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<h1><a href="../index.html">Django 1.6.7 documentation</a></h1>
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<div class="section" id="s-porting-to-python-3">
<span id="porting-to-python-3"></span><h1>Porting to Python 3<a class="headerlink" href="#porting-to-python-3" title="Permalink to this headline">¶</a></h1>
<p>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 <a class="reference external" href="http://pythonhosted.org/six/">six</a>
compatibility layer.</p>
<p>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.</p>
<div class="section" id="s-philosophy">
<span id="philosophy"></span><h2>Philosophy<a class="headerlink" href="#philosophy" title="Permalink to this headline">¶</a></h2>
<p>This document assumes that you are familiar with the changes between Python 2
and Python 3. If you aren’t, read <a class="reference external" href="http://docs.python.org/3/howto/pyporting.html">Python’s official porting guide</a> first.
Refreshing your knowledge of unicode handling on Python 2 and 3 will help; the
<a class="reference external" href="http://nedbatchelder.com/text/unipain.html">Pragmatic Unicode</a> presentation is a good resource.</p>
<p>Django uses the <em>Python 2/3 Compatible Source</em> 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.</p>
<p>Writing compatible code is much easier if you target Python ≥ 2.6. Django 1.5
introduces compatibility tools such as <a class="reference internal" href="#module-django.utils.six" title="django.utils.six"><tt class="xref py py-mod docutils literal"><span class="pre">django.utils.six</span></tt></a>, which is a
customized version of the <tt class="xref py py-mod docutils literal"><span class="pre">six</span> <span class="pre">module</span></tt>. 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.</p>
<p>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.</p>
<p>Porting tools provided by Django are inspired by this philosophy, and it’s
reflected throughout this guide.</p>
</div>
<div class="section" id="s-porting-tips">
<span id="porting-tips"></span><h2>Porting tips<a class="headerlink" href="#porting-tips" title="Permalink to this headline">¶</a></h2>
<div class="section" id="s-unicode-literals">
<span id="unicode-literals"></span><h3>Unicode literals<a class="headerlink" href="#unicode-literals" title="Permalink to this headline">¶</a></h3>
<p>This step consists in:</p>
<ul class="simple">
<li>Adding <tt class="docutils literal"><span class="pre">from</span> <span class="pre">__future__</span> <span class="pre">import</span> <span class="pre">unicode_literals</span></tt> 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;</li>
<li>Removing the <tt class="docutils literal"><span class="pre">u</span></tt> prefix before unicode strings;</li>
<li>Adding a <tt class="docutils literal"><span class="pre">b</span></tt> prefix before bytestrings.</li>
</ul>
<p>Performing these changes systematically guarantees backwards compatibility.</p>
<p>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 <tt class="docutils literal"><span class="pre">b</span></tt> prefixes.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">Python 2’s <tt class="docutils literal"><span class="pre">u</span></tt> prefix is a syntax error in Python 3.2 but it will be
allowed again in Python 3.3 thanks to <span class="target" id="index-0"></span><a class="pep reference external" href="http://www.python.org/dev/peps/pep-0414"><strong>PEP 414</strong></a>. Thus, this
transformation is optional if you target Python ≥ 3.3. It’s still
recommended, per the “write Python 3 code” philosophy.</p>
</div>
</div>
<div class="section" id="s-string-handling">
<span id="string-handling"></span><h3>String handling<a class="headerlink" href="#string-handling" title="Permalink to this headline">¶</a></h3>
<p>Python 2’s <tt class="xref py py-func docutils literal"><span class="pre">unicode()</span></tt> type was renamed <tt class="xref py py-func docutils literal"><span class="pre">str()</span></tt> in Python 3,
<tt class="xref py py-func docutils literal"><span class="pre">str()</span></tt> was renamed <tt class="docutils literal"><span class="pre">bytes()</span></tt>, and <tt class="xref py py-func docutils literal"><span class="pre">basestring()</span></tt> disappeared.
<a class="reference external" href="http://pythonhosted.org/six/">six</a> provides <a class="reference internal" href="#string-handling-with-six"><em>tools</em></a> to deal with these
changes.</p>
<p>Django also contains several string related classes and functions in the
<a class="reference internal" href="../ref/utils.html#module-django.utils.encoding" title="django.utils.encoding: A series of helper classes and function to manage character encoding."><tt class="xref py py-mod docutils literal"><span class="pre">django.utils.encoding</span></tt></a> and <a class="reference internal" href="../ref/utils.html#module-django.utils.safestring" title="django.utils.safestring: Functions and classes for working with strings that can be displayed safely without further escaping in HTML."><tt class="xref py py-mod docutils literal"><span class="pre">django.utils.safestring</span></tt></a> modules. Their
names used the words <tt class="docutils literal"><span class="pre">str</span></tt>, which doesn’t mean the same thing in Python 2
and Python 3, and <tt class="docutils literal"><span class="pre">unicode</span></tt>, which doesn’t exist in Python 3. In order to
avoid ambiguity and confusion these concepts were renamed <tt class="docutils literal"><span class="pre">bytes</span></tt> and
<tt class="docutils literal"><span class="pre">text</span></tt>.</p>
<p>Here are the name changes in <a class="reference internal" href="../ref/utils.html#module-django.utils.encoding" title="django.utils.encoding: A series of helper classes and function to manage character encoding."><tt class="xref py py-mod docutils literal"><span class="pre">django.utils.encoding</span></tt></a>:</p>
<table class="docutils">
<colgroup>
<col width="50%" />
<col width="50%" />
</colgroup>
<thead valign="bottom">
<tr class="row-odd"><th class="head">Old name</th>
<th class="head">New name</th>
</tr>
</thead>
<tbody valign="top">
<tr class="row-even"><td><tt class="docutils literal"><span class="pre">smart_str</span></tt></td>
<td><tt class="docutils literal"><span class="pre">smart_bytes</span></tt></td>
</tr>
<tr class="row-odd"><td><tt class="docutils literal"><span class="pre">smart_unicode</span></tt></td>
<td><tt class="docutils literal"><span class="pre">smart_text</span></tt></td>
</tr>
<tr class="row-even"><td><tt class="docutils literal"><span class="pre">force_unicode</span></tt></td>
<td><tt class="docutils literal"><span class="pre">force_text</span></tt></td>
</tr>
</tbody>
</table>
<p>For backwards compatibility, the old names still work on Python 2. Under
Python 3, <tt class="docutils literal"><span class="pre">smart_str</span></tt> is an alias for <tt class="docutils literal"><span class="pre">smart_text</span></tt>.</p>
<p>For forwards compatibility, the new names work as of Django 1.4.2.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last"><a class="reference internal" href="../ref/utils.html#module-django.utils.encoding" title="django.utils.encoding: A series of helper classes and function to manage character encoding."><tt class="xref py py-mod docutils literal"><span class="pre">django.utils.encoding</span></tt></a> was deeply refactored in Django 1.5 to
provide a more consistent API. Check its documentation for more
information.</p>
</div>
<p><a class="reference internal" href="../ref/utils.html#module-django.utils.safestring" title="django.utils.safestring: Functions and classes for working with strings that can be displayed safely without further escaping in HTML."><tt class="xref py py-mod docutils literal"><span class="pre">django.utils.safestring</span></tt></a> is mostly used via the
<a class="reference internal" href="../ref/utils.html#django.utils.safestring.mark_safe" title="django.utils.safestring.mark_safe"><tt class="xref py py-func docutils literal"><span class="pre">mark_safe()</span></tt></a> and
<a class="reference internal" href="../ref/utils.html#django.utils.safestring.mark_for_escaping" title="django.utils.safestring.mark_for_escaping"><tt class="xref py py-func docutils literal"><span class="pre">mark_for_escaping()</span></tt></a> functions, which didn’t
change. In case you’re using the internals, here are the name changes:</p>
<table class="docutils">
<colgroup>
<col width="50%" />
<col width="50%" />
</colgroup>
<thead valign="bottom">
<tr class="row-odd"><th class="head">Old name</th>
<th class="head">New name</th>
</tr>
</thead>
<tbody valign="top">
<tr class="row-even"><td><tt class="docutils literal"><span class="pre">EscapeString</span></tt></td>
<td><tt class="docutils literal"><span class="pre">EscapeBytes</span></tt></td>
</tr>
<tr class="row-odd"><td><tt class="docutils literal"><span class="pre">EscapeUnicode</span></tt></td>
<td><tt class="docutils literal"><span class="pre">EscapeText</span></tt></td>
</tr>
<tr class="row-even"><td><tt class="docutils literal"><span class="pre">SafeString</span></tt></td>
<td><tt class="docutils literal"><span class="pre">SafeBytes</span></tt></td>
</tr>
<tr class="row-odd"><td><tt class="docutils literal"><span class="pre">SafeUnicode</span></tt></td>
<td><tt class="docutils literal"><span class="pre">SafeText</span></tt></td>
</tr>
</tbody>
</table>
<p>For backwards compatibility, the old names still work on Python 2. Under
Python 3, <tt class="docutils literal"><span class="pre">EscapeString</span></tt> and <tt class="docutils literal"><span class="pre">SafeString</span></tt> are aliases for <tt class="docutils literal"><span class="pre">EscapeText</span></tt>
and <tt class="docutils literal"><span class="pre">SafeText</span></tt> respectively.</p>
<p>For forwards compatibility, the new names work as of Django 1.4.2.</p>
</div>
<div class="section" id="s-str-and-unicode-methods">
<span id="str-and-unicode-methods"></span><h3><tt class="xref py py-meth docutils literal"><span class="pre">__str__()</span></tt> and <tt class="xref py py-meth docutils literal"><span class="pre">__unicode__()</span></tt> methods<a class="headerlink" href="#str-and-unicode-methods" title="Permalink to this headline">¶</a></h3>
<p>In Python 2, the object model specifies <tt class="xref py py-meth docutils literal"><span class="pre">__str__()</span></tt> and
<tt class="xref py py-meth docutils literal"><span class="pre">__unicode__()</span></tt> methods. If these methods exist, they must return
<tt class="docutils literal"><span class="pre">str</span></tt> (bytes) and <tt class="docutils literal"><span class="pre">unicode</span></tt> (text) respectively.</p>
<p>The <tt class="docutils literal"><span class="pre">print</span></tt> statement and the <tt class="xref py py-func docutils literal"><span class="pre">str()</span></tt> built-in call
<tt class="xref py py-meth docutils literal"><span class="pre">__str__()</span></tt> to determine the human-readable representation of an
object. The <tt class="xref py py-func docutils literal"><span class="pre">unicode()</span></tt> built-in calls <tt class="xref py py-meth docutils literal"><span class="pre">__unicode__()</span></tt> if it
exists, and otherwise falls back to <tt class="xref py py-meth docutils literal"><span class="pre">__str__()</span></tt> and decodes the
result with the system encoding. Conversely, the
<a class="reference internal" href="../ref/models/instances.html#django.db.models.Model" title="django.db.models.Model"><tt class="xref py py-class docutils literal"><span class="pre">Model</span></tt></a> base class automatically derives
<tt class="xref py py-meth docutils literal"><span class="pre">__str__()</span></tt> from <tt class="xref py py-meth docutils literal"><span class="pre">__unicode__()</span></tt> by encoding to UTF-8.</p>
<p>In Python 3, there’s simply <tt class="xref py py-meth docutils literal"><span class="pre">__str__()</span></tt>, which must return <tt class="docutils literal"><span class="pre">str</span></tt>
(text).</p>
<p>(It is also possible to define <tt class="docutils literal"><span class="pre">__bytes__()</span></tt>, but Django application have
little use for that method, because they hardly ever deal with
<tt class="docutils literal"><span class="pre">bytes</span></tt>.)</p>
<p>Django provides a simple way to define <tt class="xref py py-meth docutils literal"><span class="pre">__str__()</span></tt> and
<tt class="xref py py-meth docutils literal"><span class="pre">__unicode__()</span></tt> methods that work on Python 2 and 3: you must
define a <tt class="xref py py-meth docutils literal"><span class="pre">__str__()</span></tt> method returning text and to apply the
<a class="reference internal" href="../ref/utils.html#django.utils.encoding.python_2_unicode_compatible" title="django.utils.encoding.python_2_unicode_compatible"><tt class="xref py py-func docutils literal"><span class="pre">python_2_unicode_compatible()</span></tt></a> decorator.</p>
<p>On Python 3, the decorator is a no-op. On Python 2, it defines appropriate
<tt class="xref py py-meth docutils literal"><span class="pre">__unicode__()</span></tt> and <tt class="xref py py-meth docutils literal"><span class="pre">__str__()</span></tt> methods (replacing the
original <tt class="xref py py-meth docutils literal"><span class="pre">__str__()</span></tt> method in the process). Here’s an example:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">__future__</span> <span class="kn">import</span> <span class="n">unicode_literals</span>
<span class="kn">from</span> <span class="nn">django.utils.encoding</span> <span class="kn">import</span> <span class="n">python_2_unicode_compatible</span>
<span class="nd">@python_2_unicode_compatible</span>
<span class="k">class</span> <span class="nc">MyClass</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">__str__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">return</span> <span class="s">"Instance of my class"</span>
</pre></div>
</div>
<p>This technique is the best match for Django’s porting philosophy.</p>
<p>For forwards compatibility, this decorator is available as of Django 1.4.2.</p>
<p>Finally, note that <tt class="xref py py-meth docutils literal"><span class="pre">__repr__()</span></tt> must return a <tt class="docutils literal"><span class="pre">str</span></tt> on all
versions of Python.</p>
</div>
<div class="section" id="s-dict-and-dict-like-classes">
<span id="dict-and-dict-like-classes"></span><h3><tt class="xref py py-class docutils literal"><span class="pre">dict</span></tt> and <tt class="xref py py-class docutils literal"><span class="pre">dict</span></tt>-like classes<a class="headerlink" href="#dict-and-dict-like-classes" title="Permalink to this headline">¶</a></h3>
<p><tt class="xref py py-meth docutils literal"><span class="pre">dict.keys()</span></tt>, <tt class="xref py py-meth docutils literal"><span class="pre">dict.items()</span></tt> and <tt class="xref py py-meth docutils literal"><span class="pre">dict.values()</span></tt> return lists in
Python 2 and iterators in Python 3. <a class="reference internal" href="../ref/request-response.html#django.http.QueryDict" title="django.http.QueryDict"><tt class="xref py py-class docutils literal"><span class="pre">QueryDict</span></tt></a> and the
<tt class="xref py py-class docutils literal"><span class="pre">dict</span></tt>-like classes defined in <a class="reference internal" href="../ref/utils.html#module-django.utils.datastructures" title="django.utils.datastructures: Data structures that aren't in Python's standard library."><tt class="xref py py-mod docutils literal"><span class="pre">django.utils.datastructures</span></tt></a>
behave likewise in Python 3.</p>
<p><a class="reference external" href="http://pythonhosted.org/six/">six</a> provides compatibility functions to work around this change:
<tt class="xref py py-func docutils literal"><span class="pre">iterkeys()</span></tt>, <tt class="xref py py-func docutils literal"><span class="pre">iteritems()</span></tt>, and <tt class="xref py py-func docutils literal"><span class="pre">itervalues()</span></tt>.
It also contains an undocumented <tt class="docutils literal"><span class="pre">iterlists</span></tt> function that works well for
<tt class="docutils literal"><span class="pre">django.utils.datastructures.MultiValueDict</span></tt> and its subclasses.</p>
</div>
<div class="section" id="s-httprequest-and-httpresponse-objects">
<span id="httprequest-and-httpresponse-objects"></span><h3><a class="reference internal" href="../ref/request-response.html#django.http.HttpRequest" title="django.http.HttpRequest"><tt class="xref py py-class docutils literal"><span class="pre">HttpRequest</span></tt></a> and <a class="reference internal" href="../ref/request-response.html#django.http.HttpResponse" title="django.http.HttpResponse"><tt class="xref py py-class docutils literal"><span class="pre">HttpResponse</span></tt></a> objects<a class="headerlink" href="#httprequest-and-httpresponse-objects" title="Permalink to this headline">¶</a></h3>
<p>According to <span class="target" id="index-1"></span><a class="pep reference external" href="http://www.python.org/dev/peps/pep-3333"><strong>PEP 3333</strong></a>:</p>
<ul class="simple">
<li>headers are always <tt class="docutils literal"><span class="pre">str</span></tt> objects,</li>
<li>input and output streams are always <tt class="docutils literal"><span class="pre">bytes</span></tt> objects.</li>
</ul>
<p>Specifically, <a class="reference internal" href="../ref/request-response.html#django.http.HttpResponse.content" title="django.http.HttpResponse.content"><tt class="xref py py-attr docutils literal"><span class="pre">HttpResponse.content</span></tt></a>
contains <tt class="docutils literal"><span class="pre">bytes</span></tt>, which may become an issue if you compare it with a
<tt class="docutils literal"><span class="pre">str</span></tt> in your tests. The preferred solution is to rely on
<a class="reference internal" href="testing/tools.html#django.test.SimpleTestCase.assertContains" title="django.test.SimpleTestCase.assertContains"><tt class="xref py py-meth docutils literal"><span class="pre">assertContains()</span></tt></a> and
<a class="reference internal" href="testing/tools.html#django.test.SimpleTestCase.assertNotContains" title="django.test.SimpleTestCase.assertNotContains"><tt class="xref py py-meth docutils literal"><span class="pre">assertNotContains()</span></tt></a>. These methods accept a
response and a unicode string as arguments.</p>
</div>
</div>
<div class="section" id="s-coding-guidelines">
<span id="coding-guidelines"></span><h2>Coding guidelines<a class="headerlink" href="#coding-guidelines" title="Permalink to this headline">¶</a></h2>
<p>The following guidelines are enforced in Django’s source code. They’re also
recommended for third-party application who follow the same porting strategy.</p>
<div class="section" id="s-syntax-requirements">
<span id="syntax-requirements"></span><h3>Syntax requirements<a class="headerlink" href="#syntax-requirements" title="Permalink to this headline">¶</a></h3>
<div class="section" id="s-unicode">
<span id="unicode"></span><h4>Unicode<a class="headerlink" href="#unicode" title="Permalink to this headline">¶</a></h4>
<p>In Python 3, all strings are considered Unicode by default. The <tt class="docutils literal"><span class="pre">unicode</span></tt>
type from Python 2 is called <tt class="docutils literal"><span class="pre">str</span></tt> in Python 3, and <tt class="docutils literal"><span class="pre">str</span></tt> becomes
<tt class="docutils literal"><span class="pre">bytes</span></tt>.</p>
<p>You mustn’t use the <tt class="docutils literal"><span class="pre">u</span></tt> prefix before a unicode string literal because it’s
a syntax error in Python 3.2. You must prefix byte strings with <tt class="docutils literal"><span class="pre">b</span></tt>.</p>
<p>In order to enable the same behavior in Python 2, every module must import
<tt class="docutils literal"><span class="pre">unicode_literals</span></tt> from <tt class="docutils literal"><span class="pre">__future__</span></tt>:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">__future__</span> <span class="kn">import</span> <span class="n">unicode_literals</span>
<span class="n">my_string</span> <span class="o">=</span> <span class="s">"This is an unicode literal"</span>
<span class="n">my_bytestring</span> <span class="o">=</span> <span class="n">b</span><span class="s">"This is a bytestring"</span>
</pre></div>
</div>
<p>If you need a byte string literal under Python 2 and a unicode string literal
under Python 3, use the <tt class="xref py py-func docutils literal"><span class="pre">str()</span></tt> builtin:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="nb">str</span><span class="p">(</span><span class="s">'my string'</span><span class="p">)</span>
</pre></div>
</div>
<p>In Python 3, there aren’t any automatic conversions between <tt class="docutils literal"><span class="pre">str</span></tt> and
<tt class="docutils literal"><span class="pre">bytes</span></tt>, and the <tt class="xref py py-mod docutils literal"><span class="pre">codecs</span></tt> module became more strict. <tt class="xref py py-meth docutils literal"><span class="pre">str.encode()</span></tt>
always returns <tt class="docutils literal"><span class="pre">bytes</span></tt>, and <tt class="docutils literal"><span class="pre">bytes.decode</span></tt> always returns <tt class="docutils literal"><span class="pre">str</span></tt>. As a
consequence, the following pattern is sometimes necessary:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">value</span> <span class="o">=</span> <span class="n">value</span><span class="o">.</span><span class="n">encode</span><span class="p">(</span><span class="s">'ascii'</span><span class="p">,</span> <span class="s">'ignore'</span><span class="p">)</span><span class="o">.</span><span class="n">decode</span><span class="p">(</span><span class="s">'ascii'</span><span class="p">)</span>
</pre></div>
</div>
<p>Be cautious if you have to <a class="reference external" href="http://docs.python.org/3/howto/pyporting.html#bytes-literals">index bytestrings</a>.</p>
</div>
<div class="section" id="s-exceptions">
<span id="exceptions"></span><h4>Exceptions<a class="headerlink" href="#exceptions" title="Permalink to this headline">¶</a></h4>
<p>When you capture exceptions, use the <tt class="docutils literal"><span class="pre">as</span></tt> keyword:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">try</span><span class="p">:</span>
<span class="o">...</span>
<span class="k">except</span> <span class="n">MyException</span> <span class="k">as</span> <span class="n">exc</span><span class="p">:</span>
<span class="o">...</span>
</pre></div>
</div>
<p>This older syntax was removed in Python 3:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">try</span><span class="p">:</span>
<span class="o">...</span>
<span class="k">except</span> <span class="n">MyException</span><span class="p">,</span> <span class="n">exc</span><span class="p">:</span> <span class="c"># Don't do that!</span>
<span class="o">...</span>
</pre></div>
</div>
<p>The syntax to reraise an exception with a different traceback also changed.
Use <tt class="xref py py-func docutils literal"><span class="pre">six.reraise()</span></tt>.</p>
</div>
</div>
<div class="section" id="s-magic-methods">
<span id="magic-methods"></span><h3>Magic methods<a class="headerlink" href="#magic-methods" title="Permalink to this headline">¶</a></h3>
<p>Use the patterns below to handle magic methods renamed in Python 3.</p>
<div class="section" id="s-iterators">
<span id="iterators"></span><h4>Iterators<a class="headerlink" href="#iterators" title="Permalink to this headline">¶</a></h4>
<div class="highlight-python"><div class="highlight"><pre><span class="k">class</span> <span class="nc">MyIterator</span><span class="p">(</span><span class="n">six</span><span class="o">.</span><span class="n">Iterator</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">__iter__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">return</span> <span class="bp">self</span> <span class="c"># implement some logic here</span>
<span class="k">def</span> <span class="nf">__next__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">raise</span> <span class="ne">StopIteration</span> <span class="c"># implement some logic here</span>
</pre></div>
</div>
</div>
<div class="section" id="s-boolean-evaluation">
<span id="boolean-evaluation"></span><h4>Boolean evaluation<a class="headerlink" href="#boolean-evaluation" title="Permalink to this headline">¶</a></h4>
<div class="highlight-python"><div class="highlight"><pre><span class="k">class</span> <span class="nc">MyBoolean</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">__bool__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">return</span> <span class="bp">True</span> <span class="c"># implement some logic here</span>
<span class="k">def</span> <span class="nf">__nonzero__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span> <span class="c"># Python 2 compatibility</span>
<span class="k">return</span> <span class="nb">type</span><span class="p">(</span><span class="bp">self</span><span class="p">)</span><span class="o">.</span><span class="n">__bool__</span><span class="p">(</span><span class="bp">self</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="section" id="s-division">
<span id="division"></span><h4>Division<a class="headerlink" href="#division" title="Permalink to this headline">¶</a></h4>
<div class="highlight-python"><div class="highlight"><pre><span class="k">class</span> <span class="nc">MyDivisible</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">__truediv__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">other</span><span class="p">):</span>
<span class="k">return</span> <span class="bp">self</span> <span class="o">/</span> <span class="n">other</span> <span class="c"># implement some logic here</span>
<span class="k">def</span> <span class="nf">__div__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">other</span><span class="p">):</span> <span class="c"># Python 2 compatibility</span>
<span class="k">return</span> <span class="nb">type</span><span class="p">(</span><span class="bp">self</span><span class="p">)</span><span class="o">.</span><span class="n">__truediv__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">other</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">__itruediv__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">other</span><span class="p">):</span>
<span class="k">return</span> <span class="bp">self</span> <span class="o">//</span> <span class="n">other</span> <span class="c"># implement some logic here</span>
<span class="k">def</span> <span class="nf">__idiv__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">other</span><span class="p">):</span> <span class="c"># Python 2 compatibility</span>
<span class="k">return</span> <span class="nb">type</span><span class="p">(</span><span class="bp">self</span><span class="p">)</span><span class="o">.</span><span class="n">__itruediv__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">other</span><span class="p">)</span>
</pre></div>
</div>
<p>Special methods are looked up on the class and not on the instance to reflect
the behavior of the Python interpreter.</p>
</div>
</div>
<div class="section" id="s-writing-compatible-code-with-six">
<span id="writing-compatible-code-with-six"></span><h3>Writing compatible code with six<a class="headerlink" href="#writing-compatible-code-with-six" title="Permalink to this headline">¶</a></h3>
<p><a class="reference external" href="http://pythonhosted.org/six/">six</a> is the canonical compatibility library for supporting Python 2 and 3 in
a single codebase. Read its documentation!</p>
<p>A <a class="reference internal" href="#module-django.utils.six" title="django.utils.six"><tt class="xref py py-mod docutils literal"><span class="pre">customized</span> <span class="pre">version</span> <span class="pre">of</span> <span class="pre">six</span></tt></a> is bundled with Django
as of version 1.4.2. You can import it as <tt class="docutils literal"><span class="pre">django.utils.six</span></tt>.</p>
<p>Here are the most common changes required to write compatible code.</p>
<div class="section" id="s-string-handling-with-six">
<span id="s-id1"></span><span id="string-handling-with-six"></span><span id="id1"></span><h4>String handling<a class="headerlink" href="#string-handling-with-six" title="Permalink to this headline">¶</a></h4>
<p>The <tt class="docutils literal"><span class="pre">basestring</span></tt> and <tt class="docutils literal"><span class="pre">unicode</span></tt> types were removed in Python 3, and the
meaning of <tt class="docutils literal"><span class="pre">str</span></tt> changed. To test these types, use the following idioms:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="nb">isinstance</span><span class="p">(</span><span class="n">myvalue</span><span class="p">,</span> <span class="n">six</span><span class="o">.</span><span class="n">string_types</span><span class="p">)</span> <span class="c"># replacement for basestring</span>
<span class="nb">isinstance</span><span class="p">(</span><span class="n">myvalue</span><span class="p">,</span> <span class="n">six</span><span class="o">.</span><span class="n">text_type</span><span class="p">)</span> <span class="c"># replacement for unicode</span>
<span class="nb">isinstance</span><span class="p">(</span><span class="n">myvalue</span><span class="p">,</span> <span class="nb">bytes</span><span class="p">)</span> <span class="c"># replacement for str</span>
</pre></div>
</div>
<p>Python ≥ 2.6 provides <tt class="docutils literal"><span class="pre">bytes</span></tt> as an alias for <tt class="docutils literal"><span class="pre">str</span></tt>, so you don’t need
<tt class="xref py py-data docutils literal"><span class="pre">six.binary_type</span></tt>.</p>
</div>
<div class="section" id="s-long">
<span id="long"></span><h4><tt class="docutils literal"><span class="pre">long</span></tt><a class="headerlink" href="#long" title="Permalink to this headline">¶</a></h4>
<p>The <tt class="docutils literal"><span class="pre">long</span></tt> type no longer exists in Python 3. <tt class="docutils literal"><span class="pre">1L</span></tt> is a syntax error. Use
<tt class="xref py py-data docutils literal"><span class="pre">six.integer_types</span></tt> check if a value is an integer or a long:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="nb">isinstance</span><span class="p">(</span><span class="n">myvalue</span><span class="p">,</span> <span class="n">six</span><span class="o">.</span><span class="n">integer_types</span><span class="p">)</span> <span class="c"># replacement for (int, long)</span>
</pre></div>
</div>
</div>
<div class="section" id="s-xrange">
<span id="xrange"></span><h4><tt class="docutils literal"><span class="pre">xrange</span></tt><a class="headerlink" href="#xrange" title="Permalink to this headline">¶</a></h4>
<p>Import <tt class="docutils literal"><span class="pre">six.moves.xrange</span></tt> wherever you use <tt class="docutils literal"><span class="pre">xrange</span></tt>.</p>
</div>
<div class="section" id="s-moved-modules">
<span id="moved-modules"></span><h4>Moved modules<a class="headerlink" href="#moved-modules" title="Permalink to this headline">¶</a></h4>
<p>Some modules were renamed in Python 3. The <tt class="docutils literal"><span class="pre">django.utils.six.moves</span></tt>
module (based on the <tt class="xref py py-mod docutils literal"><span class="pre">six.moves</span> <span class="pre">module</span></tt>) provides a
compatible location to import them.</p>
</div>
<div class="section" id="s-py2">
<span id="py2"></span><h4>PY2<a class="headerlink" href="#py2" title="Permalink to this headline">¶</a></h4>
<p>If you need different code in Python 2 and Python 3, check <tt class="xref py py-data docutils literal"><span class="pre">six.PY2</span></tt>:</p>
<div class="highlight-python"><pre>if six.PY2:
# compatibility code for Python 2</pre>
</div>
<p>This is a last resort solution when <tt class="xref py py-mod docutils literal"><span class="pre">six</span></tt> doesn’t provide an appropriate
function.</p>
<span class="target" id="module-django.utils.six"></span></div>
</div>
<div class="section" id="s-django-customized-version-of-six">
<span id="django-customized-version-of-six"></span><h3>Django customized version of six<a class="headerlink" href="#django-customized-version-of-six" title="Permalink to this headline">¶</a></h3>
<p>The version of six bundled with Django (<tt class="docutils literal"><span class="pre">django.utils.six</span></tt>) includes a few
extras.</p>
<dl class="function">
<dt id="django.utils.six.assertRaisesRegex">
<tt class="descname">assertRaisesRegex</tt>(<em>testcase</em>, <em>*args</em>, <em>**kwargs</em>)<a class="reference internal" href="../_modules/django/utils/six.html#assertRaisesRegex"><span class="viewcode-link">[source]</span></a><a class="headerlink" href="#django.utils.six.assertRaisesRegex" title="Permalink to this definition">¶</a></dt>
<dd><p>This replaces <tt class="docutils literal"><span class="pre">testcase.assertRaisesRegexp</span></tt> on Python 2, and
<tt class="docutils literal"><span class="pre">testcase.assertRaisesRegex</span></tt> on Python 3. <tt class="docutils literal"><span class="pre">assertRaisesRegexp</span></tt> still
exists in current Python 3 versions, but issues a warning.</p>
</dd></dl>
<dl class="function">
<dt id="django.utils.six.assertRegex">
<tt class="descname">assertRegex</tt>(<em>testcase</em>, <em>*args</em>, <em>**kwargs</em>)<a class="reference internal" href="../_modules/django/utils/six.html#assertRegex"><span class="viewcode-link">[source]</span></a><a class="headerlink" href="#django.utils.six.assertRegex" title="Permalink to this definition">¶</a></dt>
<dd><p>This replaces <tt class="docutils literal"><span class="pre">testcase.assertRegexpMatches</span></tt> on Python 2, and
<tt class="docutils literal"><span class="pre">testcase.assertRegex</span></tt> on Python 3. <tt class="docutils literal"><span class="pre">assertRegexpMatches</span></tt> still
exists in current Python 3 versions, but issues a warning.</p>
</dd></dl>
<p>In addition to six’ defaults moves, Django’s version provides <tt class="docutils literal"><span class="pre">thread</span></tt> as
<tt class="docutils literal"><span class="pre">_thread</span></tt> and <tt class="docutils literal"><span class="pre">dummy_thread</span></tt> as <tt class="docutils literal"><span class="pre">_dummy_thread</span></tt>.</p>
</div>
</div>
</div>
</div>
</div>
</div>
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<h3><a href="../contents.html">Table Of Contents</a></h3>
<ul>
<li><a class="reference internal" href="#">Porting to Python 3</a><ul>
<li><a class="reference internal" href="#philosophy">Philosophy</a></li>
<li><a class="reference internal" href="#porting-tips">Porting tips</a><ul>
<li><a class="reference internal" href="#unicode-literals">Unicode literals</a></li>
<li><a class="reference internal" href="#string-handling">String handling</a></li>
<li><a class="reference internal" href="#str-and-unicode-methods"><tt class="docutils literal"><span class="pre">__str__()</span></tt> and <tt class="docutils literal"><span class="pre">__unicode__()</span></tt> methods</a></li>
<li><a class="reference internal" href="#dict-and-dict-like-classes"><tt class="docutils literal"><span class="pre">dict</span></tt> and <tt class="docutils literal"><span class="pre">dict</span></tt>-like classes</a></li>
<li><a class="reference internal" href="#httprequest-and-httpresponse-objects"><tt class="docutils literal"><span class="pre">HttpRequest</span></tt> and <tt class="docutils literal"><span class="pre">HttpResponse</span></tt> objects</a></li>
</ul>
</li>
<li><a class="reference internal" href="#coding-guidelines">Coding guidelines</a><ul>
<li><a class="reference internal" href="#syntax-requirements">Syntax requirements</a><ul>
<li><a class="reference internal" href="#unicode">Unicode</a></li>
<li><a class="reference internal" href="#exceptions">Exceptions</a></li>
</ul>
</li>
<li><a class="reference internal" href="#magic-methods">Magic methods</a><ul>
<li><a class="reference internal" href="#iterators">Iterators</a></li>
<li><a class="reference internal" href="#boolean-evaluation">Boolean evaluation</a></li>
<li><a class="reference internal" href="#division">Division</a></li>
</ul>
</li>
<li><a class="reference internal" href="#writing-compatible-code-with-six">Writing compatible code with six</a><ul>
<li><a class="reference internal" href="#string-handling-with-six">String handling</a></li>
<li><a class="reference internal" href="#long"><tt class="docutils literal"><span class="pre">long</span></tt></a></li>
<li><a class="reference internal" href="#xrange"><tt class="docutils literal"><span class="pre">xrange</span></tt></a></li>
<li><a class="reference internal" href="#moved-modules">Moved modules</a></li>
<li><a class="reference internal" href="#py2">PY2</a></li>
</ul>
</li>
<li><a class="reference internal" href="#django-customized-version-of-six">Django customized version of six</a></li>
</ul>
</li>
</ul>
</li>
</ul>
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