Add --bug-report flag to help bug reporters

When invoked it will print out JSON that has all of the debugging
information needed by the maintainers to diagnose or reproduce a bug.

Closes #207

src/flake8/main/debug.py

@@ -0,0 +1,62 @@
+"""Module containing the logic for our debugging logic."""
+from __future__ import print_function
+
+import json
+import platform
+
+import setuptools
+
+
+def print_information(option, option_string, value, parser,
+                      option_manager=None):
+    """Print debugging information used in bug reports.
+
+    :param option:
+        The optparse Option instance.
+    :type option:
+        optparse.Option
+    :param str option_string:
+        The option name
+    :param value:
+        The value passed to the callback parsed from the command-line
+    :param parser:
+        The optparse OptionParser instance
+    :type parser:
+        optparse.OptionParser
+    :param option_manager:
+        The Flake8 OptionManager instance.
+    :type option_manager:
+        flake8.options.manager.OptionManager
+    """
+    if not option_manager.registered_plugins:
+        # NOTE(sigmavirus24): Flake8 parses options twice. The first time, we
+        # will not have any registered plugins. We can skip this one and only
+        # take action on the second time we're called.
+        return
+    print(json.dumps(information(option_manager), indent=2, sort_keys=True))
+    raise SystemExit(False)
+
+
+def information(option_manager):
+    """Generate the information to be printed for the bug report."""
+    return {
+        'version': option_manager.version,
+        'plugins': plugins_from(option_manager),
+        'dependencies': dependencies(),
+        'platform': {
+            'python_implementation': platform.python_implementation(),
+            'python_version': platform.python_version(),
+            'system': platform.system(),
+        },
+    }
+
+
+def plugins_from(option_manager):
+    """Generate the list of plugins installed."""
+    return [{'plugin': plugin, 'version': version}
+            for (plugin, version) in sorted(option_manager.registered_plugins)]
+
+
+def dependencies():
+    """Generate the list of dependencies we care about."""
+    return [{'dependency': 'setuptools', 'version': setuptools.__version__}]

src/flake8/main/options.py

@@ -1,5 +1,6 @@
 """Contains the logic for all of the default options for Flake8."""
 from flake8 import defaults
+from flake8.main import debug
 from flake8.main import vcs
 
 
@@ -29,6 +30,8 @@ def register_default_options(option_manager):
     - ``--append-config``
     - ``--config``
     - ``--isolated``
+    - ``--benchmark``
+    - ``--bug-report``
     """
     add_option = option_manager.add_option
 
@@ -199,3 +202,11 @@ def register_default_options(option_manager):
         '--benchmark', default=False, action='store_true',
         help='Print benchmark information about this run of Flake8',
     )
+
+    # Debugging
+
+    add_option(
+        '--bug-report', action='callback', callback=debug.print_information,
+        callback_kwargs={'option_manager': option_manager},
+        help='Print information necessary when preparing a bug report',
+    )

src/flake8/options/manager.py

@@ -240,9 +240,10 @@ class OptionManager(object):
         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'):
+    def generate_versions(self, format_str='%(name)s: %(version)s',
+                          join_on=', '):
         """Generate a comma-separated list of versions of plugins."""
-        return ', '.join(
+        return join_on.join(
             format_str % self.format_plugin(plugin)
             for plugin in self.registered_plugins
         )