vyos-documentation/docs/contributing/coding_guidelines.rst

.. _coding_guidelines:

Python Coding Guidelines
========================

The switch to the Python programming language for new code is not merely a
change of the language, but a chance to rethink and improve the programming
approach.

Let's face it: VyOS is full of spaghetti code where logic for reading the VyOS
config, generating daemon configs, and restarting processes is all mixed up.

Python (or any other language, for that matter) does not provide automatic
protection from bad design, so we need to also devise design guidelines and
follow them to keep the system extensible and maintainable.

But we are here to assist you and want to guide you through how you can become
a good VyOS contributor. The rules we have are not there to punish you - the
rules are in place to help us all. What does it mean? By having a consistent
coding style it becomes very easy for new contributors and also longtime
contributors to navigate through the sources and all the implied logic of
the spaghetti code.

Please use the following template as good starting point when developing new
modules or even rewrite a whole bunch of code in the new style XML/Pyhon
interface.

Configuration script structure and behaviour
--------------------------------------------

.. code-block:: python

  #!/usr/bin/env python3
  #
  # Copyright (C) 2019 VyOS maintainers and contributors
  #
  # This program is free software; you can redistribute it and/or modify
  # it under the terms of the GNU General Public License version 2 or later as
  # published by the Free Software Foundation.
  #
  # This program is distributed in the hope that it will be useful,
  # but WITHOUT ANY WARRANTY; without even the implied warranty of
  # MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
  # GNU General Public License for more details.
  #
  # You should have received a copy of the GNU General Public License
  # along with this program.  If not, see <http://www.gnu.org/licenses/>.

  import sys

  from vyos.config import Config
  from vyos.util import ConfigError

  def get_config():
      vc = Config()
      # Convert the VyOS config to an abstract internal representation
      config = ...
      return config

  def verify(config):
      # Verify that configuration is valid
      if invalid:
          raise ConfigError("Descriptive message")
      return True

  def generate(config):
      # Generate daemon configs
      pass

  def apply(config):
      # Apply the generated configs to the live system
      pass

  try:
      config = get_config()
      verify(config)
  except ConfigError as e:
      print(e)
      sys.exit(1)

The ``get_config()`` function must convert the VyOS config to an abstract,
internal representation. No other function is allowed to call the ``vyos.config.
Config`` object method directly. The rationale for it is that when config reads
are mixed with other logic, it's very hard to change the config syntax since
you need to weed out every occurrence of the old syntax. If syntax-specific
code is confined to a single function, the rest of the code can be left
untouched as long as the internal representation remains compatible.

Another advantage is testability of the code. Mocking the entire config
subsystem is hard, while constructing an internal representation by hand is
way simpler.

The ``verify()`` function takes your internal representation of the config and
checks if it's valid, otherwise it must raise ``ConfigError`` with an error
message that describes the problem and possibly suggests how to fix it. It must
not make any changes to the system. The rationale for it is again testability
and, in the future when the config backend is ready and every script is
rewritten in this fashion, ability to execute commit dry run ("commit test"
like in JunOS) and abort commit before making any changes to the system if an
error is found in any component.

The ``generate()`` function generates config files for system components.

The ``apply()`` function applies the generated configuration to the live
system. It should use non-disruptive reload whenever possible. It may execute
disruptive operations such as daemon process restart if a particular component
does not support non-disruptive reload, or when the expected service degradation
is minimal (for example, in case of auxiliary services such as LLDPd). In case
of high impact services such as VPN daemon and routing protocols, when non-
disruptive reload is supported for some but not all types of configuration
changes, scripts authors should make effort to determine if a configuration
change can be done in a non-disruptive way and only resort to disruptive restart
if it cannot be avoided.

Unless absolutely necessary, configuration scripts should not modify the active
configuration of system components directly. Whenever at all possible, scripts
should generate a configuration file or files that can be applied with a single
command such as reloading a service through systemd init. Inserting statements
one by one is particularly discouraged, for example, when configuring netfilter
rules, saving them to a file and loading it with iptables-restore should always
be preferred to executing iptables directly.

The ``apply()`` and ``generate()`` functions may ``raise ConfigError`` if, for
example, the daemon failed to start with the updated config. It shouldn't be a
substitute for proper config checking in the ``verify()`` function. All
reasonable effort should be made to verify that generated configuration is
valid and will be accepted by the daemon, including, when necessary, cross-
checks with other VyOS configuration subtrees.

Exceptions, including ``VyOSError`` (which is raised by ``vyos.config.Config``
on improper config operations, such as trying to use ``list_nodes()`` on a
non-tag node) should not be silenced or caught and re-raised as config error.
Sure this will not look pretty on user's screen, but it will make way better
bug reports, and help users (and most VyOS users are IT professionals) do their
own debugging as well.

For easy orientation we suggest you take a look on the ``ntp.py`` or
``interfaces-bonding.py`` (for tag nodes) implementation. Both files can be
found in the vyos-1x_ repository.


Coding guidelines
-----------------

Like any other project we have some small guidelines about our source code, too.
The rules we have are not there to punish you - the rules are in place to help
us all. By having a consistent coding style it becomes very easy for new
and also longtime contributors to navigate through the sources and all the
implied logic of any one source file..

Language
********

Python 3 **shall** be used. How long can we keep Python 2 alive anyway? No
considerations for Python 2 compatibility **should** be taken at any time.

Formatting
**********

* Python: Tabs **shall not** be used. Every indentation level should be 4 spaces
* XML: Tabs **shall not** be used. Every indentation level should be 2 spaces

.. note: There are extensions to e.g. VIM (xmllint) which will help you to get
   your indention levels correct. Add to following to your .vimrc file:
   ``au FileType xml setlocal equalprg=xmllint\ --format\ --recover\ -\
   2>/dev/null`` now you can call the linter using ``gg=G`` in command mode.

Text generation
***************

Template processor **should** be used for generating config files. Built-in
string formatting **may** be used for simple line-oriented formats where every
line is self-contained, such as iptables rules. Template processor **must** be
used for structured, multi-line formats such as those used by ISC DHCPd.

The default template processor for VyOS code is jinja2.

Code policy
-----------

When modifying the source code, remember these rules of the legacy elimination
campaign:

* No new features in Perl
* No old style command definitions
* No code incompatible with Python3

.. _process: https://blog.vyos.io/vyos-development-digest-10
.. _vyos-1x: https://github.com/vyos/vyos-1x/blob/current/schema/
.. _VyConf: https://github.com/vyos/vyconf/blob/master/data/schemata