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Porting to Python 3
Django 1.5 is the first version of Django to support Python 3. The same code
runs both on Python 2 (≥ 2.6.5) and Python 3 (≥ 3.2), thanks to the six_
compatibility layer.
.. _six:
This document is primarily targeted at authors of pluggable application
who want to support both Python 2 and 3. It also describes guidelines that
apply to Django's code.
This document assumes that you are familiar with the changes between Python 2
and Python 3. If you aren't, read `Python's official porting guide`_ first.
Refreshing your knowledge of unicode handling on Python 2 and 3 will help; the
`Pragmatic Unicode`_ presentation is a good resource.
Django uses the *Python 2/3 Compatible Source* strategy. Of course, you're
free to chose another strategy for your own code, especially if you don't need
to stay compatible with Python 2. But authors of pluggable applications are
encouraged to use the same porting strategy as Django itself.
Writing compatible code is much easier if you target Python ≥ 2.6. You will
most likely take advantage of the compatibility functions introduced in Django
1.5, like :mod:`django.utils.six`, so your application will also require
Django ≥ 1.5.
Obviously, writing compatible source code adds some overhead, and that can
cause frustration. Django's developers have found that attempting to write
Python 3 code that's compatible with Python 2 is much more rewarding than the
opposite. Not only does that make your code more future-proof, but Python 3's
advantages (like the saner string handling) start shining quickly. Dealing
with Python 2 becomes a backwards compatibility requirement, and we as
developers are used to dealing with such constraints.
Porting tools provided by Django are inspired by this philosophy, and it's
reflected throughout this guide.
.. _Python's official porting guide:
.. _Pragmatic Unicode:
Porting tips
Unicode literals
This step consists in:
- Adding ``from __future__ import unicode_literals`` at the top of your Python
modules -- it's best to put it in each and every module, otherwise you'll
keep checking the top of your files to see which mode is in effect;
- Removing the ``u`` prefix before unicode strings;
- Adding a ``b`` prefix before bytestrings.
Performing these changes systematically guarantees backwards compatibility.
However, Django applications generally don't need bytestrings, since Django
only exposes unicode interfaces to the programmer. Python 3 discourages using
bytestrings, except for binary data or byte-oriented interfaces. Python 2
makes bytestrings and unicode strings effectively interchangeable, as long as
they only contain ASCII data. Take advantage of this to use unicode strings
wherever possible and avoid the ``b`` prefixes.
.. note::
Python 2's ``u`` prefix is a syntax error in Python 3.2 but it will be
allowed again in Python 3.3 thanks to :pep:`414`. Thus, this
transformation is optional if you target Python ≥ 3.3. It's still
recommended, per the "write Python 3 code" philosophy.
String handling
Python 2's :class:`unicode` type was renamed :class:`str` in Python 3,
:class:`str` was renamed :class:`bytes`, and :class:`basestring` disappeared.
six_ provides :ref:`tools <string-handling-with-six>` to deal with these
Django also contains several string related classes and functions in the
:mod:`django.utils.encoding` and :mod:`django.utils.safestring` modules. Their
names used the words ``str``, which doesn't mean the same thing in Python 2
and Python 3, and ``unicode``, which doesn't exist in Python 3. In order to
avoid ambiguity and confusion these concepts were renamed ``bytes`` and
Here are the name changes in :mod:`django.utils.encoding`:
================== ==================
Old name New name
================== ==================
``smart_str`` ``smart_bytes``
``smart_unicode`` ``smart_text``
``force_unicode`` ``force_text``
================== ==================
For backwards compatibility, the old names still work on Python 2. Under
Python 3, ``smart_str`` is an alias for ``smart_text``.
.. note::
:mod:`django.utils.encoding` was deeply refactored in Django 1.5 to
provide a more consistent API. Check its documentation for more
:mod:`django.utils.safestring` is mostly used via the
:func:`~django.utils.safestring.mark_safe` and
:func:`~django.utils.safestring.mark_for_escaping` functions, which didn't
change. In case you're using the internals, here are the name changes:
================== ==================
Old name New name
================== ==================
``EscapeString`` ``EscapeBytes``
``EscapeUnicode`` ``EscapeText``
``SafeString`` ``SafeBytes``
``SafeUnicode`` ``SafeText``
================== ==================
For backwards compatibility, the old names still work on Python 2. Under
Python 3, ``EscapeString`` and ``SafeString`` are aliases for ``EscapeText``
and ``SafeText`` respectively.
:meth:`__str__` and :meth:`__unicode__` methods
In Python 2, the object model specifies :meth:`__str__` and
:meth:`__unicode__` methods. If these methods exist, they must return
:class:`str` (bytes) and :class:`unicode` (text) respectively.
The ``print`` statement and the :func:`str` built-in call :meth:`__str__` to
determine the human-readable representation of an object. The :func:`unicode`
built-in calls :meth:`__unicode__` if it exists, and otherwise falls back to
:meth:`__str__` and decodes the result with the system encoding. Conversely,
the :class:`~django.db.models.Model` base class automatically derives
:meth:`__str__` from :meth:`__unicode__` by encoding to UTF-8.
In Python 3, there's simply :meth:`__str__`, which must return :class:`str`
(It is also possible to define :meth:`__bytes__`, but Django application have
little use for that method, because they hardly ever deal with
Django provides a simple way to define :meth:`__str__` and :meth:`__unicode__`
methods that work on Python 2 and 3: you must define a :meth:`__str__` method
returning text and to apply the
:func:`~django.utils.encoding.python_2_unicode_compatible` decorator.
On Python 3, the decorator is a no-op. On Python 2, it defines appropriate
:meth:`__unicode__` and :meth:`__str__` methods (replacing the original
:meth:`__str__` method in the process). Here's an example::
from __future__ import unicode_literals
from django.utils.encoding import python_2_unicode_compatible
class MyClass(object):
def __str__(self):
return "Instance of my class"
This technique is the best match for Django's porting philosophy.
Finally, note that :meth:`__repr__` must return a :class:`str` on all versions
of Python.
:class:`dict` and :class:`dict`-like classes
:meth:`dict.keys`, :meth:`dict.items` and :meth:`dict.values` return lists in
Python 2 and iterators in Python 3. :class:`~django.http.QueryDict` and the
:class:`dict`-like classes defined in :mod:`django.utils.datastructures`
behave likewise in Python 3.
six_ provides compatibility functions to work around this change:
:func:`~six.iterkeys`, :func:`~six.iteritems`, and :func:`~six.itervalues`.
Django's bundled version adds :func:`~django.utils.six.iterlists` for
:class:`~django.utils.datastructures.MultiValueDict` and its subclasses.
:class:`~django.http.HttpRequest` and :class:`~django.http.HttpResponse` objects
According to :pep:`3333`:
- headers are always :class:`str` objects,
- input and output streams are always :class:`bytes` objects.
Specifically, :attr:`HttpResponse.content <django.http.HttpResponse.content>`
contains :class:`bytes`, which may require refactoring your tests.This won't
be an issue if you use :meth:`~django.test.TestCase.assertContains` and
:meth:`~django.test.TestCase.assertNotContains`: these methods expect a
unicode string.
Coding guidelines
The following guidelines are enforced in Django's source code. They're also
recommended for third-party application who follow the same porting strategy.
Syntax requirements
In Python 3, all strings are considered Unicode by default. The ``unicode``
type from Python 2 is called ``str`` in Python 3, and ``str`` becomes
You mustn't use the ``u`` prefix before a unicode string literal because it's
a syntax error in Python 3.2. You must prefix byte strings with ``b``.
In order to enable the same behavior in Python 2, every module must import
``unicode_literals`` from ``__future__``::
from __future__ import unicode_literals
my_string = "This is an unicode literal"
my_bytestring = b"This is a bytestring"
If you need a byte string literal under Python 2 and a unicode string literal
under Python 3, use the :func:`str` builtin::
str('my string')
In Python 3, there aren't any automatic conversions between :class:`str` and
:class:`bytes`, and the :mod:`codecs` module became more strict.
:meth:`str.decode` always returns :class:`bytes`, and :meth:`bytes.decode`
always returns :class:`str`. As a consequence, the following pattern is
sometimes necessary::
value = value.encode('ascii', 'ignore').decode('ascii')
Be cautious if you have to `slice bytestrings`_.
.. _slice bytestrings:
When you capture exceptions, use the ``as`` keyword::
except MyException as exc:
This older syntax was removed in Python 3::
except MyException, exc: # Don't do that!
The syntax to reraise an exception with a different traceback also changed.
Use :func:`six.reraise`.
Magic methods
Use the patterns below to handle magic methods renamed in Python 3.
class MyIterator(object):
def __iter__(self):
return self # implement some logic here
def __next__(self):
raise StopIteration # implement some logic here
next = __next__ # Python 2 compatibility
Boolean evaluation
class MyBoolean(object):
def __bool__(self):
return True # implement some logic here
__nonzero__ = __bool__ # Python 2 compatibility
class MyDivisible(object):
def __truediv__(self, other):
return self / other # implement some logic here
__div__ = __truediv__ # Python 2 compatibility
def __itruediv__(self, other):
return self // other # implement some logic here
__idiv__ = __itruediv__ # Python 2 compatibility
.. module: django.utils.six
Writing compatible code with six
six_ is the canonical compatibility library for supporting Python 2 and 3 in
a single codebase. Read its documentation!
:mod:`six` is bundled with Django: you can import it as :mod:`django.utils.six`.
Here are the most common changes required to write compatible code.
.. _string-handling-with-six:
String handling
The ``basestring`` and ``unicode`` types were removed in Python 3, and the
meaning of ``str`` changed. To test these types, use the following idioms::
isinstance(myvalue, six.string_types) # replacement for basestring
isinstance(myvalue, six.text_type) # replacement for unicode
isinstance(myvalue, bytes) # replacement for str
Python ≥ 2.6 provides ``bytes`` as an alias for ``str``, so you don't need
The ``long`` type no longer exists in Python 3. ``1L`` is a syntax error. Use
:data:`six.integer_types` check if a value is an integer or a long::
isinstance(myvalue, six.integer_types) # replacement for (int, long)
Import :func:`six.moves.xrange` wherever you use ``xrange``.
Moved modules
Some modules were renamed in Python 3. The :mod:`django.utils.six.moves
<six.moves>` module provides a compatible location to import them.
The ``urllib``, ``urllib2`` and ``urlparse`` modules were reworked in depth
and :mod:`django.utils.six.moves <six.moves>` doesn't handle them. Django
explicitly tries both locations, as follows::
from urllib.parse import urlparse, urlunparse
except ImportError: # Python 2
from urlparse import urlparse, urlunparse
If you need different code in Python 2 and Python 3, check :data:`six.PY3`::
if six.PY3:
# do stuff Python 3-wise
# do stuff Python 2-wise
This is a last resort solution when :mod:`six` doesn't provide an appropriate
.. module:: django.utils.six
Customizations of six
The version of six bundled with Django includes one extra function:
.. function:: iterlists(MultiValueDict)
Returns an iterator over the lists of values of a
:class:`~django.utils.datastructures.MultiValueDict`. This replaces
:meth:`~django.utils.datastructures.MultiValueDict.iterlists()` on Python
2 and :meth:`~django.utils.datastructures.MultiValueDict.lists()` on
Python 3.
In addition to six' defaults moves, Django's version provides ``thread`` as
``_thread`` and ``dummy_thread`` as ``_dummy_thread``.
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