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Nova Style Commandments


  • Put two newlines between top-level code (funcs, classes, etc)

  • Put one newline between methods in classes and anywhere else

  • Long lines should be wrapped in parentheses in preference to using a backslash for line continuation.

  • Do not write "except:", use "except Exception:" at the very least

  • Include your name with TODOs as in "#TODO(termie)"

  • 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


  • 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

(*) exceptions are:

  • imports from migrate package
  • imports from sqlalchemy package
  • imports from nova.db.sqlalchemy.session module


# vim: tabstop=4 shiftwidth=4 softtabstop=4
{{stdlib imports in human alphabetical order}}
{{third-party lib imports in human alphabetical order}}
{{nova 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
from nova.endpoint import cloud
import nova.flags
from nova import test



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

"""A multi line 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

If you are going to describe parameters and return values, use Sphinx, the
appropriate syntax is as follows.

:param foo: the foo parameter
:param bar: the bar parameter
: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"})

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.

For more information on creating unit tests and utilizing the testing infrastructure in OpenStack Nova, please read nova/testing/README.rst.


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

These modules are "incubating" in openstack-common and are kept in sync with the help of openstack-common's 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.

OpenStack Trademark

OpenStack is a registered trademark of OpenStack, LLC, and uses the following capitalization:


Commit Messages

Using a common format for commit messages will help keep our git history readable. Follow these guidelines:

First, provide a brief summary (it is recommended to keep the commit title under 50 chars).

The first line of the commit message should provide an accurate description of the change, not just a reference to a bug or blueprint. It must be followed by a single blank line.

If the change relates to a specific driver (libvirt, xenapi, qpid, etc...), begin the first line of the commit message with the driver name, lowercased, followed by a colon.

Following your brief summary, provide a more detailed description of the patch, manually wrapping the text at 72 characters. This description should provide enough detail that one does not have to refer to external resources to determine its high-level functionality.

Once you use 'git review', two lines will be appended to the commit message: a blank line followed by a 'Change-Id'. This is important to correlate this commit with a specific review in Gerrit, and it should not be modified.

For further information on constructing high quality commit messages, and how to split up commits into a series of changes, consult the project wiki:
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