Make Plugin.load_plugin raise a Flake8 exception

Let's catch exceptions, log them, and re-raise as a FailedToLoad
exception. This also refactors the actually loading of plugins into a
private method on the Plugin class.

flake8/exceptions.py

@@ -0,0 +1,25 @@
+"""Exception classes for all of Flake8."""
+
+
+class Flake8Exception(Exception):
+    """Plain Flake8 exception."""
+
+    pass
+
+
+class FailedToLoadPlugin(Flake8Exception):
+    """Exception raised when a plugin fails to load."""
+
+    FORMAT = 'Flake8 failed to load plugin "%(name)s" due to %(exc)s.'
+
+    def __init__(self, *args, **kwargs):
+        """Initialize our FailedToLoadPlugin exception."""
+        self.plugin = kwargs.pop('plugin')
+        self.ep_name = self.plugin.name
+        self.original_exception = kwargs.pop('exception')
+        super(FailedToLoadPlugin, self).__init__(*args, **kwargs)
+
+    def __str__(self):
+        """Return a nice string for our exception."""
+        return self.FORMAT % {'name': self.ep_name,
+                              'exc': self.original_exception}

flake8/plugins/manager.py

@@ -5,6 +5,7 @@ import logging
 import pkg_resources
 
 from flake8 import _trie
+from flake8 import exceptions
 
 LOG = logging.getLogger(__name__)
 
@@ -58,6 +59,21 @@ class Plugin(object):
         r"""Call the plugin with \*args and \*\*kwargs."""
         return self.plugin(*args, **kwargs)
 
+    def _load(self, verify_requirements):
+        # Avoid relying on hasattr() here.
+        resolve = getattr(self.entry_point, 'resolve', None)
+        require = getattr(self.entry_point, 'require', None)
+        if resolve and require:
+            if verify_requirements:
+                LOG.debug('Verifying plugin "%s"\'s requirements.',
+                          self.name)
+                require()
+            self._plugin = resolve()
+        else:
+            self._plugin = self.entry_point.load(
+                require=verify_requirements
+            )
+
     def load_plugin(self, verify_requirements=False):
         """Retrieve the plugin for this entry-point.
 
@@ -73,19 +89,16 @@ class Plugin(object):
         """
         if self._plugin is None:
             LOG.debug('Loading plugin "%s" from entry-point.', self.name)
-            # Avoid relying on hasattr() here.
-            resolve = getattr(self.entry_point, 'resolve', None)
-            require = getattr(self.entry_point, 'require', None)
-            if resolve and require:
-                if verify_requirements:
-                    LOG.debug('Verifying plugin "%s"\'s requirements.',
-                              self.name)
-                    require()
-                self._plugin = resolve()
-            else:
-                self._plugin = self.entry_point.load(
-                    require=verify_requirements
+            try:
+                self._load(verify_requirements)
+            except Exception as load_exception:
+                LOG.exception(load_exception, exc_info=True)
+                failed_to_load = exceptions.FailedToLoadPlugin(
+                    plugin=self,
+                    exception=load_exception,
                 )
+                LOG.critical(str(failed_to_load))
+                raise failed_to_load
 
     def provide_options(self, optmanager, options, extra_args):
         """Pass the parsed options and extra arguments to the plugin."""

tests/unit/test_plugin.py

@@ -1,6 +1,8 @@
 """Tests for flake8.plugins.manager.Plugin."""
 import mock
+import pytest
 
+from flake8 import exceptions
 from flake8.plugins import manager
 
 
@@ -48,6 +50,16 @@ def test_load_plugin_only_calls_require_when_verifying_requirements():
     entry_point.resolve.assert_called_once_with()
 
 
+def test_load_plugin_catches_and_reraises_exceptions():
+    """Verify we raise our own FailedToLoadPlugin."""
+    entry_point = mock.Mock(spec=['require', 'resolve'])
+    entry_point.resolve.side_effect = ValueError('Test failure')
+    plugin = manager.Plugin('T000', entry_point)
+
+    with pytest.raises(exceptions.FailedToLoadPlugin):
+        plugin.load_plugin()
+
+
 def test_plugin_property_loads_plugin_on_first_use():
     """Verify that we load our plugin when we first try to use it."""
     entry_point = mock.Mock(spec=['require', 'resolve', 'load'])