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Revision as of 09:09, 15 May 2015 by Lifeless (talk | contribs) (Dependencies)

This page tracks the progress of Python 3 effort porting for OpenStack.

Python 3

Why should OpenStack move to Python 3 right now?

Python 3 is usually seen as the new Python version which breaks compatibility and raises new Unicode issues. Python 3 is much more than that. It’s a new clean language which has a more consistent syntax. It has many new features, not less than 15 new modules. Python 3 is already well supported by major Linux distributions, whereas Python 2.7 reached its end-of-life. Slowly, some bugs cannot be fixed in Python 2.7 anymore and are only fixed in the latest Python 3 release. Python 3 is now 5 years old and considered as a mature programming language.

Python 2: 2.7? 2.6?

OpenStack Liberty targets Python 2.7.

Python 2.6 support is being dropped in OpenStack since OpenStack Juno.

Port Python 2 code to Python 3

OpenStack project chose to use the same code base for Python 2 and Python 3. The Six: Python 2 and 3 Compatibility Library helps to write code working on both versions. OpenStack supported Python 2.6 for RHEL up to Juno, but not Python 2.5 and older. Debian Stable provides Python 3 but only Python 3.2, so u'unicode' syntax should be avoided (use six.u('unicode') instead).

Common patterns

  • Replace dict.iteritems() with six.iteritems(dict)
  • Replace iterator.next() with next(iterator)
  • Replace basestring with six.string_types
  • Replace unicode with six.text_type

bytes.decode and unicode.encode

Python has a notion of "default encoding": sys.getdefaultencoding(). On Python 2, the default encoding is ASCII, whereas it is UTF-8 on Python 3.

Don't write data.decode() or text.encode() without parameter, because you will use a different encoding on Python 2 and Python 3.

Use an explicit encoding instead. Example: data.decode('utf-8') or text.encode('utf-8'). The right encoding depends on the use case, but UTF-8 is usually a good candidate (it is a superset of ASCII).


Olso Incubator has a function safe_decode() which can be used to decode a bytes string and pass text strings unchanged.

The default encoding is sys.stdin.encoding or sys.getdefaultencoding():

  • Python 3: the locale encoding, or UTF-8 if sys.stdin is "mocked" (io.StringIO instance)
  • Python 2: the locale encoding, or ASCII if stdin is not a TTY or if sys.stdin is "mocked" (StringIO.StringIO instance)

It's safer to explicit the encoding to not rely on the locale encoding and have the same behaviour even if sys.stdin is "mocked".

Safe usage:

  • safe_decode(data, 'utf-8'): decode bytes from UTF-8 or returns data unchanged if it's already a text string

Unsafe usage:

  • safe_decode(data)

By default, the decoder is strict. You can specify a different error handler using the optional errors parameter. Example: safe_decode(b'[\xff]', 'ascii', 'ignore') returns '[]'.


Olso Incubator has a function safe_encode() which can be used to encode a string. Its usage is tricky and you should understand how it works and which encodings are used.

  • safe_encode(text) encodes text to the output encoding
  • safe_encode(bytes) may decode the string and then reencode to a different encoding if input and output encodings are different

The default input encoding (incomding parameter) is sys.stdin.encoding or sys.getdefaultencoding():

  • Python 3: the locale encoding, or UTF-8 if sys.stdin is "mocked" (io.StringIO instance)
  • Python 2: the locale encoding, or ASCII if stdin is not a TTY or if sys.stdin is "mocked" (StringIO.StringIO instance)

The default output encoding (encoding parameter) is UTF-8.

It's safer to explicit the input encoding to not rely on the locale encoding and have the same behaviour even if sys.stdin is "mocked".

Safe usage:

  • safe_encode(data, incoming='utf-8'): encode text to UTF-8 or returns data unchanged if it's already a bytes string (since the input and output encoding are UTF-8)

Unsafe usage:

  • safe_encode(data)


  • safe_encode(b'\xe9', incoming='latin-1') returns b'\xc3\xa9'.

By default, the encoder and the decoder are strict. You can specify a different error handler using the optional errors parameter. Example: safe_encode(b'[\xff]', incoming='ascii', errors='ignore') returns b'[]'.

logging module and format exceptions

On Python 2, the logging module accepts bytes and text strings. On Python 3, it only accepts text strings. For example, logging.error(b'hello') logs b'hello' instead of 'hello'.

There is no clear rule for format exceptions yet. There are different choices depending on the project:

  • str(exc): native string, so use bytes on Python 2
  • six.text_type(exc): always use Unicode. It may raise unicode error depending on the exception, be careful. Example of such error in python 2: unicode(Exception("nonascii:\xe9")).
  • six.u(str(exc)): unsafe on Python 2 if str(exc) contains non-ASCII bytes, ex: unicode(str(Exception("\xff")))
  • LOG.exception(_LE("... %(exc)s ..."), {"exc": exc, ...})

Since logging functions expect text strings on Python 3, logged exceptions should be formatted using str(exc). Example: LOG.debug(str(exc)).


The HTTP protocol is based on bytes:

  • HTTP body contains bytes. For example, use io.BytesIO for a stream storing an HTTP body.
  • HTTPConnection.getresponse().read() returns bytes (in Python 3, str which is bytes in Python 2)
  • On Python 3, the http.client accepts text for HTTP headers: keys are encoded to ASCII and values to ISO 8859-1 (which is only a small subset of the Unicode charset)
  • It looks like Swift encodes internally HTTP headers to UTF-8 (directly using the UTF-8 encoding, not using a MIME encoding like =?UTF-8?Q?...?=. See the HTTP [RFC 2047 http://www.ietf.org/rfc/rfc2047.txt] and HTTP header should use what character encoding?

References to port Python 2 code to Python 3

Common pitfalls

What is a string ?

You should definitely not talk about "strings" in your commit logs/reviews. In Python 2, a 'string' is bytes; in Python 3, it's a Unicode text string. The following code snippet may help in understanding the difference:

Python 2:

   >>> type('foo')
   <type 'str'>
   >>> type(u'foo')
   <type 'unicode'>
   >>> type(b'foo')
   <type 'str'>
   >>> isinstance('foo', six.text_type)
   >>> isinstance(u'foo', six.text_type)
   >>> bytes is str
   >>> b'foo'[0]

Python 3:

   >>> type('foo')
   <class 'str'>
   >>> type(u'foo')
   <class 'str'>
   >>> type(b'foo')
   <class 'bytes'>
   >>> isinstance('foo', six.text_type)
   >>> isinstance(b'foo', six.text_type)
   >>> bytes is str
   >>> b'foo'[0]

tox/testr error: db type could not be determined

The "db type could not be determined" error comes from .testrepository/times.dbm used by testr.

Workaround: "rm -rf .testrepository/".

Python 3 Status of OpenStack projects

Common Libraries (Oslo Projects)

For the list of Common Libraries, see http://git.openstack.org/cgit/openstack/governance/tree/reference/programs.yaml#n160

Project Python 3 compatibility Comment
cliff Yes
oslo.concurrency Yes

oslo.concurrency 1.9 supports Python 3, next version will support fully Python 3 in processutils (execute() and ssh_execute() will use Unicode).

oslo-incubator Yes py34 gate is voting, py3 classifiers are missing: https://review.openstack.org/182968
oslo.config Yes
oslo.context Yes
oslo.db Yes On Python 3, PyMySQL driver is used instead of MySQL-Python to test MySQL. setup.cfg contains the Python 3 classifier.
oslo.i18n Yes
oslo.log Yes
oslo.messaging Yes, except of Qpid & AMQP drivers The development version of oslo.messaging fully works on Python 3, except of Qpid and AMQP 1.0 transports which are Python 2 only. the py3 gate is voting.
oslo.middleware Yes
oslo.rootwrap Yes
oslo.serialization Yes
oslosphinx  ? The project only contains two short .py files, it looks to be Python 3 compatible. Is Sphinx Python 3 compatible?
oslotest Yes
oslo.versionedobjects Yes
oslo.vmware Yes
oslo.utils Yes
oslo.version Yes not released on PyPI yet
pylockfile Yes
stevedore Yes
taskflow Yes

Development tools

Project Python 3 compatibility Comment
cookiecutter yes
hacking yes py33 gate is not voting
pbr yes
stackforge/python-jenkins yes py33 gate is voting
openstack-infra/jenkins-job-builder partial https://review.openstack.org/172238

OpenStack clients

Project Python 3 compatibility CI tests running? Python 3 classifiers ? Blocked by Comment
keystonemiddleware Yes, but requires patched python-memcached No Yes python-memcached keystonemiddleware 1.6.1 supports Python 3, but it is still blocked by python-memcached which has pending pull request for Python 3.
python-barbicanclient Yes Voting On PyPI
python-ceilometerclient Yes Voting On PyPI
python-cinderclient Yes Voting On PyPI
python-ganttclient  ?  ?  ?  ?
python-glanceclient Yes Voting On PyPI
python-heatclient Yes Voting On PyPI
python-ironicclient Yes Voting On PyPI
python-keystoneclient Yes Voting On PyPI
python-marconiclient Yes Voting Yes
python-melangeclient  ?  ?  ?
python-novaclient Yes Voting On PyPII
python-neutronclient Yes Voting Yes
python-openstackclient OK Voting Yes As of 0.9
python-savannaaclient In progress Non-voting Yes python-savannaclient: enable the py33 gates abandonned: "We haven't enough tests to ensure that savanna client works correctly on py33, so, it's kind of premature step. We already have py33 and pypy jobs in experimental pipeline."
python-saharaclient OK Voting In the git repo
python-swiftclient Yes Voting On PyPI
python-tuskarclient Yes Voting On PyPI
python-troveclient Yes Voting On PyPI


There are two ways to express dependencies that should differ between Python 2.x and 3.x. We currently support using multiple requirements files, and will shortly be supporting and recommending the use of environment markers. Recent releases of all the tools in our ecosystem are needed for environment markers.


This approach generates a different requirements list depending on what Python version is running when the egg info is created (which goes into wheels and sdists). As a consequence this can't work with tox, publishing sdists to PyPI, or wheels. It is however useful for anything installing from git. To use it, create two requirements files:

  • requirements-py2.txt: all dependencies for Python 2 (not only dependencies specific to Python 2)
  • requirements-py3.txt: all dependencies for Python 3 (not only dependencies specific to Python 3)
  • (same for test-requirements.txt)

You have to edit tox.ini to specify the right requirements file. Extract of a tox.ini file:

deps = -r{toxinidir}/requirements-py3.txt

Environment markers

Environment markers provide a mini-language to express when a dependency is relevant. We hope the next release of pbr will support them. Recent pip, setuptools and wheel versions do support them, but until pbr exports the requirements metadata with them intact and structured correctly for setuptools, we can't use them anywhere.

When placed in a requirements.txt file, the markers are interpreted directly by pip in `pip install -r requirements.txt`, and indirectly via setuptools when installing an sdist or wheel (including `pip install .`. For instance:

futures; python_version < '3.2'

See also:

Oslo Messaging issue:

  • futures is not needed on Python 3, since concurrent.futures is now part of the Pyhon stdlib since Python 3.2
  • futures was added to requirements-py3.txt because tox creates the source tarball using Python 2 which reads requirements.txt
  • The correct fix would be to use environment markers on futures

Dependencies status

Porting status for global-requirement.txt.

Project Python 3 compatibility CI tests running? Python 3 classifiers ? Blocked by Comment
boto Yes N/A Yes See https://github.com/boto/boto3 (experimental) <- This seems dead, and https://github.com/boto/boto works with Python 3.x (since 2.32).
django-compressor Yes N/A Yes Requirements upgraded: https://review.openstack.org/94357
django-openstack-auth Yes N/A Yes

As of 1.1.6

dnspython Yes N/A Yes Must use the Python 3 version, see https://github.com/rthalley/dnspython/issues/60
ecdsa Yes N/A Yes Py3 support merge before the 0.10 release (see https://github.com/warner/python-ecdsa/commits/master)
eventlet Yes Yes Yes eventlet 0.17.3 now fully support Python 3
jsonrpclib No N/A No The project seems dead :(
libvirt-python Yes N/A Yes in Git
MySQL-Python No, but mysqlclient or PyMySQL are compatible N/A No


MySQL-Python: 2 pending pull requests for Python 3.

INADA Naoki, the maintainer of PyMySQL, wrote mysqlclient: fork of MySQL-Python using libmysqlclient.so which is Python 3 compatible. He recommends to use mysqlclient instead of MySQL-python even on Python 2. mysqlclient fixes some bugs, build issues and it support Python 3. For example: support MariaDB's libmysqlclient.so, support microsecond in TIME column.

PyMySQL is compatible with Python 3: see PyMySQL evaluation discussion to eventually replace mysql-python with PyMySQL in OpenStack.

netifaces Yes N/A Yes Patch sent by Victor Stinner (in private): netifaces_python3.patch, Debian has patches too. Python 3 support as of 0.10.4. Pushed to requirements: https://review.openstack.org/94358 .
nose-exclude No No No https://bitbucket.org/kgrandis/nose-exclude/issue/10/test-failures-with-python-3
nosehtmloutput No No No
  • nose-exclude (tests only)
  • openstack.nose-plugin
nosexcover No N/A On PyPI Python 3 support since 1.0.9
openstack.nose-plugin No No No
oslo.vmware No Voting Yes suds

Patch: Use suds-jurko on Python 2

oslo.config Yes Voting On PyPI
pam No for pam, Yes for simplepam NA No The fork simplepam works on Python 2 and 3
paramiko Yes N/A On PyPI   Requirements upgraded: https://review.openstack.org/#/c/81132/
paste Yes N/A Yes Compatible with Python 3 since Paste 2.0
pycadf Yes Yes Yes
python-ldap No No No The project seems dead. See ldap3 which is Python 3 compatible (license: LGPL, one of the optional deps is BSD-4-Clause).
python-memcached No, but there is a pending pull request No No

python-memcached 1.54 includes many fixes for Python 3, but it doesn't work on Python 3 yet. Pull request: Port memcache to Python 3 #67.

Julien Danjou ported pymemcache to Python 3, another memcached client, he suggests to use this one instead

qpid-python No No No Required if using oslo.messaging's qpid backend. That is the old pure python amqp 0-10 client library. That only speaks 0-10 and works with qpidd broker. It is used by the impl_qpid.py driver in oslo.messaging. That client is legacy - it isn't being ported to use AMQP 1.0
pyngus No No No

pyngus is required if using oslo.messaging's AMQP 1.0 driver. pyngus is a wrapper for proton and proton has not been ported to python 3 yet: see Proton issue #490.

Proton is a C library with various bindings in other languages. The language bindings are auto generated via the SWIG tool, plus a the python wrapper bits. All the python stuff is found under proton-c/bindings/python in the git tree. The first bit of work is to refactor the SWIG file cproton.i to work for both Python 2 and 3.

rtslib-fb No No No
Routes Yes using 2to3 Yes Yes Routes doesn't work on Python 3 when building wheel packages using Python 2, see: Port Routes to Python 3
sphinxcontrib-docbookrestapi Yes Yes In the Git repo, not on PyPI
sphinxcontrib-httpdomain Yes N/A No
sphinxcontrib-pecanwsme No No No
sqlalchemy-migrate Yes N/A Patch sent

MySQL is not supported yet on Python 3 (MySQL-python is still used). Patches:

suds Yes for the fork suds-jurko No No "Lightweight SOAP client". Last commit 2 years ago: https://fedorahosted.org/suds/browser See also this fork which is promising: jurko/suds. Note in global requirements: "suds is not python 3.x compatible, suds-jurko is a fork that works with py3x. oslo.vmware would convert to suds-jurko first then nova and cinder would follow. suds should be remove immediately once those projects move to suds-jurko for all jobs."
taskflow Yes Yes Yes
thrift No No No
websockify No No No

OpenStack applications

Completely updated on Monday, September the 29th, 2014. Partially updated later.

Project Python 3 compatibility CI tests running? Trove classifiers Blocked by Comment
ceilometer No No No


  • croniter
  • thrift (which is blocking happybase)

Requirements for tests

  • MySQL-python
  • sphinxcontrib-docbookrestapi
  • sphinxcontrib-httpdomain
  • sphinxcontrib-pecanwsme
cinder No No No


  • rtslib-fb

Requirements for tests

  • MySQL-python
glance No No No


  • glance_store (partial py3 support, blocked by suds)

Requirements for tests

  • MySQL-python
  • qpid-python
gnocchi Yes Yes  ?
heat work in progress No No


  • qpid-python

Requirements for tests

  • MySQL-python

Sirushti Murugesan proposed Add Python 3.4 support spec for Liberty.

horizon No No No


  • django-pyscss

Requirements for tests

  • nodeenv
  • nose-exclude
  • nosehtmloutput
  • openstack.nose_plugin
ironic Yes Yes  ?

Python 3.4 unit tests are now being run for openstack/ironic. The unit tests are a voting job. Thanks to Victor Sergeyev for all of his work to update the Ironic code to make it pass the unit tests using Python 3.4: Run tests in py34 environment

keystone No No No

Requirements for tests

  • ldappool
  • pysaml2?
  • python-ldap
  • MySQL-python
neutron work in progress No No


  • jsonrpclib
  • MySQL-python
Porting to Python 3 spec accepted for Liberty, port in progress. Patches:
nova work in progress No No
  • suds
  • websockify

Requirements for tests

  • MySQL-python
Adding Python 3.4 support to Nova spec accepted for Liberty. Port in progress.

Patches: see master (bp/nova-python3).

swift No No No


Requirements for tests:

  • nosehtmloutput
  • openstack.nose_plugin

Number of core OpenStack projetcs blocked by each dependency:

     5 MySQL-python
     1 suds
     2 qpid-python
     2 openstack.nose_plugin
     2 nosehtmloutput
     1 xstatic-spin
     1 xstatic-rickshaw
     1 xstatic-qunit
     1 xstatic-jsencrypt
     1 xstatic-jquery.tablesorter
     1 xstatic-jquery.quicksearch
     1 xstatic-jquery-migrate
     1 xstatic-jquery
     1 xstatic-jasmine
     1 xstatic-hogan
     1 xstatic-font-awesome
     1 xstatic-d3
     1 xstatic-bootstrap-datepicker
     1 xstatic-angular-mock
     1 xstatic-angular-cookies
     1 xstatic-angular
     1 xstatic
     1 websockify
     1 thrift
     1 sphinxcontrib-pecanwsme
     1 sphinxcontrib-httpdomain
     1 sphinxcontrib-docbookrestapi
     1 rtslib-fb
     1 python-ldap
     1 nose-exclude
     1 nodeenv
     1 ldappool
     1 jsonrpclib
     1 glance_store
     1 django-pyscss
     1 croniter

Status of Python 3 in Linux distributions

Reports at OpenStack Summits

Pycon Montreal 2014: Sprint Port OpenStack to Python 3

Enovance organized a sprint to Port OpenStack to Python 3 during 4 days: between April, 14 (Monday) and April, 17 (Thursday) 2014. See the page Python3/SprintPycon2014.