mirror of
https://github.com/django/django.git
synced 2024-11-29 14:46:18 +01:00
408 lines
14 KiB
Plaintext
408 lines
14 KiB
Plaintext
===================
|
||
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: http://pythonhosted.org/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.
|
||
|
||
Philosophy
|
||
==========
|
||
|
||
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. Django 1.5
|
||
introduces compatibility tools such as :mod:`django.utils.six`, which is a
|
||
customized version of the :mod:`six module <six>`. For convenience,
|
||
forwards-compatible aliases were introduced in Django 1.4.2. If your
|
||
application takes advantage of these tools, it will require Django ≥ 1.4.2.
|
||
|
||
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: http://docs.python.org/3/howto/pyporting.html
|
||
.. _Pragmatic Unicode: http://nedbatchelder.com/text/unipain.html
|
||
|
||
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 `unicode`_ type was renamed :class:`str` in Python 3,
|
||
``str()`` was renamed :class:`bytes`, and `basestring`_ disappeared.
|
||
six_ provides :ref:`tools <string-handling-with-six>` to deal with these
|
||
changes.
|
||
|
||
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
|
||
``text``.
|
||
|
||
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``.
|
||
|
||
For forwards compatibility, the new names work as of Django 1.4.2.
|
||
|
||
.. note::
|
||
|
||
:mod:`django.utils.encoding` was deeply refactored in Django 1.5 to
|
||
provide a more consistent API. Check its documentation for more
|
||
information.
|
||
|
||
: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.
|
||
|
||
For forwards compatibility, the new names work as of Django 1.4.2.
|
||
|
||
:meth:`~object.__str__` and ` __unicode__()`_ methods
|
||
-----------------------------------------------------
|
||
|
||
In Python 2, the object model specifies :meth:`~object.__str__` and
|
||
` __unicode__()`_ methods. If these methods exist, they must return
|
||
``str`` (bytes) and ``unicode`` (text) respectively.
|
||
|
||
The ``print`` statement and the :class:`str` built-in call
|
||
:meth:`~object.__str__` to determine the human-readable representation of an
|
||
object. The ``unicode`` built-in calls ` __unicode__()`_ if it
|
||
exists, and otherwise falls back to :meth:`~object.__str__` and decodes the
|
||
result with the system encoding. Conversely, the
|
||
:class:`~django.db.models.Model` base class automatically derives
|
||
:meth:`~object.__str__` from ` __unicode__()`_ by encoding to UTF-8.
|
||
|
||
In Python 3, there's simply :meth:`~object.__str__`, which must return ``str``
|
||
(text).
|
||
|
||
(It is also possible to define :meth:`~object.__bytes__`, but Django application
|
||
have little use for that method, because they hardly ever deal with ``bytes``.)
|
||
|
||
Django provides a simple way to define :meth:`~object.__str__` and
|
||
` __unicode__()`_ methods that work on Python 2 and 3: you must
|
||
define a :meth:`~object.__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
|
||
` __unicode__()`_ and :meth:`~object.__str__` methods (replacing the
|
||
original :meth:`~object.__str__` method in the process). Here's an example::
|
||
|
||
from __future__ import unicode_literals
|
||
from django.utils.encoding import python_2_unicode_compatible
|
||
|
||
@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.
|
||
|
||
For forwards compatibility, this decorator is available as of Django 1.4.2.
|
||
|
||
Finally, note that :meth:`~object.__repr__` must return a ``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`.
|
||
It also contains an undocumented ``iterlists`` function that works well for
|
||
``django.utils.datastructures.MultiValueDict`` and its subclasses.
|
||
|
||
:class:`~django.http.HttpRequest` and :class:`~django.http.HttpResponse` objects
|
||
--------------------------------------------------------------------------------
|
||
|
||
According to :pep:`3333`:
|
||
|
||
- headers are always ``str`` objects,
|
||
- input and output streams are always ``bytes`` objects.
|
||
|
||
Specifically, :attr:`HttpResponse.content <django.http.HttpResponse.content>`
|
||
contains ``bytes``, which may become an issue if you compare it with a
|
||
``str`` in your tests. The preferred solution is to rely on
|
||
:meth:`~django.test.SimpleTestCase.assertContains` and
|
||
:meth:`~django.test.SimpleTestCase.assertNotContains`. These methods accept a
|
||
response and a unicode string as arguments.
|
||
|
||
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
|
||
-------------------
|
||
|
||
Unicode
|
||
~~~~~~~
|
||
|
||
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
|
||
``bytes``.
|
||
|
||
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 :class:`str` builtin::
|
||
|
||
str('my string')
|
||
|
||
In Python 3, there aren't any automatic conversions between ``str`` and
|
||
``bytes``, and the :mod:`codecs` module became more strict. :meth:`str.encode`
|
||
always returns ``bytes``, and ``bytes.decode`` always returns ``str``. As a
|
||
consequence, the following pattern is sometimes necessary::
|
||
|
||
value = value.encode('ascii', 'ignore').decode('ascii')
|
||
|
||
Be cautious if you have to `index bytestrings`_.
|
||
|
||
.. _index bytestrings: https://docs.python.org/3/howto/pyporting.html#indexing-bytes-objects
|
||
|
||
Exceptions
|
||
~~~~~~~~~~
|
||
|
||
When you capture exceptions, use the ``as`` keyword::
|
||
|
||
try:
|
||
...
|
||
except MyException as exc:
|
||
...
|
||
|
||
This older syntax was removed in Python 3::
|
||
|
||
try:
|
||
...
|
||
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.
|
||
|
||
Iterators
|
||
~~~~~~~~~
|
||
|
||
::
|
||
|
||
class MyIterator(six.Iterator):
|
||
def __iter__(self):
|
||
return self # implement some logic here
|
||
|
||
def __next__(self):
|
||
raise StopIteration # implement some logic here
|
||
|
||
Boolean evaluation
|
||
~~~~~~~~~~~~~~~~~~
|
||
|
||
::
|
||
|
||
class MyBoolean(object):
|
||
|
||
def __bool__(self):
|
||
return True # implement some logic here
|
||
|
||
def __nonzero__(self): # Python 2 compatibility
|
||
return type(self).__bool__(self)
|
||
|
||
Division
|
||
~~~~~~~~
|
||
|
||
::
|
||
|
||
class MyDivisible(object):
|
||
|
||
def __truediv__(self, other):
|
||
return self / other # implement some logic here
|
||
|
||
def __div__(self, other): # Python 2 compatibility
|
||
return type(self).__truediv__(self, other)
|
||
|
||
def __itruediv__(self, other):
|
||
return self // other # implement some logic here
|
||
|
||
def __idiv__(self, other): # Python 2 compatibility
|
||
return type(self).__itruediv__(self, other)
|
||
|
||
Special methods are looked up on the class and not on the instance to reflect
|
||
the behavior of the Python interpreter.
|
||
|
||
.. 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!
|
||
|
||
A :mod:`customized version of six <django.utils.six>` is bundled with Django
|
||
as of version 1.4.2. You can import it as ``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
|
||
:data:`six.binary_type`.
|
||
|
||
``long``
|
||
~~~~~~~~
|
||
|
||
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)
|
||
|
||
``xrange``
|
||
~~~~~~~~~~
|
||
|
||
Import ``six.moves.xrange`` wherever you use ``xrange``.
|
||
|
||
Moved modules
|
||
~~~~~~~~~~~~~
|
||
|
||
Some modules were renamed in Python 3. The ``django.utils.six.moves``
|
||
module (based on the :mod:`six.moves module <six.moves>`) provides a
|
||
compatible location to import them.
|
||
|
||
PY2
|
||
~~~
|
||
|
||
If you need different code in Python 2 and Python 3, check :data:`six.PY2`::
|
||
|
||
if six.PY2:
|
||
# compatibility code for Python 2
|
||
|
||
This is a last resort solution when :mod:`six` doesn't provide an appropriate
|
||
function.
|
||
|
||
.. module:: django.utils.six
|
||
|
||
Django customized version of six
|
||
--------------------------------
|
||
|
||
The version of six bundled with Django (``django.utils.six``) includes a few
|
||
extras.
|
||
|
||
.. function:: assertRaisesRegex(testcase, *args, **kwargs)
|
||
|
||
This replaces ``testcase.assertRaisesRegexp`` on Python 2, and
|
||
``testcase.assertRaisesRegex`` on Python 3. ``assertRaisesRegexp`` still
|
||
exists in current Python 3 versions, but issues a warning.
|
||
|
||
.. function:: assertRegex(testcase, *args, **kwargs)
|
||
|
||
This replaces ``testcase.assertRegexpMatches`` on Python 2, and
|
||
``testcase.assertRegex`` on Python 3. ``assertRegexpMatches`` still
|
||
exists in current Python 3 versions, but issues a warning.
|
||
|
||
|
||
In addition to six' defaults moves, Django's version provides ``thread`` as
|
||
``_thread`` and ``dummy_thread`` as ``_dummy_thread``.
|
||
|
||
.. _unicode: http://docs.python.org/2/library/functions.html#unicode
|
||
.. _ __unicode__(): https://docs.python.org/2/reference/datamodel.html#object.__unicode__
|
||
.. _basestring: http://docs.python.org/2/library/functions.html#basestring
|