Fix #411. Fix extracting generator without parens

Extracting generator comprehension without selecting the surrounding
parentheses may produce invalid code. This commit adds surrounding
parentheses when it detects that the selected code is a bare generator
comprehension.

rope/refactor/extract.py

@@ -251,6 +251,20 @@ def returning_named_expr(self):
             )
         return self._returning_named_expr
 
+    _returning_generator = None
+
+    @property
+    def returning_generator_exp(self):
+        """Does the extracted piece contains a generator expression"""
+        if self._returning_generator is None:
+            self._returning_generator = (
+                isinstance(self._parsed_extracted, ast.Module)
+                and isinstance(self._parsed_extracted.body[0], ast.Expr)
+                and isinstance(self._parsed_extracted.body[0].value, ast.GeneratorExp)
+            )
+
+        return self._returning_generator
+
 
 class _ExtractCollector:
     """Collects information needed for performing the extract"""
@@ -1103,6 +1117,6 @@ def _get_single_expression_body(extracted, info):
     if not large_multiline:
         extracted = _join_lines(extracted)
     multiline_expression = "\n" in extracted
-    if info.returning_named_expr or (multiline_expression and not large_multiline):
+    if info.returning_named_expr or info.returning_generator_exp or (multiline_expression and not large_multiline):
         extracted = "(" + extracted + ")"
     return extracted

ropetest/refactor/extracttest.py

@@ -2708,6 +2708,27 @@ def _a(y):
         """)
         self.assertEqual(expected, refactored)
 
+    def test_extract_with_generator_2(self):
+        code = dedent("""\
+            def f():
+                y = [1,2,3,4]
+                a = sum(x for x in y)
+        """)
+        extract_target = "x for x in y"
+        start, end = code.index(extract_target), code.index(extract_target) + len(
+            extract_target
+        )
+        refactored = self.do_extract_method(code, start, end, "_a")
+        expected = dedent("""\
+            def f():
+                y = [1,2,3,4]
+                a = sum(_a(y))
+
+            def _a(y):
+                return (x for x in y)
+        """)
+        self.assertEqual(expected, refactored)
+
     def test_extract_with_set_comprehension(self):
         code = dedent("""\
             def f():