Make styles able to be extended by users

README.rst

@@ -56,6 +56,9 @@ the code being developed.  This option is only supported if using the
 * ``edited`` - style as ``smarkets`` only with `import` statements for packages local to your company or organisation coming after `import` statements for third-party packages, see an `example <https://github.com/PyCQA/flake8-import-order/blob/master/tests/test_cases/complete_edited.py>`__
 * ``pep8`` - style that only enforces groups without enforcing the order within the groups
 
+You can also `add your own style <#extending-styles>`_ by extending ``Style``
+class.
+
 Limitations
 -----------
 
@@ -82,5 +85,45 @@ is also the same for all Python versions because otherwise it would
 be impossible to write programs that work under both Python 2 and 3
 *and* pass the import order check.
 
+Extending styles
+----------------
+
+You can add your own style by extending ``flake8_import_order.styles.Style``
+class.  Here's an example:
+
+.. code-block:: python
+
+    from flake8_import_order.styles import Cryptography
+
+
+    class ReversedCryptography(Cryptography):
+        # Note that Cryptography is a subclass of Style.
+
+        @staticmethod
+        def sorted_names(names):
+            return reversed(Cryptography.sorted_names(names))
+
+To make flake8-import-order able to discover your extended style, you need to
+register it as ``flake8_import_order.styles`` using setuptools' `entry points
+<https://setuptools.readthedocs.io/en/latest/pkg_resources.html#entry-points>`__
+mechanism:
+
+.. code-block:: python
+
+    # setup.py of your style package
+    setup(
+        name='flake8-import-order-reversed-cryptography',
+        ...,
+        entry_points={
+            'flake8_import_order.styles': [
+                'reversed = reversedcryptography:ReversedCryptography',
+                # 'reversed' is a style name.  You can pass it to
+                # --import-order-style option
+                # 'reversedcryptography:ReversedCryptography' is an import path
+                # of your extended style class.
+            ]
+        }
+    )
+
 .. |Build Status| image:: https://travis-ci.org/PyCQA/flake8-import-order.png?branch=master
    :target: https://travis-ci.org/PyCQA/flake8-import-order

flake8_import_order/checker.py

@@ -1,11 +1,10 @@
 import ast
 
+import pkg_resources
+
 import pycodestyle
 
 from flake8_import_order import ImportVisitor
-from flake8_import_order.styles import (
-    AppNexus, Cryptography, Edited, Google, PEP8, Smarkets,
-)
 
 DEFAULT_IMPORT_ORDER_STYLE = 'cryptography'
 
@@ -59,20 +58,18 @@ def check_order(self):
             if not pycodestyle.noqa(self.lines[import_.lineno - 1]):
                 imports.append(import_)
 
-        if style_option == 'cryptography':
-            style = Cryptography(imports)
-        elif style_option == 'google':
-            style = Google(imports)
-        elif style_option == 'pep8':
-            style = PEP8(imports)
-        elif style_option == 'smarkets':
-            style = Smarkets(imports)
-        elif style_option == 'appnexus':
-            style = AppNexus(imports)
-        elif style_option == 'edited':
-            style = Edited(imports)
-        else:
+        try:
+            style_entry_point = next(
+                pkg_resources.iter_entry_points(
+                    'flake8_import_order.styles',
+                    name=style_option
+                )
+            )
+        except StopIteration:
             raise AssertionError("Unknown style {}".format(style_option))
+        else:
+            style_cls = style_entry_point.load()
+            style = style_cls(imports)
 
         for error in style.check():
             yield self.error(error)

flake8_import_order/flake8_linter.py

@@ -2,6 +2,8 @@
 
 import optparse
 
+import pkg_resources
+
 from flake8_import_order import __version__
 from flake8_import_order.checker import (
     DEFAULT_IMPORT_ORDER_STYLE, ImportOrderChecker,
@@ -46,11 +48,19 @@ def add_options(cls, parser):
             default=DEFAULT_IMPORT_ORDER_STYLE,
             action="store",
             type="string",
-            help=("Style to follow. Available: "
-                  "cryptography, google, smarkets, appnexus, pep8"),
+            help=("Style to follow. Available: " +
+                  ", ".join(cls.list_available_styles())),
             parse_from_config=True,
         )
 
+    @staticmethod
+    def list_available_styles():
+        entry_points = pkg_resources.iter_entry_points(
+            'flake8_import_order.styles'
+        )
+        for entry_point in entry_points:
+            yield entry_point.name
+
     @classmethod
     def parse_options(cls, options):
         optdict = {}

setup.py

@@ -29,7 +29,8 @@
     zip_safe=False,
 
     install_requires=[
-        "pycodestyle"
+        "pycodestyle",
+        "setuptools",
     ],
 
     tests_require=[
@@ -41,6 +42,14 @@
 
     py_modules=['flake8_import_order'],
     entry_points={
+        'flake8_import_order.styles': [
+            'cryptography = flake8_import_order.styles:Cryptography',
+            'google = flake8_import_order.styles:Google',
+            'pep8 = flake8_import_order.styles:PEP8',
+            'smarkets = flake8_import_order.styles:Smarkets',
+            'appnexus = flake8_import_order.styles:AppNexus',
+            'edited = flake8_import_order.styles:Edited',
+        ],
         'flake8.extension': [
             'I10 = flake8_import_order.flake8_linter:Linter',
         ],