listmaker: (test|star_expr) ( old_comp_for | (',' (test|star_expr))* [','] )

testlist_gexp: (test|star_expr) ( old_comp_for | (',' (test|star_expr))* [','] )

comp_for: [ASYNC] 'for' exprlist 'in' or_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]

inner_args.append(Node(syms.old_comp_if, [if_leaf, test]))

inner = Node(syms.listmaker, [xp, Node(syms.old_comp_for, inner_args)])

class TestGeneratorExpressions(GrammarTest):

def test_trailing_comma_after_generator_expression_argument_works(self):

# BPO issue 27494

self.validate("set(x for x in [],)")

Make 2to3 accept a trailing comma in generator expressions. For example, ``set(x

for x in [],)`` is now allowed.