Restructure the Python 2 grammar to be closer to the original Python 2 grammar in case of list comprehensions.

parso/python/grammar27.txt

@@ -103,10 +103,11 @@ term: factor (('*'|'/'|'%'|'//') factor)*
 factor: ('+'|'-'|'~') factor | power
 power: atom trailer* ['**' factor]
 atom: ('(' [yield_expr|testlist_comp] ')' |
-       '[' [testlist_comp] ']' |
+       '[' [listmaker] ']' |
        '{' [dictorsetmaker] '}' |
        '`' testlist1 '`' |
        NAME | NUMBER | STRING+)
+listmaker: test ( list_for | (',' test)* [','] )
 testlist_comp: test ( comp_for | (',' test)* [','] )
 lambdef: 'lambda' [varargslist] ':' test
 trailer: '(' [arglist] ')' | '[' subscriptlist ']' | '.' NAME
@@ -127,8 +128,12 @@ arglist: (argument ',')* (argument [',']
 # results in an ambiguity. ast.c makes sure it's a NAME.
 argument: test [comp_for] | test '=' test
 
+list_iter: list_for | list_if
+list_for: 'for' exprlist 'in' testlist_safe [list_iter]
+list_if: 'if' old_test [list_iter]
+
 comp_iter: comp_for | comp_if
-comp_for: 'for' exprlist 'in' testlist_safe [comp_iter]
+comp_for: 'for' exprlist 'in' or_test [comp_iter]
 comp_if: 'if' old_test [comp_iter]
 
 testlist1: test (',' test)*

parso/python/parser.py

@@ -38,6 +38,10 @@ class Parser(BaseParser):
         'while_stmt': tree.WhileStmt,
         'try_stmt': tree.TryStmt,
         'comp_for': tree.CompFor,
+        # Not sure if this is the best idea, but IMO it's the easiest way to
+        # avoid extreme amounts of work around the subtle difference of 2/3
+        # grammar in list comoprehensions.
+        'list_for': tree.CompFor,
         'decorator': tree.Decorator,
         'lambdef': tree.Lambda,
         'old_lambdef': tree.Lambda,
@@ -101,6 +105,12 @@ class Parser(BaseParser):
                 # ones and therefore have pseudo start/end positions and no
                 # prefixes. Just ignore them.
                 children = [children[0]] + children[2:-1]
+            elif symbol == 'list_if':
+                # Make transitioning from 2 to 3 easier.
+                symbol = 'comp_if'
+            elif symbol == 'listmaker':
+                # Same as list_if above.
+                symbol = 'testlist_comp'
             return self.default_node(symbol, children)
 
     def convert_leaf(self, pgen_grammar, type, value, prefix, start_pos):