Add B018 check to find useless string declarations

README.rst

@@ -130,6 +130,8 @@ Either assert for a more specific exception (builtin or custom), use
 (``with self.assertRaises(Exception) as ex:``) with an assertion against the
 data available in ``ex``.
 
+**B018**: Found useless expression. Either assign it to a variable or remove it.
+
 
 Opinionated warnings
 ~~~~~~~~~~~~~~~~~~~~

bugbear.py

@@ -278,10 +278,12 @@ def visit_FunctionDef(self, node):
         self.check_for_b901(node)
         self.check_for_b902(node)
         self.check_for_b006(node)
+        self.check_for_b018(node)
         self.generic_visit(node)
 
     def visit_ClassDef(self, node):
         self.check_for_b903(node)
+        self.check_for_b018(node)
         self.generic_visit(node)
 
     def visit_Try(self, node):
@@ -575,6 +577,11 @@ def check_for_b903(self, node):
 
         self.errors.append(B903(node.lineno, node.col_offset))
 
+    def check_for_b018(self, node):
+        for subnode in node.body[1:]:
+            if isinstance(subnode, ast.Expr) and isinstance(subnode.value, ast.Str):
+                self.errors.append(B018(subnode.lineno, subnode.col_offset))
+
 
 @attr.s
 class NameFinder(ast.NodeVisitor):
@@ -767,6 +774,11 @@ def visit(self, node):
         "context manager form of assertRaises."
     )
 )
+B018 = Error(
+    message=(
+        "B018 Found useless expression. Either assign it to a variable or remove it."
+    )
+)
 
 # Warnings disabled by default.
 B901 = Error(

tests/b018.py

@@ -0,0 +1,35 @@
+"""
+Should emit:
+B018 - on lines 14, 19, 30, 35
+"""
+
+
+def foo1():
+    """my docstring"""
+
+
+def foo2():
+    """my docstring"""
+    a = 2
+    "str"
+
+
+def foo3():
+    a = 2
+    "str"
+
+
+class Foo1:
+    """abc"""
+
+
+class Foo2:
+    """abc"""
+
+    a = 2
+    "str"
+
+
+class Foo3:
+    a = 2
+    "str"

tests/test_bugbear.py

@@ -28,6 +28,7 @@
     B015,
     B016,
     B017,
+    B018,
     B904,
     B901,
     B902,
@@ -211,6 +212,14 @@ def test_b017(self):
         expected = self.errors(B017(22, 8))
         self.assertEqual(errors, expected)
 
+    def test_b018(self):
+        filename = Path(__file__).absolute().parent / "b018.py"
+        bbc = BugBearChecker(filename=str(filename))
+        errors = list(bbc.run())
+        self.assertEqual(
+            errors, self.errors(B018(14, 4), B018(19, 4), B018(30, 4), B018(35, 4))
+        )
+
     def test_b901(self):
         filename = Path(__file__).absolute().parent / "b901.py"
         bbc = BugBearChecker(filename=str(filename))