django/docs/topics/python3.txt

======================
Python 3 compatibility
======================

Django 1.5 introduces a compatibility layer that allows the code to be run both
in Python 2 (2.6/2.7) and Python 3 (>= 3.2) (*work in progress*).

This document is not meant as a complete Python 2 to Python 3 migration guide.
There are many existing resources you can read. But we describe some utilities
and guidelines that we recommend you should use when you want to ensure your
code can be run with both Python 2 and 3.

* http://docs.python.org/py3k/howto/pyporting.html
* http://python3porting.com/

django.utils.py3
================

Whenever a symbol or module has different semantics or different locations on
Python 2 and Python 3, you can import it from ``django.utils.py3`` where it
will be automatically converted depending on your current Python version.

PY3
---

If you need to know anywhere in your code if you are running Python 3 or a
previous Python 2 version, you can check the ``PY3`` boolean variable::

    from django.utils.py3 import PY3

    if PY3:
        # Do stuff Python 3-wise
    else:
        # Do stuff Python 2-wise

This should be considered as a last resort solution when it is not possible
to import a compatible name from django.utils.py3, as described in the sections
below.

String handling
===============

In Python 3, all strings are considered Unicode strings by default. Byte strings
have to be prefixed with the letter 'b'. To mimic the same behaviour in Python 2,
we recommend you import ``unicode_literals`` from the ``__future__`` library::

    from __future__ import unicode_literals

    my_string = "This is an unicode literal"
    my_bytestring = b"This is a bytestring"

Be cautious if you have to slice bytestrings.
See http://docs.python.org/py3k/howto/pyporting.html#bytes-literals

Different expected strings
--------------------------

Some method parameters have changed the expected string type of a parameter.
For example, ``strftime`` format parameter expects a bytestring on Python 2 but
a normal (Unicode) string on Python 3. For these cases, ``django.utils.py3``
provides a ``n()`` function which encodes the string parameter only with
Python 2.

    >>> from __future__ import unicode_literals
    >>> from datetime import datetime

    >>> print(datetime.date(2012, 5, 21).strftime(n("%m → %Y")))
    05 → 2012

Renamed types
=============

Several types are named differently in Python 2 and Python 3. In order to keep
compatibility while using those types, import their corresponding aliases from
``django.utils.py3``.

===========  =========  =====================
Python 2     Python 3   django.utils.py3
===========  =========  =====================
basestring,  str,       string_types (tuple)
unicode      str        text_type
int, long    int,       integer_types (tuple)
long         int        long_type
===========  =========  =====================

String aliases
--------------

Code sample::

    if isinstance(foo, basestring):
        print("foo is a string")

    # I want to convert a number to a Unicode string
    bar = 45
    bar_string = unicode(bar)

Should be replaced by::

    from django.utils.py3 import string_types, text_type

    if isinstance(foo, string_types):
        print("foo is a string")

    # I want to convert a number to a Unicode string
    bar = 45
    bar_string = text_type(bar)

No more long type
-----------------

``long`` and ``int`` types have been unified in Python 3, meaning that  ``long``
is no longer available. ``django.utils.py3`` provides both ``long_type`` and
``integer_types`` aliases. For example:

.. code-block:: python

    # Old Python 2 code
    my_var = long(333463247234623)
    if isinstance(my_var, (int, long)):
        # ...

Should be replaced by:

.. code-block:: python

    from django.utils.py3 import long_type, integer_types

    my_var = long_type(333463247234623)
    if isinstance(my_var, integer_types):
        # ...


Changed module locations
========================

The following modules have changed their location in Python 3. Therefore, it is
recommended to import them from the ``django.utils.py3`` compatibility layer:

=============================== ======================================  ======================
Python 2                        Python3                                 django.utils.py3
=============================== ======================================  ======================
Cookie                          http.cookies                            cookies

urlparse.urlparse               urllib.parse.urlparse                   urlparse
urlparse.urlunparse             urllib.parse.urlunparse                 urlunparse
urlparse.urljoin                urllib.parse.urljoin                    urljoin
urlparse.urlsplit               urllib.parse.urlsplit                   urlsplit
urlparse.urlunsplit             urllib.parse.urlunsplit                 urlunsplit
urlparse.urldefrag              urllib.parse.urldefrag                  urldefrag
urlparse.parse_qsl              urllib.parse.parse_qsl                  parse_qsl
urllib.quote                    urllib.parse.quote                      quote
urllib.unquote                  urllib.parse.unquote                    unquote
urllib.quote_plus               urllib.parse.quote_plus                 quote_plus
urllib.unquote_plus             urllib.parse.unquote_plus               unquote_plus
urllib.urlencode                urllib.parse.urlencode                  urlencode
urllib.urlopen                  urllib.request.urlopen                  urlopen
urllib.url2pathname             urllib.request.url2pathname             url2pathname
urllib.urlretrieve              urllib.request.urlretrieve              urlretrieve
urllib2                         urllib.request                          urllib2
urllib2.Request                 urllib.request.Request                  Request
urllib2.OpenerDirector          urllib.request.OpenerDirector           OpenerDirector
urllib2.UnknownHandler          urllib.request.UnknownHandler           UnknownHandler
urllib2.HTTPHandler             urllib.request.HTTPHandler              HTTPHandler
urllib2.HTTPSHandler            urllib.request.HTTPSHandler             HTTPSHandler
urllib2.HTTPDefaultErrorHandler urllib.request.HTTPDefaultErrorHandler  HTTPDefaultErrorHandler
urllib2.FTPHandler              urllib.request.FTPHandler               FTPHandler
urllib2.HTTPError               urllib.request.HTTPError                HTTPError
urllib2.HTTPErrorProcessor      urllib.request.HTTPErrorProcessor       HTTPErrorProcessor

htmlentitydefs.name2codepoint   html.entities.name2codepoint            name2codepoint
HTMLParser                      html.parser                             HTMLParser
cPickle/pickle                  pickle                                  pickle
thread/dummy_thread             _thread/_dummy_thread                   thread

os.getcwdu                      os.getcwd                               getcwdu
itertools.izip                  zip                                     zip
sys.maxint                      sys.maxsize                             maxsize
unichr                          chr                                     unichr
xrange                          range                                   xrange
=============================== ======================================  ======================


Ouptut encoding now Unicode
===========================

If you want to catch stdout/stderr output, the output content is UTF-8 encoded
in Python 2, while it is Unicode strings in Python 3. You can use the OutputIO
stream to capture this output::

    from django.utils.py3 import OutputIO

    try:
        old_stdout = sys.stdout
        out = OutputIO()
        sys.stdout = out
        # Do stuff which produces standard output
        result = out.getvalue()
    finally:
        sys.stdout = old_stdout

Dict iteritems/itervalues/iterkeys
==================================

The iteritems(), itervalues() and iterkeys() methods of dictionaries do not
exist any more in Python 3, simply because they represent the default items()
values() and keys() behavior in Python 3. Therefore, to keep compatibility,
use similar functions from ``django.utils.py3``::

    from django.utils.py3 import iteritems, itervalues, iterkeys

    my_dict = {'a': 21, 'b': 42}
    for key, value in iteritems(my_dict):
        # ...
    for value in itervalues(my_dict):
        # ...
    for key in iterkeys(my_dict):
        # ...

Note that in Python 3, dict.keys(), dict.items() and dict.values() return
"views" instead of lists. Wrap them into list() if you really need their return
values to be in a list.

http://docs.python.org/release/3.0.1/whatsnew/3.0.html#views-and-iterators-instead-of-lists

Metaclass
=========

The syntax for declaring metaclasses has changed in Python 3.
``django.utils.py3`` offers a compatible way to declare metaclasses::

    from django.utils.py3 import with_metaclass

    class MyClass(with_metaclass(SubClass1, SubClass2,...)):
        # ...

Re-raising exceptions
=====================

One of the syntaxes to raise exceptions (raise E, V, T) is gone in Python 3.
This is especially used in very specific cases where you want to re-raise a
different exception that the initial one, while keeping the original traceback.
So, instead of::

    raise Exception, Exception(msg), traceback

Use::

    from django.utils.py3 import reraise

    reraise(Exception, Exception(msg), traceback)