Improve B018 further (#202)

Catch also useless expressions on the first node in a body if the node is not a string (docstring).

bugbear.py

@@ -578,8 +578,12 @@ 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(
+        for index, subnode in enumerate(node.body):
+            if not isinstance(subnode, ast.Expr):
+                continue
+            if index == 0 and isinstance(subnode.value, ast.Str):
+                continue  # most likely a docstring
+            if isinstance(
                 subnode.value,
                 (
                     ast.Str,

tests/b018_classes.py

@@ -1,6 +1,6 @@
 """
 Should emit:
-B018 - on lines 15-26, 31
+B018 - on lines 15-26, 30, 32
 """
 
 
@@ -27,5 +27,6 @@ class Foo2:
 
 
 class Foo3:
+    123
     a = 2
     "str"

tests/b018_functions.py

@@ -1,6 +1,6 @@
 """
 Should emit:
-B018 - on lines 14-25, 30
+B018 - on lines 14-25, 29, 31
 """
 
 
@@ -26,5 +26,6 @@ def foo2():
 
 
 def foo3():
+    123
     a = 2
     "str"

tests/test_bugbear.py

@@ -218,7 +218,8 @@ def test_b018_functions(self):
         errors = list(bbc.run())
 
         expected = [B018(line, 4) for line in range(14, 26)]
-        expected.append(B018(30, 4))
+        expected.append(B018(29, 4))
+        expected.append(B018(31, 4))
         self.assertEqual(errors, self.errors(*expected))
 
     def test_b018_classes(self):
@@ -227,7 +228,8 @@ def test_b018_classes(self):
         errors = list(bbc.run())
 
         expected = [B018(line, 4) for line in range(15, 27)]
-        expected.append(B018(31, 4))
+        expected.append(B018(30, 4))
+        expected.append(B018(32, 4))
         self.assertEqual(errors, self.errors(*expected))
 
     def test_b901(self):