155 lines
4.5 KiB
Plaintext
155 lines
4.5 KiB
Plaintext
======================
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Python 3 compatibility
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======================
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Django 1.5 is the first version of Django to support Python 3. The same code
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runs both on Python 2 (≥ 2.6.5) and Python 3 (≥ 3.2), thanks to the six_
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compatibility layer and ``unicode_literals``.
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.. _six: http://packages.python.org/six/
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This document is not meant as a Python 2 to Python 3 migration guide. There
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are many existing resources, including `Python's official porting guide`_.
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Rather, it describes guidelines that apply to Django's code and are
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recommended for pluggable apps that run with both Python 2 and 3.
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.. _Python's official porting guide: http://docs.python.org/py3k/howto/pyporting.html
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Syntax requirements
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===================
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Unicode
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-------
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In Python 3, all strings are considered Unicode by default. The ``unicode``
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type from Python 2 is called ``str`` in Python 3, and ``str`` becomes
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``bytes``.
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You mustn't use the ``u`` prefix before a unicode string literal because it's
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a syntax error in Python 3.2. You must prefix byte strings with ``b``.
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In order to enable the same behavior in Python 2, every module must import
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``unicode_literals`` from ``__future__``::
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from __future__ import unicode_literals
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my_string = "This is an unicode literal"
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my_bytestring = b"This is a bytestring"
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In classes, define ``__str__`` methods returning unicode strings and apply the
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:func:`~django.utils.encoding.python_2_unicode_compatible` decorator. It will
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define appropriate ``__unicode__`` and ``__str__`` in Python 2::
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from __future__ import unicode_literals
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from django.utils.encoding import python_2_unicode_compatible
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@python_2_unicode_compatible
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class MyClass(object):
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def __str__(self):
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return "Instance of my class"
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If you need a byte string literal under Python 2 and a unicode string literal
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under Python 3, use the :func:`str` builtin::
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str('my string')
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Be cautious if you have to `slice bytestrings`_.
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.. _slice bytestrings: http://docs.python.org/py3k/howto/pyporting.html#bytes-literals
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Exceptions
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----------
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When you capture exceptions, use the ``as`` keyword::
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try:
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...
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except MyException as exc:
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...
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This older syntax was removed in Python 3::
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try:
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...
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except MyException, exc:
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...
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The syntax to reraise an exception with a different traceback also changed.
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Use :func:`six.reraise`.
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.. module: django.utils.six
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Writing compatible code with six
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================================
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six_ is the canonical compatibility library for supporting Python 2 and 3 in
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a single codebase. Read its documentation!
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:mod:`six` is bundled with Django: you can import it as :mod:`django.utils.six`.
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Here are the most common changes required to write compatible code.
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String types
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------------
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The ``basestring`` and ``unicode`` types were removed in Python 3, and the
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meaning of ``str`` changed. To test these types, use the following idioms::
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isinstance(myvalue, six.string_types) # replacement for basestring
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isinstance(myvalue, six.text_type) # replacement for unicode
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isinstance(myvalue, bytes) # replacement for str
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Python ≥ 2.6 provides ``bytes`` as an alias for ``str``, so you don't need
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:attr:`six.binary_type`.
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``long``
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--------
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The ``long`` type no longer exists in Python 3. ``1L`` is a syntax error. Use
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:data:`six.integer_types` check if a value is an integer or a long::
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isinstance(myvalue, six.integer_types) # replacement for (int, long)
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``xrange``
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----------
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Import :func:`six.moves.xrange` wherever you use ``xrange``.
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Moved modules
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-------------
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Some modules were renamed in Python 3. The :mod:`django.utils.six.moves
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<six.moves>` module provides a compatible location to import them.
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In addition to six' defaults, Django's version provides ``dummy_thread`` as
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``_dummy_thread``.
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PY3
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---
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If you need different code in Python 2 and Python 3, check :data:`six.PY3`::
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if six.PY3:
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# do stuff Python 3-wise
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else:
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# do stuff Python 2-wise
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This is a last resort solution when :mod:`six` doesn't provide an appropriate
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function.
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.. module:: django.utils.six
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Customizations of six
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=====================
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The version of six bundled with Django includes a few additional tools:
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.. function:: iterlists(MultiValueDict)
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Returns an iterator over the lists of values of a
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:class:`~django.utils.datastructures.MultiValueDict`. This replaces
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:meth:`~django.utils.datastructures.MultiValueDict.iterlists()` on Python
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2 and :meth:`~django.utils.datastructures.MultiValueDict.lists()` on
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Python 3.
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