback (most recent call last):
-SyntaxError: invalid syntax
+SyntaxError: expected ':'
 
 >>> p = p =
 Traceback (most recent call last):
@@ -331,7 +331,7 @@ SyntaxError: Generator expression must be parenthesized
 >>> class C(x for x in L):
 ...     pass
 Traceback (most recent call last):
-SyntaxError: invalid syntax
+SyntaxError: expected ':'
 
 >>> def g(*args, **kwargs):
 ...     print(args, sorted(kwargs.items()))
@@ -708,6 +708,107 @@ leading to spurious errors.
      ...
    SyntaxError: cannot assign to function call
 
+    Missing ':' before suites:
+
+    >>> def f()
+    ...     pass
+    Traceback (most recent call last):
+    SyntaxError: expected ':'
+
+    >>> class A
+    ...     pass
+    Traceback (most recent call last):
+    SyntaxError: expected ':'
+
+   >>> if 1
+   ...   pass
+   ... elif 1:
+   ...   pass
+   ... else:
+   ...   x() = 1
+   Traceback (most recent call last):
+   SyntaxError: expected ':'
+
+   >>> if 1:
+   ...   pass
+   ... elif 1
+   ...   pass
+   ... else:
+   ...   x() = 1
+   Traceback (most recent call last):
+   SyntaxError: expected ':'
+
+   >>> if 1:
+   ...   pass
+   ... elif 1:
+   ...   pass
+   ... else
+   ...   x() = 1
+   Traceback (most recent call last):
+   SyntaxError: expected ':'
+
+   >>> for x in range(10)
+   ...   pass
+   Traceback (most recent call last):
+   SyntaxError: expected ':'
+
+   >>> while True
+   ...   pass
+   Traceback (most recent call last):
+   SyntaxError: expected ':'
+
+   >>> with blech as something
+   ...   pass
+   Traceback (most recent call last):
+   SyntaxError: expected ':'
+
+   >>> with blech
+   ...   pass
+   Traceback (most recent call last):
+   SyntaxError: expected ':'
+
+   >>> with blech, block as something
+   ...   pass
+   Traceback (most recent call last):
+   SyntaxError: expected ':'
+
+   >>> with blech, block as something, bluch
+   ...   pass
+   Traceback (most recent call last):
+   SyntaxError: expected ':'
+
+   >>> with (blech as something)
+   ...   pass
+   Traceback (most recent call last):
+   SyntaxError: expected ':'
+
+   >>> with (blech)
+   ...   pass
+   Traceback (most recent call last):
+   SyntaxError: expected ':'
+
+   >>> with (blech, block as something)
+   ...   pass
+   Traceback (most recent call last):
+   SyntaxError: expected ':'
+
+   >>> with (blech, block as something, bluch)
+   ...   pass
+   Traceback (most recent call last):
+   SyntaxError: expected ':'
+
+   >>> try
+   ...   pass
+   Traceback (most recent call last):
+   SyntaxError: expected ':'
+
+   >>> try:
+   ...   pass
+   ... except
+   ...   pass
+   Traceback (most recent call last):
+   SyntaxError: expected ':'
+
 Make sure that the old "raise X, Y[, Z]" form is gone:
    >>> raise X, Y
    Traceback (most recent call last):
@@ -992,7 +1093,7 @@ def func2():
     finally:
         pass
 """
-        self._check_error(code, "invalid syntax")
+        self._check_error(code, "expected ':'")
 
     def test_invalid_line_continuation_left_recursive(self):
         # Check bpo-42218: SyntaxErrors following left-recursive rules

Misc/NEWS.d/next/Core and Builtins/2021-01-24-20-19-55.bpo-42997.QUOPgP.rst

@@ -0,0 +1 @@
+Improve error message for missing ":" before blocks. Patch by Pablo Galindo.

Parser/pegen.c

@@ -782,7 +782,6 @@ _PyPegen_is_memoized(Parser *p, int type, void *pres)
     return 0;
 }
 
-
 int
 _PyPegen_lookahead_with_name(int positive, expr_ty (func)(Parser *), Parser *p)
 {
@@ -836,6 +835,28 @@ _PyPegen_expect_token(Parser *p, int type)
     return t;
 }
 
+Token *
+_PyPegen_expect_forced_token(Parser *p, int type, const char* expected) {
+
+    if (p->error_indicator == 1) {
+        return NULL;
+    }
+
+    if (p->mark == p->fill) {
+        if (_PyPegen_fill_token(p) < 0) {
+            p->error_indicator = 1;
+            return NULL;
+        }
+    }
+    Token *t = p->tokens[p->mark];
+    if (t->type != type) {
+        RAISE_SYNTAX_ERROR_KNOWN_LOCATION(t, "expected '%s'", expected);
+        return NULL;
+    }
+    p->mark += 1;
+    return t;
+}
+
 expr_ty
 _PyPegen_expect_soft_keyword(Parser *p, const char *keyword)
 {

Parser/pegen.h

@@ -102,10 +102,7 @@ typedef struct {
     arg_ty kwarg;
 } StarEtc;
 
-typedef struct {
-    operator_ty kind;
-} AugOperator;
-
+typedef struct { operator_ty kind; } AugOperator; 
 typedef struct {
     void *element;
     int is_keyword;
@@ -118,12 +115,14 @@ int _PyPegen_insert_memo(Parser *p, int mark, int type, void *node);
 int _PyPegen_update_memo(Parser *p, int mark, int type, void *node);
 int _PyPegen_is_memoized(Parser *p, int type, void *pres);
 
+
 int _PyPegen_lookahead_with_name(int, expr_ty (func)(Parser *), Parser *);
 int _PyPegen_lookahead_with_int(int, Token *(func)(Parser *, int), Parser *, int);
 int _PyPegen_lookahead_with_string(int , expr_ty (func)(Parser *, const char*), Parser *, const char*);
 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);
 Token *_PyPegen_get_last_nonnwhitespace_token(Parser *);
 int _PyPegen_fill_token(Parser *p);

Tools/peg_generator/pegen/c_generator.py

@@ -8,6 +8,7 @@ from pegen import grammar
 from pegen.grammar import (
     Alt,
     Cut,
+    Forced,
     Gather,
     GrammarVisitor,
     Group,
@@ -252,6 +253,24 @@ class CCallMakerVisitor(GrammarVisitor):
     def visit_NegativeLookahead(self, node: NegativeLookahead) -> FunctionCall:
         return self.lookahead_call_helper(node, 0)
 
+    def visit_Forced(self, node: Forced) -> FunctionCall:
+        call = self.generate_call(node.node)
+        if call.nodetype == NodeTypes.GENERIC_TOKEN:
+            val = ast.literal_eval(node.node.value)
+            assert val in self.exact_tokens, f"{node.value} is not a known literal"
+            type = self.exact_tokens[val]
+            return FunctionCall(
+                assigned_variable="_literal",
+                function=f"_PyPegen_expect_forced_token",
+                arguments=["p", type, f'"{val}"'],
+                nodetype=NodeTypes.GENERIC_TOKEN,
+                return_type="Token *",
+                comment=f"forced_token='{val}'",
+            )
+        else:
+            raise NotImplementedError(
+                    f"Forced tokens don't work with {call.nodetype} tokens")
+
     def visit_Opt(self, node: Opt) -> FunctionCall:
         call = self.generate_call(node.node)
         return FunctionCall(

Tools/peg_generator/pegen/grammar.py

@@ -288,6 +288,23 @@ class NamedItem:
         gen.callmakervisitor.visit(self.item)
 
 
+class Forced:
+    def __init__(self, node: Plain):
+        self.node = node
+
+    def __str__(self) -> str:
+        return f"&&{self.node}"
+
+    def __iter__(self) -> Iterator[Plain]:
+        yield self.node
+
+    def nullable_visit(self, rules: Dict[str, Rule]) -> bool:
+        return True
+
+    def initial_names(self) -> AbstractSet[str]:
+        return set()
+
+
 class Lookahead:
     def __init__(self, node: Plain, sign: str):
         self.node = node
@@ -459,7 +476,7 @@ class Cut:
 
 
 Plain = Union[Leaf, Group]
-Item = Union[Plain, Opt, Repeat, Lookahead, Rhs, Cut]
+Item = Union[Plain, Opt, Repeat, Forced, Lookahead, Rhs, Cut]
 RuleName = Tuple[str, str]
 MetaTuple = Tuple[str, Optional[str]]
 MetaList = List[MetaTuple]

Tools/peg_generator/pegen/grammar_parser.py

@@ -13,6 +13,7 @@ from ast import literal_eval
 from pegen.grammar import (
     Alt,
     Cut,
+    Forced,
     Gather,
     Group,
     Item,
@@ -402,7 +403,7 @@ class GeneratedParser(Parser):
 
     @memoize
     def named_item(self) -> Optional[NamedItem]:
-        # named_item: NAME '[' NAME '*' ']' '=' ~ item | NAME '[' NAME ']' '=' ~ item | NAME '=' ~ item | item | lookahead
+        # named_item: NAME '[' NAME '*' ']' '=' ~ item | NAME '[' NAME ']' '=' ~ item | NAME '=' ~ item | item | forced_atom | lookahead
         mark = self.mark()
         cut = False
         if (
@@ -465,6 +466,13 @@ class GeneratedParser(Parser):
         self.reset(mark)
         if cut: return None
         cut = False
+        if (
+            (it := self.forced_atom())
+        ):
+            return NamedItem ( None , it )
+        self.reset(mark)
+        if cut: return None
+        cut = False
         if (
             (it := self.lookahead())
         ):
@@ -473,6 +481,25 @@ class GeneratedParser(Parser):
         if cut: return None
         return None
 
+    @memoize
+    def forced_atom(self) -> Optional[NamedItem]:
+        # forced_atom: '&' '&' ~ atom
+        mark = self.mark()
+        cut = False
+        if (
+            (literal := self.expect('&'))
+            and
+            (literal_1 := self.expect('&'))
+            and
+            (cut := True)
+            and
+            (atom := self.atom())
+        ):
+            return Forced ( atom )
+        self.reset(mark)
+        if cut: return None
+        return None
+
     @memoize
     def lookahead(self) -> Optional[LookaheadOrCut]:
         # lookahead: '&' ~ atom | '!' ~ atom | '~'

Tools/peg_generator/pegen/metagrammar.gram

@@ -4,6 +4,7 @@ from ast import literal_eval
 from pegen.grammar import (
     Alt,
     Cut,
+    Forced,
     Gather,
     Group,
     Item,
@@ -87,8 +88,12 @@ named_item[NamedItem]:
     | NAME '[' type=NAME ']' '=' ~ item {NamedItem(name.string, item, type.string)}
     | NAME '=' ~ item {NamedItem(name.string, item)}
     | item {NamedItem(None, item)}
+    | it=forced_atom {NamedItem(None, it)}
     | it=lookahead {NamedItem(None, it)}
 
+forced_atom[NamedItem]:
+    | '&''&' ~ atom {Forced(atom)}
+
 lookahead[LookaheadOrCut]:
     | '&' ~ atom {PositiveLookahead(atom)}
     | '!' ~ atom {NegativeLookahead(atom)}