OpenStack Style Guidelines

General

  • [H903] Use only UNIX style newlines (\n), not Windows style (\r\n)

  • [H904] Wrap long lines in parentheses and not a backslash for line continuation.

  • [H201] Do not write except:, use except Exception: at the very least

  • [H101] Include your name with TODOs as in # TODO(yourname)

  • 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
    

Imports

  • [H302] Do not import objects, only modules (*)
  • [H301] Do not import more than one module per line (*)
  • [H303] Do not use wildcard * import (*)
  • [H304] Do not make relative imports
  • Order your imports by the full module path
  • [H305 H306 H307] Organize your imports according to the Import order template and Real-world Import Order Examples below.

(*) exceptions are:

  • imports from migrate package
  • imports from sqlalchemy package
  • imports from oslo-incubator.openstack.common.gettextutils module

Import order template

{{stdlib imports in human alphabetical order}}
\n
{{third-party lib imports in human alphabetical order}}
\n
{{project imports in human alphabetical order}}
\n
\n
{{begin your code}}

Real-world Import Order Examples

Example:

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

Docstrings

  • [H401] Docstrings should not start with a space.
  • [H402] The first line of a docstring should end with punctuation.
  • [H403] Multi line docstrings should end on a new line.
  • [H404] Multi line docstrings should start without a leading new line.
  • [H405] Multi line docstrings should start with a one line summary followed by an empty line.

Example:

"""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
"""

Dictionaries/Lists

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.

Example:

my_dictionary = {
    "image": {
        "name": "Just a Snapshot",
        "size": 2749573,
        "properties": {
            "user_id": 12,
            "arch": "x86_64",
        },
        "things": [
            "thing_one",
            "thing_two",
        ],
        "status": "ACTIVE",
    },
}
  • [H501] 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',
                                 kwarg1=constants.ACTIVE,
                                 kwarg2=['a', 'b', 'c'])

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

list_of_strings = [
    'what_a_long_string',
    'not as long',
]

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

object_one.call_a_method('string three',
                         'string four',
                         kwarg1=list_of_strings,
                         kwarg2=dict_of_numbers)

Internationalization (i18n) Strings

In order to support multiple languages, we have a mechanism to support automatic translations of exception and log strings.

Example:

msg = _("An error occurred")
raise HTTPBadRequest(explanation=msg)
  • [H702] If you have a variable to place within the string, first internationalize the template string then do the replacement.

    Example:

    msg = _("Missing parameter: %s") % ("flavor")
    LOG.error(msg)
    
  • [H703] If you have multiple variables to place in the string, use keyword parameters. This helps our translators reorder parameters when needed.

    Example:

    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. In order to start making code Python 3.x compatible before it can be is fully Python 3.x compatible, we have checks for Python 2.x-only constructs:

  • [H231] except. Instead of:

    except x,y:
    

    Use:

    except x as y:
    
  • [H232] 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.

  • [H233] The print operator can be avoided by using:

    from __future__ import print_function
    

    at the top of your module.

  • [H234] assertEquals() logs a DeprecationWarning in Python 3.x, use assertEqual() instead. The same goes for assertNotEquals().

  • [H235] assert_() is deprecated in Python 3.x, use assertTrue() instead.

  • [H236] Use six.add_metaclass instead of __metaclass__.

    Example:

    import six
    
    @six.add_metaclass(Meta)
    class YourClass():
    

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.

Unit Tests and assertRaises

A properly written test asserts that particular behavior occurs. This can be a success condition or a failure condition, including an exception. When asserting that a particular exception is raised, the most specific exception possible should be used.

  • [H202] Testing for Exception being raised is almost always a mistake since it will match (almost) every exception, even those unrelated to the exception intended to be tested.

    This applies to catching exceptions manually with a try/except block, or using assertRaises().

    Example:

    self.assertRaises(exception.InstanceNotFound, db.instance_get_by_uuid,
                          elevated, instance_uuid)
    

oslo-incubator

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

OpenStack Licensing

  • [H102 H103] 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.
    
  • [H104] Files with no code shouldn’t contain any license header nor comments, and must be left completely empty.

Commit Messages

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

  • [H802] First, provide a brief summary of 50 characters or less. Summaries of greater then 72 characters will be rejected by the gate.
  • [H801] The first line of the commit message should provide an accurate description of the change, not just a reference to a bug or blueprint.
  • [H803] The first line of the commit message must not end with a period and must be followed by a single blank line.

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