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

7 files changed, 215 insertions, 2 deletions

diff --git a/docs/source/release-notes/3.1.0.rst b/docs/source/release-notes/3.1.0.rst
new file mode 100644
index 0000000..7df073a
--- /dev/null
+++ b/docs/source/release-notes/3.1.0.rst
@@ -0,0 +1,4 @@
+3.1.0 -- 2016-yy-xx
+-------------------
+
+- Add ``--bug-report`` flag to make issue reporters' lives easier.
diff --git a/docs/source/release-notes/index.rst b/docs/source/release-notes/index.rst
index 07b9cd6..c59a64f 100644
--- a/docs/source/release-notes/index.rst
+++ b/docs/source/release-notes/index.rst
@@ -6,6 +6,7 @@ All of the release notes that have been recorded for Flake8 are organized here
 with the newest releases first.
 
 .. toctree::
+    3.1.0
     3.0.4
     3.0.3
     3.0.2
diff --git a/docs/source/user/options.rst b/docs/source/user/options.rst
index acaa67c..c139f69 100644
--- a/docs/source/user/options.rst
+++ b/docs/source/user/options.rst
@@ -728,3 +728,51 @@
         flake8 --benchmark dir/
 
     This **can not** be specified in config files.
+
+
+.. option:: --bug-report
+
+    Generate information necessary to file a complete bug report for Flake8.
+    This will pretty-print a JSON blob that should be copied and pasted into a
+    bug report for Flake8.
+
+    Command-line usage:
+
+    .. prompt:: bash
+
+        flake8 --bug-report
+
+    The output should look vaguely like:
+
+    .. code-block:: js
+
+        {
+          "dependencies": [
+            {
+              "dependency": "setuptools",
+              "version": "25.1.1"
+            }
+          ],
+          "platform": {
+            "python_implementation": "CPython",
+            "python_version": "2.7.12",
+            "system": "Darwin"
+          },
+          "plugins": [
+            {
+              "plugin": "mccabe",
+              "version": "0.5.1"
+            },
+            {
+              "plugin": "pycodestyle",
+              "version": "2.0.0"
+            },
+            {
+              "plugin": "pyflakes",
+              "version": "1.2.3"
+            }
+          ],
+          "version": "3.1.0.dev0"
+        }
+
+    This **can not** be specified in config files.
diff --git a/src/flake8/main/debug.py b/src/flake8/main/debug.py
new file mode 100644
index 0000000..e6ea141
--- /dev/null
+++ b/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__}]
diff --git a/src/flake8/main/options.py b/src/flake8/main/options.py
index c725c38..47fb30c 100644
--- a/src/flake8/main/options.py
+++ b/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',
+    )
diff --git a/src/flake8/options/manager.py b/src/flake8/options/manager.py
index de9356e..b536907 100644
--- a/src/flake8/options/manager.py
+++ b/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
         )
diff --git a/tests/unit/test_debug.py b/tests/unit/test_debug.py
new file mode 100644
index 0000000..dbafe2e
--- /dev/null
+++ b/tests/unit/test_debug.py
@@ -0,0 +1,86 @@
+"""Tests for our debugging module."""
+import mock
+import pytest
+import setuptools
+
+from flake8.main import debug
+
+
+def test_dependencies():
+    """Verify that we format our dependencies appropriately."""
+    expected = [{'dependency': 'setuptools',
+                 'version': setuptools.__version__}]
+    assert expected == debug.dependencies()
+
+
+@pytest.mark.parametrize('plugins, expected', [
+    ([], []),
+    ([('pycodestyle', '2.0.0')], [{'plugin': 'pycodestyle',
+                                   'version': '2.0.0'}]),
+    ([('pycodestyle', '2.0.0'), ('mccabe', '0.5.9')],
+        [{'plugin': 'mccabe', 'version': '0.5.9'},
+         {'plugin': 'pycodestyle', 'version': '2.0.0'}]),
+])
+def test_plugins_from(plugins, expected):
+    """Test that we format plugins appropriately."""
+    option_manager = mock.Mock(registered_plugins=set(plugins))
+    assert expected == debug.plugins_from(option_manager)
+
+
+@mock.patch('platform.python_implementation', return_value='CPython')
+@mock.patch('platform.python_version', return_value='3.5.3')
+@mock.patch('platform.system', return_value='Linux')
+def test_information(system, pyversion, pyimpl):
+    """Verify that we return all the information we care about."""
+    expected = {
+        'version': '3.1.0',
+        'plugins': [{'plugin': 'mccabe', 'version': '0.5.9'},
+                    {'plugin': 'pycodestyle', 'version': '2.0.0'}],
+        'dependencies': [{'dependency': 'setuptools',
+                          'version': setuptools.__version__}],
+        'platform': {
+            'python_implementation': 'CPython',
+            'python_version': '3.5.3',
+            'system': 'Linux',
+        },
+    }
+    option_manager = mock.Mock(
+        registered_plugins=set([('pycodestyle', '2.0.0'),
+                                ('mccabe', '0.5.9')]),
+        version='3.1.0',
+    )
+    assert expected == debug.information(option_manager)
+    pyimpl.assert_called_once_with()
+    pyversion.assert_called_once_with()
+    system.assert_called_once_with()
+
+
+@mock.patch('flake8.main.debug.print')
+@mock.patch('flake8.main.debug.information', return_value={})
+@mock.patch('json.dumps', return_value='{}')
+def test_print_information_no_plugins(dumps, information, print_mock):
+    """Verify we print and exit only when we have plugins."""
+    plugins = []
+    option_manager = mock.Mock(registered_plugins=set(plugins))
+    assert debug.print_information(
+        None, None, None, None, option_manager=option_manager,
+    ) is None
+    assert dumps.called is False
+    assert information.called is False
+    assert print_mock.called is False
+
+
+@mock.patch('flake8.main.debug.print')
+@mock.patch('flake8.main.debug.information', return_value={})
+@mock.patch('json.dumps', return_value='{}')
+def test_print_information(dumps, information, print_mock):
+    """Verify we print and exit only when we have plugins."""
+    plugins = [('pycodestyle', '2.0.0'), ('mccabe', '0.5.9')]
+    option_manager = mock.Mock(registered_plugins=set(plugins))
+    with pytest.raises(SystemExit):
+        debug.print_information(
+            None, None, None, None, option_manager=option_manager,
+        )
+    print_mock.assert_called_once_with('{}')
+    dumps.assert_called_once_with({}, indent=2, sort_keys=True)
+    information.assert_called_once_with(option_manager)