include tuples in b018 useless-statement check (#432)

* include tuples in b018 useless-statement check

* annotate the bad and good test lines

* update readme w/ notes about comma

README.rst

@@ -137,6 +137,9 @@ using ``pytest.raises``), or use the context manager form with a target
 (e.g. ``with self.assertRaises(Exception) as ex:``).
 
 **B018**: Found useless expression. Either assign it to a variable or remove it.
+Note that dangling commas will cause things to be interpreted as useless tuples.
+For example, in the statement ``print(".."),`` is the same as ``(print(".."),)``
+which is an unassigned tuple. Simply remove the comma to clear the error.
 
 **B019**: Use of ``functools.lru_cache`` or ``functools.cache`` on methods
 can lead to memory leaks. The cache may retain instance references, preventing

bugbear.py

@@ -1173,6 +1173,7 @@ def check_for_b018(self, node):
                     ast.List,
                     ast.Set,
                     ast.Dict,
+                    ast.Tuple,
                 ),
             ) or (
                 isinstance(subnode.value, ast.Constant)

tests/b018_classes.py

@@ -30,3 +30,6 @@ class Foo3:
     a = 2
     "str"
     1
+    (1,)  # bad
+    (2, 3)  # bad
+    t = (4, 5)  # good

tests/b018_functions.py

@@ -29,3 +29,6 @@ def foo3():
     a = 2
     "str"
     3
+    (1,)  # bad
+    (2, 3)  # bad
+    t = (4, 5)  # good

tests/b018_modules.py

@@ -16,3 +16,6 @@
 [1, 2]  # list
 {1, 2}  # set
 {"foo": "bar"}  # dict
+(1,)  # bad
+(2, 3)  # bad
+t = (4, 5)  # good

tests/test_bugbear.py

@@ -270,6 +270,8 @@ def test_b018_functions(self):
         expected = [B018(line, 4) for line in range(15, 25)]
         expected.append(B018(28, 4))
         expected.append(B018(31, 4))
+        expected.append(B018(32, 4))
+        expected.append(B018(33, 4))
         self.assertEqual(errors, self.errors(*expected))
 
     def test_b018_classes(self):
@@ -280,14 +282,16 @@ def test_b018_classes(self):
         expected = [B018(line, 4) for line in range(16, 26)]
         expected.append(B018(29, 4))
         expected.append(B018(32, 4))
+        expected.append(B018(33, 4))
+        expected.append(B018(34, 4))
         self.assertEqual(errors, self.errors(*expected))
 
     def test_b018_modules(self):
         filename = Path(__file__).absolute().parent / "b018_modules.py"
         bbc = BugBearChecker(filename=str(filename))
         errors = list(bbc.run())
 
-        expected = [B018(line, 0) for line in range(9, 19)]
+        expected = [B018(line, 0) for line in range(9, 21)]
         self.assertEqual(errors, self.errors(*expected))
 
     def test_b019(self):