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Being pedantic about pedanticism: HACKING styleguide


Just now I set out to merge a recent style guide change from python-novaclient into the hacking docs of other OpenStack projects. My patch didn't apply, though, because each project has subtly diverging HACKING files.

Rather than contribute to this divergence, I've now read and compared the style guides from Nova, Glance, Keystone, python-keystoneclient, and python-novaclient. From these diffs I've created a file (attached) that encompasses the total of all guidelines from all projects. Remarkably, this merge produced only minor disagreements, described below under the heading FLAMEBAIT.

I propose that this unified style guide be copied into each of the above projects, with a mandate to maintain consistency henceforth. Any objections?


FLAMEBAIT (docstring format):

The only explicit contradiction I came across is regarding docstring formatting. Glance says this:

**DO NOT** leave an extra newline before the closing triple-double-quote.

Nova, this:

  A docstring ends with an empty line before the closing quotations.

I propose that we just pick one or the other, or remove both prescriptions.

FLAMEBAIT (filename):

Some projects have a HACKING file, and some have a HACKING.rst file. Doesn't .rst generally indicate a REST API definition? I propose that the file be called HACKING.

OpenStack Style Commandments

- Step 1: Read http://www.python.org/dev/peps/pep-0008/
- Step 2: Read http://www.python.org/dev/peps/pep-0008/ again
- Step 3: Read on

- Put two newlines between top-level code (funcs, classes, etc)
- Put one newline between methods in classes and anywhere else
- Do not write "except:", use "except Exception:" at the very least
- Include your name with TODOs as in "#TODO(termie)"
- When defining global constants, define them before functions and classes
- Do not shadow a built-in or reserved word. Example::

    def list():
        return [1, 2, 3]

    mylist = list() # BAD, shadows `list` built-in

    class Foo(object):
        def list(self):
            return [1, 2, 3]

    mylist = Foo().list() # OKAY, does not shadow built-in


- TODO(name): implies that something should be done (cleanup, refactoring,
  etc), but is expected to be functional.
- FIXME(name): implies that the method/function/etc shouldn't be used until
  that code is resolved and bug fixed.

- Do not import objects, only modules
- Do not import more than one module per line
- Do not make relative imports
- Order your imports by the full module path
- Organize your imports according to the following template


  # vim: tabstop=4 shiftwidth=4 softtabstop=4
  {{stdlib imports in human alphabetical order}}
  {{third-party lib imports in human alphabetical order}}
  {{project imports in human alphabetical order}}
  {{begin your code}}

Human Alphabetical Order Examples

  import httplib
  import logging
  import random
  import StringIO
  import time
  import unittest

  import eventlet
  import webob.exc

  import nova.api.ec2
  from nova.api import openstack
  from nova.auth import users
  import nova.flags
  from nova.endpoint import cloud
  from nova import test


  """A one line docstring looks like this and ends in a period."""

  """A multiline docstring has a one-line summary, less than 80 characters.

  Then a new paragraph after a newline that explains in more detail any
  general information about the function, class or method. Example usages
  are also great to have here if it is a complex class for function.

  When writing the docstring for a class, an extra line should be placed
  after the closing quotations. For more in-depth explanations for these
  decisions see http://www.python.org/dev/peps/pep-0257/

  A docstring ends with an empty line before the closing quotations.

  Describe parameters and return values, using the Sphinx format; the
  appropriate syntax is as follows.

  :param foo: the foo parameter
  :param bar: the bar parameter
  :type bar: parameter type for 'bar'
  :returns: return_type -- description of the return value
  :returns: description of the return value
  :raises: AttributeError, KeyError


If a dictionary (dict) or list object is longer than 80 characters, its items
should be split with newlines. Embedded iterables should have their items
indented. Additionally, the last item in the dictionary should have a trailing
comma. This increases readability and simplifies future diffs.


  my_dictionary = {
      "image": {
          "name": "Just a Snapshot",
          "size": 2749573,
          "properties": {
               "user_id": 12,
               "arch": "x86_64",
          "things": [
          "status": "ACTIVE",

Calling Methods
Calls to methods 80 characters or longer should format each argument with
newlines. This is not a requirement, but a guideline::

    unnecessarily_long_function_name('string one',
                                     'string two',
                                     kwarg2=['a', 'b', 'c'])

Rather than constructing parameters inline, it is better to break things up::

    list_of_strings = [
        'not as long',

    dict_of_numbers = {
        'one': 1,
        'two': 2,
        'twenty four': 24,

    object_one.call_a_method('string three',
                             'string four',

Internationalization (i18n) Strings
In order to support multiple languages, we have a mechanism to support
automatic translations of exception and log strings.


    msg = _("An error occurred")
    raise HTTPBadRequest(explanation=msg)

If you have a variable to place within the string, first internationalize the
template string then do the replacement.


    msg = _("Missing parameter: %s") % ("flavor",)

If you have multiple variables to place in the string, use keyword parameters.
This helps our translators reorder parameters when needed.


    msg = _("The server with id %(s_id)s has no key %(m_key)s")
    LOG.error(msg % {"s_id": "1234", "m_key": "imageId"})

Text encoding
- All text within python code should be of type 'unicode'.


    >>> s = 'foo'
    >>> s
    >>> type(s)
    <type 'str'>


    >>> u = u'foo'
    >>> u
    >>> type(u)
    <type 'unicode'>

- Transitions between internal unicode and external strings should always
  be immediately and explicitly encoded or decoded.

- All external text that is not explicitly encoded (database storage,
  commandline arguments, etc.) should be presumed to be encoded as utf-8.


    mystring = infile.readline()
    myreturnstring = do_some_magic_with(mystring)


    mystring = infile.readline()
    mytext = s.decode('utf-8')
    returntext = do_some_magic_with(mytext)
    returnstring = returntext.encode('utf-8')

Creating Unit Tests
For every new feature, unit tests should be created that both test and
(implicitly) document the usage of said feature. If submitting a patch for a
bug that had no unit test, a new passing unit test should be added. If a
submitted bug fix does have a unit test, be sure to add a new one that fails
without the patch and passes with the patch.


A number of modules from openstack-common are imported into this project.

These modules are "incubating" in openstack-common and are kept in sync
with the help of openstack-common's update.py script. See:


The copy of the code should never be directly modified here. Please
always update openstack-common first and then run the script to copy
the changes across.

Style consistency across OpenStack

If you modify this style-guide, it is your responsibility to
duplicate the change throughout the other OpenStack style
guides.  This HACKING file should remain identical among
all projects.

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