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 Python 2.7Python 3.2Python 3.3
Windows (32-bit)
Windows (64-bit)
Mac OS X (10.5+)
0.7.0 Failed View build log
Linux (32-bit)
0.7.0 Failed View build log
Linux (64-bit)
0.7.0 Failed View build log
Lastest release
version 0.7.0 on Aug 12th, 2013


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

  • Use only UNIX style newlines ("n"), not Windows style ("rn")

  • 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
  • Use the "is not" operator when testing for unequal identities. Example:

    if not X is Y:  # BAD, intended behavior is ambiguous
    if X is not Y:  # OKAY, intuitive
  • Use the "not in" operator for evaluating membership in a collection. Example:

    if not X in Y:  # BAD, intended behavior is ambiguous
    if X not in Y:  # OKAY, intuitive
    if not (X in Y or X in Z):  # OKAY, still better than all those 'not's


  • Do not import objects, only modules (*)
  • Do not import more than one module per line (*)
  • Do not use wildcard * import (*)
  • 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
  • imports from nova.db.sqlalchemy.migration.versioning_api package


# 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
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 http://www.python.org/dev/peps/pep-0257/

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",

Do not use locals() for formatting strings, it is not clear as using explicit dictionaries and can hide errors during refactoring.

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"})

Python 3.x compatibility

OpenStack code should become Python 3.x compatible. That means all Python 2.x-only constructs or dependencies should be avoided. An example is

except x,y:


except x as y:

instead. Also Python 3.x has become more strict regarding octal string literals. Use "0o755" instead of "0755". Similarly, explicit use of long literals (01234L) should be avoided.

Other Python 3.x compatibility issues, like e.g. print operator can be avoided in new code by using

from __future__ import print_function

at the top of your module.

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 oslo-incubator are imported into the project.

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


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

OpenStack Trademark

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


OpenStack Licensing

Newly contributed Source Code should be licensed under the Apache 2.0 license. All source files should have the following header:

# Licensed under the Apache License, Version 2.0 (the "License"); you may # not use this file except in compliance with the License. You may obtain # a copy of the License at # # http://www.apache.org/licenses/LICENSE-2.0 # # Unless required by applicable law or agreed to in writing, software # distributed under the License is distributed on an "AS IS" BASIS, WITHOUT # WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the # License for the specific language governing permissions and limitations # under the License.

Commit Messages

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

First, provide a brief summary of 50 characters or less. Summaries of greater then 72 characters will be rejected by the gate.

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 not end with a period and 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:


Further Reading

System Message: WARNING/2 (<string>, line 324)

Title underline too short.

Further Reading

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Last updated Aug 12th, 2013

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