Revert "closes bpo-27494: Fix 2to3 handling of trailing comma after a generator expression (GH-3771)" (#8241)

This reverts commit af810b35b4.

This is not valid syntax (see bpo-32012).

Lib/lib2to3/Grammar.txt

@@ -111,8 +111,8 @@ atom: ('(' [yield_expr|testlist_gexp] ')' |
        '{' [dictsetmaker] '}' |
        '`' testlist1 '`' |
        NAME | NUMBER | STRING+ | '.' '.' '.')
-listmaker: (test|star_expr) ( old_comp_for | (',' (test|star_expr))* [','] )
-testlist_gexp: (test|star_expr) ( old_comp_for | (',' (test|star_expr))* [','] )
+listmaker: (test|star_expr) ( comp_for | (',' (test|star_expr))* [','] )
+testlist_gexp: (test|star_expr) ( comp_for | (',' (test|star_expr))* [','] )
 lambdef: 'lambda' [varargslist] ':' test
 trailer: '(' [arglist] ')' | '[' subscriptlist ']' | '.' NAME
 subscriptlist: subscript (',' subscript)* [',']
@@ -142,28 +142,9 @@ argument: ( test [comp_for] |
 	    star_expr )
 
 comp_iter: comp_for | comp_if
-comp_for: [ASYNC] 'for' exprlist 'in' or_test [comp_iter]
+comp_for: [ASYNC] 'for' exprlist 'in' testlist_safe [comp_iter]
 comp_if: 'if' old_test [comp_iter]
 
-# As noted above, testlist_safe extends the syntax allowed in list
-# comprehensions and generators. We can't use it indiscriminately in all
-# derivations using a comp_for-like pattern because the testlist_safe derivation
-# contains comma which clashes with trailing comma in arglist.
-#
-# This was an issue because the parser would not follow the correct derivation
-# when parsing syntactically valid Python code. Since testlist_safe was created
-# specifically to handle list comprehensions and generator expressions enclosed
-# with parentheses, it's safe to only use it in those. That avoids the issue; we
-# can parse code like set(x for x in [],).
-#
-# The syntax supported by this set of rules is not a valid Python 3 syntax,
-# hence the prefix "old".
-#
-# See https://bugs.python.org/issue27494
-old_comp_iter: old_comp_for | old_comp_if
-old_comp_for: [ASYNC] 'for' exprlist 'in' testlist_safe [old_comp_iter]
-old_comp_if: 'if' old_test [old_comp_iter]
-
 testlist1: test (',' test)*
 
 # not used in grammar, but may appear in "node" passed from Parser to Compiler

Lib/lib2to3/fixer_util.py

@@ -101,8 +101,8 @@ def ListComp(xp, fp, it, test=None):
         test.prefix = " "
         if_leaf = Leaf(token.NAME, "if")
         if_leaf.prefix = " "
-        inner_args.append(Node(syms.old_comp_if, [if_leaf, test]))
-    inner = Node(syms.listmaker, [xp, Node(syms.old_comp_for, inner_args)])
+        inner_args.append(Node(syms.comp_if, [if_leaf, test]))
+    inner = Node(syms.listmaker, [xp, Node(syms.comp_for, inner_args)])
     return Node(syms.atom,
                        [Leaf(token.LBRACE, "["),
                         inner,
@@ -208,7 +208,7 @@ def attr_chain(obj, attr):
         next = getattr(next, attr)
 
 p0 = """for_stmt< 'for' any 'in' node=any ':' any* >
-        | old_comp_for< 'for' any 'in' node=any any* >
+        | comp_for< 'for' any 'in' node=any any* >
      """
 p1 = """
 power<

Lib/lib2to3/fixes/fix_dict.py

@@ -83,7 +83,7 @@ class FixDict(fixer_base.BaseFix):
     p1 = patcomp.compile_pattern(P1)
 
     P2 = """for_stmt< 'for' any 'in' node=any ':' any* >
-            | old_comp_for< 'for' any 'in' node=any any* >
+            | comp_for< 'for' any 'in' node=any any* >
          """
     p2 = patcomp.compile_pattern(P2)
 

Lib/lib2to3/fixes/fix_paren.py

@@ -15,7 +15,7 @@ class FixParen(fixer_base.BaseFix):
     PATTERN = """
         atom< ('[' | '(')
             (listmaker< any
-                old_comp_for<
+                comp_for<
                     'for' NAME 'in'
                     target=testlist_safe< any (',' any)+ [',']
                      >
@@ -24,7 +24,7 @@ class FixParen(fixer_base.BaseFix):
             >
             |
             testlist_gexp< any
-                old_comp_for<
+                comp_for<
                     'for' NAME 'in'
                     target=testlist_safe< any (',' any)+ [',']
                      >

Lib/lib2to3/fixes/fix_xrange.py

@@ -55,7 +55,7 @@ class FixXrange(fixer_base.BaseFix):
     p1 = patcomp.compile_pattern(P1)
 
     P2 = """for_stmt< 'for' any 'in' node=any ':' any* >
-            | old_comp_for< 'for' any 'in' node=any any* >
+            | comp_for< 'for' any 'in' node=any any* >
             | comparison< any 'in' node=any any*>
          """
     p2 = patcomp.compile_pattern(P2)

Lib/lib2to3/tests/test_parser.py

@@ -612,13 +612,6 @@ class TestLiterals(GrammarTest):
         self.validate(s)
 
 
-class TestGeneratorExpressions(GrammarTest):
-
-    def test_trailing_comma_after_generator_expression_argument_works(self):
-        # BPO issue 27494
-        self.validate("set(x for x in [],)")
-
-
 def diff_texts(a, b, filename):
     a = a.splitlines()
     b = b.splitlines()

Misc/NEWS.d/next/Library/2018-07-11-10-03-21.bpo-27494.04OWkW.rst

@@ -0,0 +1,2 @@
+Reverted :issue:`27494`. 2to3 rejects now a trailing comma in generator
+expressions.