query backends from installed packages

bin/netjsonconfig

@@ -1,9 +1,11 @@
 #!/usr/bin/env python
 
+import argparse
 import os
 import sys
+
 import six
-import argparse
+
 import netjsonconfig
 
 description = """
@@ -56,7 +58,7 @@ output = parser.add_argument_group('output')
 
 output.add_argument('--backend', '-b',
                     required=True,
-                    choices=['openwrt', 'openwisp', 'openvpn'],
+                    choices=netjsonconfig.get_backends().keys(),
                     action='store',
                     type=str,
                     help='Configuration backend')
@@ -166,13 +168,8 @@ context = dict(os.environ)
 method = args.method
 method_arguments = parse_method_arguments(args.args)
 
-backends = {
-    'openwrt': netjsonconfig.OpenWrt,
-    'openwisp': netjsonconfig.OpenWisp,
-    'openvpn': netjsonconfig.OpenVpn
-}
 
-backend_class = backends[args.backend]
+backend_class = netjsonconfig.get_backends()[args.backend]
 try:
     options = dict(templates=templates, context=context)
     if args.config:

docs/source/backends/create_your_backend.rst

@@ -0,0 +1,96 @@
+
+===================
+Create your backend
+===================
+
+.. include:: ../_github.rst
+
+Every backend is based on the common ground of some elements provided by the 
+netjsonconfig library. The `BaseBackend`, `BaseConverter`, `BaseParser` and
+`BaseRenderer` are a battle proven set of tools that can be extended when
+creating you backend.
+
+But the netjsonconfig package is not a playground to experiment, your contributions
+to a new backend should start elsewhere, a different package, where you are in control
+and can make errors and experiment more.
+
+Netjsonconfig can now discover packages that provides a custom backend using
+a feature available in the Python packaging ecosystem which is called `entry_points`.
+
+To create a new backend start from scratch with a new folder and add this file to your
+project root directory.
+
+.. code-block:: python
+
+    # setup.py
+    from setuptools import setup, find_packages
+
+    setup(
+        name='example_backend',
+        version='0.0.0',
+        description='an example to illustrate a netjsonconfig backend as an external module',
+        packages=find_packages(),
+        entry_points={
+            'netjsonconfig.backends': [
+                'example=example_backend.__init__:ExampleBackend',
+            ]
+        }
+    )
+
+this file can be used to create a package that can be installed using pip or other tools
+in the python ecosystem. You can find more information about Python packaging
+`at packaging.python.org <https://packaging.python.org/>`_
+and `at the hitchhikers guide to packaging <https://the-hitchhikers-guide-to-packaging.readthedocs.io/en/latest/>`_.
+
+The most important part is to give your package a good name, a well thought description and
+to add the `entry_points` keyword argument with the following code
+
+..  code-block:: python
+
+        {
+                # this is used by netjsonconfig
+                # to find your backend
+                'netjsonconfig.backends': [
+                        ...
+                ]
+        }
+
+Now your package will be in the list of backends that netjsonconfig can use!
+
+But we still have to give us a name to be unique! Netjsonconfig already
+defined the names `openwisp`, `openwrt` and `openvpn` but you can choose
+whatever you like most.
+
+The name `netjsonconfig.backends` will be associated with a list of classes
+from your package that will be presented to netjconfig at runtime. To specify
+which classes you want to expose write the triple `name`, `path` and `class_name`
+using the format `name=path:class_name` as in the example below.
+
+The `path` part is simply the path to the file that contains the class
+you want to expose and the `class_name` is the name of the class.
+
+..  code-block:: python
+
+        {
+                'netjsonconfig.backends': [
+                        # name=path:class_name
+                        'example=example_backend.__init__:ExampleBackend',
+                ]
+        }
+
+The previous example can be used with the following class definition
+
+.. code-block:: python
+
+        # example_backend/__init__.py
+        from netjsonconfig.backends.base.backend import BaseBackend
+        from netjsonconfig.backends.base.renderer import BaseRenderer
+        from netjsonconfig.backends.base.parser import BaseParser
+
+        from netjsonconfig.schema import schema as default_schema
+
+        class ExampleBackend(BaseBackend):
+            schema = default_schema
+            converter = []
+            parser = BaseParser
+            renderer = BaseRenderer

docs/source/index.rst

@@ -55,6 +55,7 @@ Contents:
    /backends/openwrt
    /backends/openwisp
    /backends/openvpn
+   /backends/create_your_backend
    /general/commandline_utility
    /general/running_tests
    /general/contributing

netjsonconfig/__init__.py

@@ -1,5 +1,24 @@
+from pkg_resources import iter_entry_points
+import logging
+
 from .version import VERSION, __version__, get_version  # noqa
 
 from .backends.openwrt.openwrt import OpenWrt  # noqa
 from .backends.openwisp.openwisp import OpenWisp  # noqa
 from .backends.openvpn.openvpn import OpenVpn  # noqa
+
+
+def get_backends():
+    default = {
+        'openwrt': OpenWrt,
+        'openwisp': OpenWisp,
+        'openvpn': OpenVpn,
+    }
+    logger = logging.getLogger(__name__)
+
+    for entry_point in iter_entry_points('netjsonconfig.backends'):
+        try:
+            default.update({entry_point.name.lower(): entry_point.load()})
+        except ImportError as e:  # noqa
+            logger.error("Error loading backend {}".format(entry_point.name.lower()))
+    return default

netjsonconfig/backends/openwrt/schema.py

@@ -6,7 +6,6 @@
 from ..openvpn.schema import base_openvpn_schema
 from .timezones import timezones
 
-
 default_radio_driver = "mac80211"