linters: Add flake8-bugbear for finding bugs and design problems

Also, make appropriate changes to satisfy issues flagged by
`flake8-bugbear`.

src/flake8/checker.py

@@ -503,7 +503,7 @@ class FileChecker(object):
                 runner = checker.run()
             except AttributeError:
                 runner = checker
-            for (line_number, offset, text, check) in runner:
+            for (line_number, offset, text, _) in runner:
                 self.report(
                     error_code=None,
                     line_number=line_number,
@@ -668,11 +668,14 @@ def _run_checks(checker):
 
 
 def find_offset(offset, mapping):
+    # type: (int, processor._LogicalMapping) -> Tuple[int, int]
     """Find the offset tuple for a single offset."""
     if isinstance(offset, tuple):
         return offset
 
-    for token_offset, position in mapping:
+    for token in mapping:
+        token_offset = token[0]
         if offset <= token_offset:
+            position = token[1]
             break
     return (position[0], position[1] + offset - token_offset)

tests/unit/test_base_formatter.py

@@ -147,7 +147,7 @@ class AfterInitFormatter(base.BaseFormatter):
 def test_after_init_is_always_called():
     """Verify after_init is called."""
     formatter = AfterInitFormatter(options())
-    assert getattr(formatter, 'post_initialized') is True
+    assert formatter.post_initialized is True
 
 
 class FormatFormatter(base.BaseFormatter):

tox.ini

@@ -45,6 +45,7 @@ basepython = python3
 skip_install = true
 deps =
     flake8
+    flake8-bugbear
     flake8-colors
     flake8-docstrings>=1.3.1
     flake8-import-order>=0.9