Fill in most of the legacy API

This does not handle setting custom options via the parameters to
get_style_guide.

src/flake8/api/__init__.py

@@ -3,8 +3,3 @@
 This is the only submodule in Flake8 with a guaranteed stable API. All other
 submodules are considered internal only and are subject to change.
 """
-
-
-def get_style_guide(**kwargs):
-    """Stub out the only function I'm aware of people using."""
-    pass

src/flake8/api/legacy.py

@@ -0,0 +1,142 @@
+"""Module containing shims around Flake8 2.0 behaviour."""
+import os.path
+
+from flake8.formatting import base as formatter
+from flake8.main import application as app
+
+
+def get_style_guide(**kwargs):
+    """Stub out the only function I'm aware of people using."""
+    application = app.Application()
+    application.initialize([])
+    return StyleGuide(application)
+
+
+class StyleGuide(object):
+    """Public facing object that mimic's Flake8 2.0's StyleGuide.
+
+    .. note::
+
+        There are important changes in how this object behaves compared to
+        the StyleGuide object provided in Flake8 2.x.
+
+    .. warning::
+
+        This object should not be instantiated directly by users.
+
+    .. versionchanged:: 3.0.0
+    """
+
+    def __init__(self, application):
+        """Initialize our StyleGuide."""
+        self._application = application
+        self._file_checker_manager = application.file_checker_manager
+
+    @property
+    def options(self):
+        """The parsed options.
+
+        An instance of :class:`optparse.Values` containing parsed options.
+        """
+        return self._application.options
+
+    @property
+    def paths(self):
+        """The extra arguments passed as paths."""
+        return self._application.paths
+
+    def check_files(self, paths=None):
+        """Run collected checks on the files provided.
+
+        This will check the files passed in and return a :class:`Report`
+        instance.
+
+        :param list paths:
+            List of filenames (or paths) to check.
+        :returns:
+            Object that mimic's Flake8 2.0's Reporter class.
+        :rtype:
+            flake8.api.legacy.Report
+        """
+        self._application.run_checks(paths)
+        self._application.report_errors()
+        return Report(self._application)
+
+    def excluded(self, filename, parent=None):
+        """Determine if a file is excluded.
+
+        :param str filename:
+            Path to the file to check if it is excluded.
+        :param str parent:
+            Name of the parent directory containing the file.
+        :returns:
+            True if the filename is excluded, False otherwise.
+        :rtype:
+            bool
+        """
+        return (self._file_checker_manager.is_path_excluded(filename) or
+                (parent and
+                    self._file_checker_manager.is_path_excluded(
+                        os.path.join(parent, filename))))
+
+    def init_report(self, reporter=None):
+        """Set up a formatter for this run of Flake8."""
+        if (reporter is not None and
+                not issubclass(reporter, formatter.BaseFormatter)):
+            raise ValueError("Report should be subclass of "
+                             "flake8.formatter.BaseFormatter.")
+        self._application.make_formatter(reporter)
+        self._application.guide = None
+        # NOTE(sigmavirus24): This isn't the intended use of
+        # Application#make_guide but it works pretty well.
+        # Stop cringing... I know it's gross.
+        self._application.make_guide()
+
+    def input_file(self, filename, lines=None, expected=None, line_offset=0):
+        """Run collected checks on a single file.
+
+        This will check the file passed in and return a :class:`Report`
+        instance.
+
+        :param str filename:
+            The path to the file to check.
+        :param list lines:
+            Ignored since Flake8 3.0.
+        :param expected:
+            Ignored since Flake8 3.0.
+        :param int line_offset:
+            Ignored since Flake8 3.0.
+        :returns:
+            Object that mimic's Flake8 2.0's Reporter class.
+        :rtype:
+            flake8.api.legacy.Report
+        """
+        return self.check_files([filename])
+
+
+class Report(object):
+    """Public facing object that mimic's Flake8 2.0's API.
+
+    .. note::
+
+        There are important changes in how this object behaves compared to
+        the object provided in Flake8 2.x.
+
+    .. versionchanged:: 3.0.0
+    """
+
+    def __init__(self, application):
+        """Initialize the Report for the user.
+
+        .. warning:: This should not be instantiated by users.
+        """
+        self._application = application
+
+    @property
+    def total_errors(self):
+        """The total number of errors found by Flake8."""
+        return self._application.result_count
+
+    def get_statistics(self, violation):
+        """Get the number of occurences of a violation."""
+        raise NotImplementedError('Statistics capturing needs to happen first')

src/flake8/api/style_guide.py

@@ -1,47 +0,0 @@
-"""Module containing shims around Flake8 2.0 behaviour."""
-import os.path
-
-from flake8.formatting import base as formatter
-
-
-class LegacyStyleGuide(object):
-    """Public facing object that mimic's Flake8 2.0's StyleGuide."""
-
-    def __init__(self, application):
-        self._application = application
-        self._file_checker_manager = application.file_checker_manager
-
-    @property
-    def options(self):
-        """The parsed options.
-
-        An instance of :class:`optparse.Values` containing parsed options.
-        """
-        return self._application.options
-
-    @property
-    def paths(self):
-        """The extra arguments passed as paths."""
-        return self._application.paths
-
-    def check_files(self, paths=None):
-        raise NotImplementedError('This should be easy')
-
-    def excluded(self, filename, parent=None):
-        return (self._file_checker_manager.is_path_excluded(filename) or
-                (parent and
-                    self._file_checker_manager.is_path_excluded(
-                        os.path.join(parent, filename))))
-
-    def init_report(self, reporter=None):
-        if (reporter is not None and
-                isinstance(reporter, formatter.BaseFormatter)):
-            self._application.formatter = reporter
-            self._application.guide = None
-            # NOTE(sigmavirus24): This isn't the intended use of
-            # Application#make_guide but it works pretty well.
-            # Stop cringing... I know it's gross.
-            self._application.make_guide()
-
-    def input_file(self, filename, lines=None, expected=None, line_offset=0):
-        raise NotImplementedError('This should be a pain')

src/flake8/main/application.py

@@ -177,13 +177,15 @@ class Application(object):
                                                 self.options,
                                                 self.args)
 
-    def make_formatter(self):
+    def make_formatter(self, formatter_class=None):
         # type: () -> NoneType
         """Initialize a formatter based on the parsed options."""
         if self.formatter is None:
-            self.formatter = self.formatting_plugins.get(
+            if formatter_class is None:
-                self.options.format, self.formatting_plugins['default']
+                formatter_class = self.formatting_plugins.get(
-            ).execute(self.options)
+                    self.options.format, self.formatting_plugins['default']
+                ).execute
+            self.formatter = formatter_class(self.options)
 
     def make_notifier(self):
         # type: () -> NoneType
@@ -212,15 +214,17 @@ class Application(object):
                 checker_plugins=self.check_plugins,
             )
 
-    def run_checks(self):
+    def run_checks(self, files=None):
-        # type: () -> NoneType
+        # type: (Union[List[str], NoneType]) -> NoneType
         """Run the actual checks with the FileChecker Manager.
 
         This method encapsulates the logic to make a
         :class:`~flake8.checker.Manger` instance run the checks it is
         managing.
+
+        :param list files:
+            List of filenames to process
         """
-        files = None
         if self.running_against_diff:
             files = list(sorted(self.parsed_diff.keys()))
         self.file_checker_manager.start(files)