Enable plugins automatically during registration

Previously the --select was only ever populated to E,F,W,C and so
plugins would not be reported when not off-by-default. This adds a
tiny shim so that we enable plugins that are not off-by-default and
:x
:x

src/flake8/options/aggregator.py

@@ -53,7 +53,14 @@ def aggregate_options(manager, arglist=None, values=None):
               list(extended_default_ignore))
     extended_default_ignore.update(default_values.ignore)
     default_values.ignore = list(extended_default_ignore)
+
+    extended_default_select = manager.extended_default_select.copy()
+    LOG.debug('Extended default select list: %s',
+              list(extended_default_select))
+    extended_default_select.update(default_values.select)
+    default_values.select = list(extended_default_select)
     LOG.debug('Merged default ignore list: %s', default_values.ignore)
+    LOG.debug('Merged default select list: %s', default_values.select)
 
     # Merge values parsed from config onto the default values returned
     for config_name, value in parsed_config.items():

src/flake8/options/manager.py

@@ -163,6 +163,7 @@ def __init__(self, prog=None, version=None,
         self.version = version
         self.registered_plugins = set()
         self.extended_default_ignore = set()
+        self.extended_default_select = set()
 
     @staticmethod
     def format_plugin(plugin_tuple):
@@ -201,8 +202,8 @@ def remove_from_default_ignore(self, error_codes):
         LOG.debug('Removing %r from the default ignore list', error_codes)
         for error_code in error_codes:
             try:
-                self.extend_default_ignore.remove(error_code)
-            except ValueError:
+                self.extended_default_ignore.remove(error_code)
+            except (ValueError, KeyError):
                 LOG.debug('Attempted to remove %s from default ignore'
                           ' but it was not a member of the list.', error_code)
 
@@ -216,6 +217,16 @@ def extend_default_ignore(self, error_codes):
         LOG.debug('Extending default ignore list with %r', error_codes)
         self.extended_default_ignore.update(error_codes)
 
+    def extend_default_select(self, error_codes):
+        """Extend the default select list with the error codes provided.
+
+        :param list error_codes:
+            List of strings that are the error/warning codes with which
+            to extend the default select list.
+        """
+        LOG.debug('Extending default select list with %r', error_codes)
+        self.extended_default_select.update(error_codes)
+
     def generate_versions(self, format_str='%(name)s: %(version)s'):
         """Generate a comma-separated list of versions of plugins."""
         return ', '.join(

src/flake8/plugins/manager.py

@@ -161,6 +161,7 @@ def load_plugin(self, verify_requirements=False):
     def enable(self, optmanager):
         """Remove plugin name from the default ignore list."""
         optmanager.remove_from_default_ignore([self.name])
+        optmanager.extend_default_select([self.name])
 
     def disable(self, optmanager):
         """Add the plugin name to the default ignore list."""
@@ -418,6 +419,16 @@ def checks_expecting(self, argument_name):
             if argument_name == plugin.parameters[0]:
                 yield plugin
 
+    def register_options(self, optmanager):
+        """Register all of the checkers' options to the OptionManager."""
+        super(Checkers, self).register_options(optmanager)
+
+        def conditionally_enable(plugin):
+            if plugin.group() is None and not plugin.off_by_default:
+                plugin.enable(optmanager)
+
+        list(self.manager.map(conditionally_enable))
+
     @property
     def ast_plugins(self):
         """List of plugins that expect the AST tree."""