Refactor logging handling

Old system was using manual string formatting for log messages.
This caused issues with common operations like exception logging
because often they need to be handled differently for Py2/Py3
compatibility. In order to unify the effort:

 - All logging is changed to `logging.level(msg, arg1, arg2)` style.
 - A `SafeLogger` is implemented to auto-decode exceptions properly
in the args (ref #1403).
 - Custom formatters were overriding useful logging functionality
like traceback outputing (ref #1402). They are refactored to be
more transparent. Traceback information is provided in `--debug`
mode for `read_file` errors in generators.
 - Formatters will now auto-format multiline log messages in order
to make them look related. Similarly, traceback will be formatted in
the same fashion.
 - `pelican.log.LimitFilter` was (ab)using logging message which
would result in awkward syntax for argumented logging style. This
functionality is moved to `extra` keyword argument.
 - Levels for errors that would result skipping a file (`read_file`)
changed from `warning` to `error` in order to make them stand out
among other logs.
 - Small consistency changes to log messages (i.e. changing all
to start with an uppercase letter) and quality-of-life improvements
(some log messages were dumping raw object information).

pelican/generators.py

@@ -22,13 +22,14 @@ from jinja2 import (Environment, FileSystemLoader, PrefixLoader, ChoiceLoader,
 from pelican.contents import Article, Draft, Page, Static, is_valid_content
 from pelican.readers import Readers
 from pelican.utils import (copy, process_translations, mkdir_p, DateFormatter,
-                           FileStampDataCacher)
+                           FileStampDataCacher, python_2_unicode_compatible)
 from pelican import signals
 
 
 logger = logging.getLogger(__name__)
 
 
+@python_2_unicode_compatible
 class Generator(object):
     """Baseclass generator"""
 
@@ -67,7 +68,7 @@ class Generator(object):
             extensions=self.settings['JINJA_EXTENSIONS'],
         )
 
-        logger.debug('template list: {0}'.format(self.env.list_templates()))
+        logger.debug('Template list: %s', self.env.list_templates())
 
         # provide utils.strftime as a jinja filter
         self.env.filters.update({'strftime': DateFormatter()})
@@ -153,6 +154,10 @@ class Generator(object):
                 value = list(value.items())  # py3k safeguard for iterators
             self.context[item] = value
 
+    def __str__(self):
+        # return the name of the class for logging purposes
+        return self.__class__.__name__
+
 
 class CachingGenerator(Generator, FileStampDataCacher):
     '''Subclass of Generator and FileStampDataCacher classes
@@ -470,7 +475,8 @@ class ArticlesGenerator(CachingGenerator):
                         context_signal=signals.article_generator_context,
                         context_sender=self)
                 except Exception as e:
-                    logger.warning('Could not process {}\n{}'.format(f, e))
+                    logger.error('Could not process %s\n%s', f, e,
+                        exc_info=self.settings.get('DEBUG', False))
                     continue
 
                 if not is_valid_content(article, f):
@@ -492,9 +498,8 @@ class ArticlesGenerator(CachingGenerator):
                     context_sender=self)
                 all_drafts.append(draft)
             else:
-                logger.warning("Unknown status %s for file %s, skipping it." %
-                               (repr(article.status),
-                                repr(f)))
+                logger.error("Unknown status '%s' for file %s, skipping it.",
+                               article.status, f)
 
         self.articles, self.translations = process_translations(all_articles)
         self.drafts, self.drafts_translations = \
@@ -594,7 +599,8 @@ class PagesGenerator(CachingGenerator):
                         context_signal=signals.page_generator_context,
                         context_sender=self)
                 except Exception as e:
-                    logger.warning('Could not process {}\n{}'.format(f, e))
+                    logger.error('Could not process %s\n%s', f, e,
+                        exc_info=self.settings.get('DEBUG', False))
                     continue
 
                 if not is_valid_content(page, f):
@@ -609,9 +615,8 @@ class PagesGenerator(CachingGenerator):
             elif page.status == "hidden":
                 hidden_pages.append(page)
             else:
-                logger.warning("Unknown status %s for file %s, skipping it." %
-                               (repr(page.status),
-                                repr(f)))
+                logger.error("Unknown status '%s' for file %s, skipping it.",
+                               page.status, f)
 
         self.pages, self.translations = process_translations(all_pages)
         self.hidden_pages, self.hidden_translations = (
@@ -679,7 +684,7 @@ class StaticGenerator(Generator):
             save_as = os.path.join(self.output_path, sc.save_as)
             mkdir_p(os.path.dirname(save_as))
             shutil.copy2(source_path, save_as)
-            logger.info('copying {} to {}'.format(sc.source_path, sc.save_as))
+            logger.info('Copying %s to %s', sc.source_path, sc.save_as)
 
 
 class SourceFileGenerator(Generator):
@@ -694,7 +699,7 @@ class SourceFileGenerator(Generator):
         copy(obj.source_path, dest)
 
     def generate_output(self, writer=None):
-        logger.info(' Generating source files...')
+        logger.info('Generating source files...')
         for obj in chain(self.context['articles'], self.context['pages']):
             self._create_source(obj)
             for obj_trans in obj.translations: