Prevent unintended behaviour modifying options.ignorefeature/register-optional-checks

Previously, it was entirely plausible for us to remove something from
options.ignore overzealously. This is more confined logic and much more akin
to what I was intending the behaviour to be.

1 files changed, 7 insertions, 5 deletions

diff --git a/flake8/engine.py b/flake8/engine.py
index b77199b..cdad7f9 100644
--- a/flake8/engine.py
+++ b/flake8/engine.py
@@ -101,11 +101,13 @@ class StyleGuide(pep8.StyleGuide):
 
 
 def _disable_extensions(parser, options):
-    select = set(options.select)
-    ignore = set(options.ignore)
-    ignore.update(getattr(parser, 'ignored_extensions', []))
-    ignore -= select
-    options.ignore = tuple(ignore)
+    ignored_extensions = set(getattr(parser, 'ignored_extensions', []))
+    # Remove any of the selected extensions from the extensions ignored by
+    # default.
+    ignored_extensions -= set(options.select)
+    # Whatever is left afterwards should be unioned with options.ignore and
+    # options.ignore should be updated with that.
+    options.ignore = tuple(ignored_extensions.union(options.ignore))
 
 
 def get_style_guide(**kwargs):