Further refine our logic handling selection

There was a *very* subtle bug in how we handle blanket select statements
with error codes that are in our DEFAULT_IGNORE. In the specific case,
users were specifying ``--select E`` and E126 was not being properly
reported. This unveiled further logic bugs as we refined this.

Closes #318

2 files changed, 13 insertions, 5 deletions

diff --git a/src/flake8/style_guide.py b/src/flake8/style_guide.py
index fb0df3a..38fc29a 100644
--- a/src/flake8/style_guide.py
+++ b/src/flake8/style_guide.py
@@ -74,6 +74,10 @@ class StyleGuide(object):
             reverse=True,
         ))
         self._ignored = tuple(sorted(options.ignore, reverse=True))
+        self._using_default_ignore = set(self._ignored) == set(defaults.IGNORE)
+        self._using_default_select = (
+            set(self._selected) == set(defaults.SELECT)
+        )
         self._decision_cache = {}
         self._parsed_diff = {}
 
@@ -135,14 +139,15 @@ class StyleGuide(object):
         ignore = find_first_match(code, self._ignored)
 
         if select and ignore:
+            if self._using_default_ignore and not self._using_default_select:
+                return Decision.Selected
             return find_more_specific(select, ignore)
         if extra_select and ignore:
             return find_more_specific(extra_select, ignore)
-        if select or (extra_select and self._selected == defaults.SELECT):
+        if select or (extra_select and self._using_default_select):
             return Decision.Selected
-        if select is None and extra_select is None and ignore is not None:
-            return Decision.Ignored
-        if self._selected != defaults.SELECT and select is None:
+        if ((select is None and extra_select is None) or
+                (not self._using_default_ignore and select is None)):
             return Decision.Ignored
         return Decision.Selected
 
diff --git a/tests/unit/test_style_guide.py b/tests/unit/test_style_guide.py
index 7357b25..10550f8 100644
--- a/tests/unit/test_style_guide.py
+++ b/tests/unit/test_style_guide.py
@@ -125,6 +125,9 @@ def test_is_user_selected_excludes_errors(select_list, error_code):
     (['E41'], ['E2', 'E12', 'E4'], 'E410', style_guide.Decision.Selected),
     (['E'], ['F'], 'E410', style_guide.Decision.Selected),
     (['F'], [], 'E410', style_guide.Decision.Ignored),
+    (['E'], defaults.IGNORE, 'E126', style_guide.Decision.Selected),
+    (['W'], defaults.IGNORE, 'E126', style_guide.Decision.Ignored),
+    (['E'], defaults.IGNORE, 'W391', style_guide.Decision.Ignored),
 ])
 def test_should_report_error(select_list, ignore_list, error_code, expected):
     """Verify we decide when to report an error."""
@@ -141,7 +144,7 @@ def test_should_report_error(select_list, ignore_list, error_code, expected):
         (defaults.SELECT, [], ['I1'], [], 'I100',
             style_guide.Decision.Selected),
         (defaults.SELECT, [], ['I1'], [], 'I201',
-            style_guide.Decision.Selected),
+            style_guide.Decision.Ignored),
         (defaults.SELECT, ['I2'], ['I1'], [], 'I101',
             style_guide.Decision.Selected),
         (defaults.SELECT, ['I2'], ['I1'], [], 'I201',