Add B018 check to find useless string declarations (#196)

* Add B018 check to find useless string declarations

This will find all sorts of useless string declarations in functions and classes.
It will also prevent to use multiline strings as comments

Resolves #195

* Better error message

* Fix black

Author: kasium

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))