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<li class="toctree-l1"><a class="reference internal" href="what_else.html">What else you need to know</a><ul>
<li class="toctree-l2"><a class="reference internal" href="what_else.html#bytes">bytes</a></li>
<li class="toctree-l2"><a class="reference internal" href="what_else.html#str">str</a></li>
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<li class="toctree-l3"><a class="reference internal" href="what_else.html#memory-efficiency-and-alternatives">Memory-efficiency and alternatives</a></li>
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<li class="toctree-l2"><a class="reference internal" href="what_else.html#int">int</a></li>
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<li class="toctree-l2"><a class="reference internal" href="what_else.html#passing-data-to-from-python-2-libraries">Passing data to/from Python 2 libraries</a></li>
<li class="toctree-l2"><a class="reference internal" href="what_else.html#native-string-type">Native string type</a></li>
<li class="toctree-l2"><a class="reference internal" href="what_else.html#open">open()</a></li>
<li class="toctree-l2"><a class="reference internal" href="what_else.html#custom-str-methods">Custom __str__ methods</a></li>
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<li class="toctree-l2"><a class="reference internal" href="what_else.html#binding-a-method-to-a-class">Binding a method to a class</a></li>
<li class="toctree-l2"><a class="reference internal" href="what_else.html#metaclasses">Metaclasses</a></li>
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<li class="toctree-l1 current"><a class="current reference internal" href="">Automatic conversion to Py2/3</a><ul>
<li class="toctree-l2"><a class="reference internal" href="#futurize-py2-to-py2-3"><code class="docutils literal"><span class="pre">futurize</span></code>: Py2 to Py2/3</a><ul>
<li class="toctree-l3"><a class="reference internal" href="#stage-1-safe-fixes">Stage 1: &#8220;safe&#8221; fixes</a></li>
<li class="toctree-l3"><a class="reference internal" href="#stage-2-py3-style-code-with-wrappers-for-py2">Stage 2: Py3-style code with wrappers for Py2</a></li>
<li class="toctree-l3"><a class="reference internal" href="#separating-text-from-bytes">Separating text from bytes</a></li>
<li class="toctree-l3"><a class="reference internal" href="#post-conversion">Post-conversion</a></li>
</ul>
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<li class="toctree-l2"><a class="reference internal" href="#futurize-quick-start-guide"><code class="docutils literal"><span class="pre">futurize</span></code> quick-start guide</a><ul>
<li class="toctree-l3"><a class="reference internal" href="#step-0-setup">Step 0: setup</a></li>
<li class="toctree-l3"><a class="reference internal" href="#step-1-modern-py2-code">Step 1: modern Py2 code</a></li>
<li class="toctree-l3"><a class="reference internal" href="#step-2-working-py3-code-that-still-supports-py2">Step 2: working Py3 code that still supports Py2</a></li>
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<li class="toctree-l2"><a class="reference internal" href="#pasteurize-py3-to-py2-3"><code class="docutils literal"><span class="pre">pasteurize</span></code>: Py3 to Py2/3</a></li>
<li class="toctree-l2"><a class="reference internal" href="#known-limitations">Known limitations</a></li>
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<li class="toctree-l1"><a class="reference internal" href="faq.html">Frequently Asked Questions (FAQ)</a><ul>
<li class="toctree-l2"><a class="reference internal" href="faq.html#who-is-this-for">Who is this for?</a></li>
<li class="toctree-l2"><a class="reference internal" href="faq.html#why-upgrade-to-python-3">Why upgrade to Python 3?</a></li>
<li class="toctree-l2"><a class="reference internal" href="faq.html#porting-philosophy">Porting philosophy</a><ul>
<li class="toctree-l3"><a class="reference internal" href="faq.html#why-write-python-3-style-code">Why write Python 3-style code?</a></li>
<li class="toctree-l3"><a class="reference internal" href="faq.html#can-t-i-just-roll-my-own-py2-3-compatibility-layer">Can&#8217;t I just roll my own Py2/3 compatibility layer?</a></li>
<li class="toctree-l3"><a class="reference internal" href="faq.html#what-inspired-this-project">What inspired this project?</a></li>
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<li class="toctree-l2"><a class="reference internal" href="faq.html#maturity">Maturity</a><ul>
<li class="toctree-l3"><a class="reference internal" href="faq.html#how-well-has-it-been-tested">How well has it been tested?</a></li>
<li class="toctree-l3"><a class="reference internal" href="faq.html#is-the-api-stable">Is the API stable?</a></li>
</ul>
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<li class="toctree-l2"><a class="reference internal" href="faq.html#relationship-between-python-future-and-other-compatibility-tools">Relationship between python-future and other compatibility tools</a><ul>
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<span id="automatic-conversion"></span><h1>Automatic conversion to Py2/3<a class="headerlink" href="#automatic-conversion-to-py2-3" title="Permalink to this headline">¶</a></h1>
<p>The <code class="docutils literal"><span class="pre">future</span></code> source tree includes scripts called <code class="docutils literal"><span class="pre">futurize</span></code> and
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<code class="docutils literal"><span class="pre">lib2to3</span></code> and use fixers from <code class="docutils literal"><span class="pre">2to3</span></code>, <code class="docutils literal"><span class="pre">3to2</span></code>, and <code class="docutils literal"><span class="pre">python-modernize</span></code>.</p>
<p><code class="docutils literal"><span class="pre">futurize</span></code> passes Python 2 code through all the appropriate fixers to turn it
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<p>For conversions from Python 3 code to Py2/3, use the <code class="docutils literal"><span class="pre">pasteurize</span></code> script
instead. This converts Py3-only constructs (e.g. new metaclass syntax) and adds
<code class="docutils literal"><span class="pre">__future__</span></code> and <code class="docutils literal"><span class="pre">future</span></code> imports to the top of each module.</p>
<p>In both cases, the result should be relatively clean Py3-style code that runs
mostly unchanged on both Python 2 and Python 3.</p>
<div class="section" id="futurize-py2-to-py2-3">
<span id="forwards-conversion"></span><h2><code class="docutils literal"><span class="pre">futurize</span></code>: Py2 to Py2/3<a class="headerlink" href="#futurize-py2-to-py2-3" title="Permalink to this headline">¶</a></h2>
<p>The <code class="docutils literal"><span class="pre">futurize</span></code> script passes Python 2 code through all the appropriate fixers
to turn it into valid Python 3 code, and then adds <code class="docutils literal"><span class="pre">__future__</span></code> and
<code class="docutils literal"><span class="pre">future</span></code> package imports to re-enable compatibility with Python 2.</p>
<p>For example, running <code class="docutils literal"><span class="pre">futurize</span></code> turns this Python 2 code:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">ConfigParser</span>                 <span class="c"># Py2 module name</span>

<span class="k">class</span> <span class="nc">Upper</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
    <span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">iterable</span><span class="p">):</span>
        <span class="bp">self</span><span class="o">.</span><span class="n">_iter</span> <span class="o">=</span> <span class="nb">iter</span><span class="p">(</span><span class="n">iterable</span><span class="p">)</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="c"># Py2-style iterator interface</span>
        <span class="k">return</span> <span class="nb">next</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">_iter</span><span class="p">)</span><span class="o">.</span><span class="n">upper</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="n">itr</span> <span class="o">=</span> <span class="n">Upper</span><span class="p">(</span><span class="s">&#39;hello&#39;</span><span class="p">)</span>
<span class="k">print</span> <span class="nb">next</span><span class="p">(</span><span class="n">itr</span><span class="p">),</span>
<span class="k">for</span> <span class="n">letter</span> <span class="ow">in</span> <span class="n">itr</span><span class="p">:</span>
    <span class="k">print</span> <span class="n">letter</span><span class="p">,</span>                   <span class="c"># Py2-style print statement</span>
</pre></div>
</div>
<p>into this code which runs on both Py2 and Py3:</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">print_function</span>
<span class="kn">from</span> <span class="nn">future</span> <span class="kn">import</span> <span class="n">standard_library</span>
<span class="n">standard_library</span><span class="o">.</span><span class="n">install_aliases</span><span class="p">()</span>
<span class="kn">from</span> <span class="nn">future.builtins</span> <span class="kn">import</span> <span class="nb">next</span>
<span class="kn">from</span> <span class="nn">future.builtins</span> <span class="kn">import</span> <span class="nb">object</span>
<span class="kn">import</span> <span class="nn">configparser</span>                 <span class="c"># Py3-style import</span>

<span class="k">class</span> <span class="nc">Upper</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
    <span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">iterable</span><span class="p">):</span>
        <span class="bp">self</span><span class="o">.</span><span class="n">_iter</span> <span class="o">=</span> <span class="nb">iter</span><span class="p">(</span><span class="n">iterable</span><span class="p">)</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="c"># Py3-style iterator interface</span>
        <span class="k">return</span> <span class="nb">next</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">_iter</span><span class="p">)</span><span class="o">.</span><span class="n">upper</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="n">itr</span> <span class="o">=</span> <span class="n">Upper</span><span class="p">(</span><span class="s">&#39;hello&#39;</span><span class="p">)</span>
<span class="k">print</span><span class="p">(</span><span class="nb">next</span><span class="p">(</span><span class="n">itr</span><span class="p">),</span> <span class="n">end</span><span class="o">=</span><span class="s">&#39; &#39;</span><span class="p">)</span>           <span class="c"># Py3-style print function</span>
<span class="k">for</span> <span class="n">letter</span> <span class="ow">in</span> <span class="n">itr</span><span class="p">:</span>
    <span class="k">print</span><span class="p">(</span><span class="n">letter</span><span class="p">,</span> <span class="n">end</span><span class="o">=</span><span class="s">&#39; &#39;</span><span class="p">)</span>
</pre></div>
</div>
<p>To write out all the changes to your Python files that <code class="docutils literal"><span class="pre">futurize</span></code> suggests,
use the <code class="docutils literal"><span class="pre">-w</span></code> flag.</p>
<p>For complex projects, it is probably best to divide the porting into two stages.
Stage 1 is for &#8220;safe&#8221; changes that modernize the code but do not break Python
2.6 compatibility or introduce a depdendency on the <code class="docutils literal"><span class="pre">future</span></code> package. Stage 2
is to complete the process.</p>
<div class="section" id="stage-1-safe-fixes">
<span id="forwards-conversion-stage1"></span><h3>Stage 1: &#8220;safe&#8221; fixes<a class="headerlink" href="#stage-1-safe-fixes" title="Permalink to this headline">¶</a></h3>
<p>Run the first stage of the conversion process with:</p>
<div class="highlight-python"><div class="highlight"><pre>futurize --stage1 mypackage/*.py
</pre></div>
</div>
<p>or, if you are using zsh, recursively:</p>
<div class="highlight-python"><div class="highlight"><pre>futurize --stage1 mypackage/**/*.py
</pre></div>
</div>
<p>This applies fixes that modernize Python 2 code without changing the effect of
the code. With luck, this will not introduce any bugs into the code, or will at
least be trivial to fix. The changes are those that bring the Python code
up-to-date without breaking Py2 compatibility. The resulting code will be
modern Python 2.6-compatible code plus <code class="docutils literal"><span class="pre">__future__</span></code> imports from the
following set:</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">absolute_import</span>
<span class="kn">from</span> <span class="nn">__future__</span> <span class="kn">import</span> <span class="n">division</span>
<span class="kn">from</span> <span class="nn">__future__</span> <span class="kn">import</span> <span class="n">print_function</span>
</pre></div>
</div>
<p>Only those <code class="docutils literal"><span class="pre">__future__</span></code> imports deemed necessary will be added unless
the <code class="docutils literal"><span class="pre">--all-imports</span></code> command-line option is passed to <code class="docutils literal"><span class="pre">futurize</span></code>, in
which case they are all added.</p>
<p>The <code class="docutils literal"><span class="pre">from</span> <span class="pre">__future__</span> <span class="pre">import</span> <span class="pre">unicode_literals</span></code> declaration is not added
unless the <code class="docutils literal"><span class="pre">--unicode-literals</span></code> flag is passed to <code class="docutils literal"><span class="pre">futurize</span></code>.</p>
<p>The changes include:</p>
<div class="highlight-python"><div class="highlight"><pre>- except MyException, e:
+ except MyException as e:

- print &gt;&gt;stderr, &quot;Blah&quot;
+ from __future__ import print_function
+ print(&quot;Blah&quot;, stderr)

- class MyClass:
+ class MyClass(object):

- def next(self):
+ def __next__(self):

- if d.has_key(key):
+ if key in d:
</pre></div>
</div>
<p>Implicit relative imports fixed, e.g.:</p>
<div class="highlight-python"><div class="highlight"><pre>- import mymodule
+ from __future__ import absolute_import
+ from . import mymodule
</pre></div>
</div>
<p>Stage 1 does not add any imports from the <code class="docutils literal"><span class="pre">future</span></code> package. The output of
stage 1 will probably not (yet) run on Python 3.</p>
<p>The goal for this stage is to create most of the <code class="docutils literal"><span class="pre">diff</span></code> for the entire
porting process, but without introducing any bugs. It should be uncontroversial
and safe to apply to every Python 2 package. The subsequent patches introducing
Python 3 compatibility should then be shorter and easier to review.</p>
<p>The complete set of fixers applied by <code class="docutils literal"><span class="pre">futurize</span> <span class="pre">--stage1</span></code> is:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">lib2to3</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_apply</span>
<span class="n">lib2to3</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_except</span>
<span class="n">lib2to3</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_exitfunc</span>
<span class="n">lib2to3</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_funcattrs</span>
<span class="n">lib2to3</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_has_key</span>
<span class="n">lib2to3</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_idioms</span>
<span class="n">lib2to3</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_intern</span>
<span class="n">lib2to3</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_isinstance</span>
<span class="n">lib2to3</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_methodattrs</span>
<span class="n">lib2to3</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_ne</span>
<span class="n">lib2to3</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_numliterals</span>
<span class="n">lib2to3</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_paren</span>
<span class="n">lib2to3</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_reduce</span>
<span class="n">lib2to3</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_renames</span>
<span class="n">lib2to3</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_repr</span>
<span class="n">lib2to3</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_standarderror</span>
<span class="n">lib2to3</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_sys_exc</span>
<span class="n">lib2to3</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_throw</span>
<span class="n">lib2to3</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_tuple_params</span>
<span class="n">lib2to3</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_types</span>
<span class="n">lib2to3</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_ws_comma</span>
<span class="n">lib2to3</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_xreadlines</span>
<span class="n">libfuturize</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_absolute_import</span>
<span class="n">libfuturize</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_next_call</span>
<span class="n">libfuturize</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_print_with_import</span>
<span class="n">libfuturize</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_raise</span>
</pre></div>
</div>
<p>The following fixers from <code class="docutils literal"><span class="pre">lib2to3</span></code> are not applied:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">lib2to3</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_import</span>
</pre></div>
</div>
<p>The <code class="docutils literal"><span class="pre">fix_absolute_import</span></code> fixer in <code class="docutils literal"><span class="pre">libfuturize.fixes</span></code> is applied instead of
<code class="docutils literal"><span class="pre">lib2to3.fixes.fix_import</span></code>. The new fixer both makes implicit relative
imports explicit and adds the declaration <code class="docutils literal"><span class="pre">from</span> <span class="pre">__future__</span> <span class="pre">import</span>
<span class="pre">absolute_import</span></code> at the top of each relevant module.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">lib2to3</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_next</span>
</pre></div>
</div>
<p>The <code class="docutils literal"><span class="pre">fix_next_call</span></code> fixer in <code class="docutils literal"><span class="pre">libfuturize.fixes</span></code> is applied instead of
<code class="docutils literal"><span class="pre">fix_next</span></code> in stage 1. The new fixer changes any <code class="docutils literal"><span class="pre">obj.next()</span></code> calls to
<code class="docutils literal"><span class="pre">next(obj)</span></code>, which is Py2/3 compatible, but doesn&#8217;t change any <code class="docutils literal"><span class="pre">next</span></code> method
names to <code class="docutils literal"><span class="pre">__next__</span></code>, which would break Py2 compatibility.</p>
<p><code class="docutils literal"><span class="pre">fix_next</span></code> is applied in stage 2.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">lib2to3</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_print</span>
</pre></div>
</div>
<p>The <code class="docutils literal"><span class="pre">fix_print_with_import</span></code> fixer in <code class="docutils literal"><span class="pre">libfuturize.fixes</span></code> changes the code to
use print as a function and also adds <code class="docutils literal"><span class="pre">from</span> <span class="pre">__future__</span> <span class="pre">import</span>
<span class="pre">print_function</span></code> to the top of modules using <code class="docutils literal"><span class="pre">print()</span></code>.</p>
<p>In addition, it avoids adding an extra set of parentheses if these already
exist. So <code class="docutils literal"><span class="pre">print(x)</span></code> does not become <code class="docutils literal"><span class="pre">print((x))</span></code>.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">lib2to3</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_raise</span>
</pre></div>
</div>
<p>This fixer translates code to use the Python 3-only <code class="docutils literal"><span class="pre">with_traceback()</span></code>
method on exceptions.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">lib2to3</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_set_literal</span>
</pre></div>
</div>
<p>This converts <code class="docutils literal"><span class="pre">set([1,</span> <span class="pre">2,</span> <span class="pre">3]</span></code>) to <code class="docutils literal"><span class="pre">{1,</span> <span class="pre">2,</span> <span class="pre">3}</span></code>, breaking Python 2.6 support.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">lib2to3</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_ws_comma</span>
</pre></div>
</div>
<p>This performs cosmetic changes. This is not applied by default because it
does not serve to improve Python 2/3 compatibility. (In some cases it may
also reduce readability: see issue #58.)</p>
</div>
<div class="section" id="stage-2-py3-style-code-with-wrappers-for-py2">
<span id="forwards-conversion-stage2"></span><h3>Stage 2: Py3-style code with wrappers for Py2<a class="headerlink" href="#stage-2-py3-style-code-with-wrappers-for-py2" title="Permalink to this headline">¶</a></h3>
<p>Run stage 2 of the conversion process with:</p>
<div class="highlight-python"><div class="highlight"><pre>futurize --stage2 myfolder/*.py
</pre></div>
</div>
<p>This stage adds a dependency on the <code class="docutils literal"><span class="pre">future</span></code> package. The goal for stage 2 is
to make further mostly safe changes to the Python 2 code to use Python 3-style
code that then still runs on Python 2 with the help of the appropriate builtins
and utilities in <code class="docutils literal"><span class="pre">future</span></code>.</p>
<p>For example:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">name</span> <span class="o">=</span> <span class="nb">raw_input</span><span class="p">(</span><span class="s">&#39;What is your name?</span><span class="se">\n</span><span class="s">&#39;</span><span class="p">)</span>

<span class="k">for</span> <span class="n">k</span><span class="p">,</span> <span class="n">v</span> <span class="ow">in</span> <span class="n">d</span><span class="o">.</span><span class="n">iteritems</span><span class="p">():</span>
    <span class="k">assert</span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">v</span><span class="p">,</span> <span class="nb">basestring</span><span class="p">)</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">__unicode__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
        <span class="k">return</span> <span class="s">u&#39;My object&#39;</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="nb">unicode</span><span class="p">(</span><span class="bp">self</span><span class="p">)</span><span class="o">.</span><span class="n">encode</span><span class="p">(</span><span class="s">&#39;utf-8&#39;</span><span class="p">)</span>
</pre></div>
</div>
<p>would be converted by Stage 2 to this code:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">builtins</span> <span class="kn">import</span> <span class="nb">input</span>
<span class="kn">from</span> <span class="nn">builtins</span> <span class="kn">import</span> <span class="nb">str</span>
<span class="kn">from</span> <span class="nn">future.utils</span> <span class="kn">import</span> <span class="n">iteritems</span><span class="p">,</span> <span class="n">python_2_unicode_compatible</span>

<span class="n">name</span> <span class="o">=</span> <span class="nb">input</span><span class="p">(</span><span class="s">&#39;What is your name?</span><span class="se">\n</span><span class="s">&#39;</span><span class="p">)</span>

<span class="k">for</span> <span class="n">k</span><span class="p">,</span> <span class="n">v</span> <span class="ow">in</span> <span class="n">iteritems</span><span class="p">(</span><span class="n">d</span><span class="p">):</span>
    <span class="k">assert</span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">v</span><span class="p">,</span> <span class="p">(</span><span class="nb">str</span><span class="p">,</span> <span class="nb">bytes</span><span class="p">))</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">u&#39;My object&#39;</span>
</pre></div>
</div>
<p>Stage 2 also renames standard-library imports to their Py3 names and adds these
two lines:</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">standard_library</span>
<span class="n">standard_library</span><span class="o">.</span><span class="n">install_aliases</span><span class="p">()</span>
</pre></div>
</div>
<p>For example:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">ConfigParser</span>
</pre></div>
</div>
<p>becomes:</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">standard_library</span>
<span class="n">standard_library</span><span class="o">.</span><span class="n">install_aliases</span><span class="p">()</span>
<span class="kn">import</span> <span class="nn">configparser</span>
</pre></div>
</div>
<p>The complete list of fixers applied in Stage 2 is:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">lib2to3</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_basestring</span>
<span class="n">lib2to3</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_dict</span>
<span class="n">lib2to3</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_exec</span>
<span class="n">lib2to3</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_getcwdu</span>
<span class="n">lib2to3</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_input</span>
<span class="n">lib2to3</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_itertools</span>
<span class="n">lib2to3</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_itertools_imports</span>
<span class="n">lib2to3</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_filter</span>
<span class="n">lib2to3</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_long</span>
<span class="n">lib2to3</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_map</span>
<span class="n">lib2to3</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_nonzero</span>
<span class="n">lib2to3</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_operator</span>
<span class="n">lib2to3</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_raw_input</span>
<span class="n">lib2to3</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_zip</span>

<span class="n">libfuturize</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_cmp</span>
<span class="n">libfuturize</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_division</span>
<span class="n">libfuturize</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_execfile</span>
<span class="n">libfuturize</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_future_builtins</span>
<span class="n">libfuturize</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_future_standard_library</span>
<span class="n">libfuturize</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_future_standard_library_urllib</span>
<span class="n">libfuturize</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_metaclass</span>
<span class="n">libpasteurize</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_newstyle</span>
<span class="n">libfuturize</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_object</span>
<span class="n">libfuturize</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_unicode_keep_u</span>
<span class="n">libfuturize</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_xrange_with_import</span>
</pre></div>
</div>
<p>Not applied:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">lib2to3</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_buffer</span>    <span class="c"># Perhaps not safe. Test this.</span>
<span class="n">lib2to3</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_callable</span>  <span class="c"># Not needed in Py3.2+</span>
<span class="n">lib2to3</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_execfile</span>  <span class="c"># Some problems: see issue #37.</span>
                            <span class="c"># We use the custom libfuturize.fixes.fix_execfile instead.</span>
<span class="n">lib2to3</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_future</span>    <span class="c"># Removing __future__ imports is bad for Py2 compatibility!</span>
<span class="n">lib2to3</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_imports</span>   <span class="c"># Called by libfuturize.fixes.fix_future_standard_library</span>
<span class="n">lib2to3</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_imports2</span>  <span class="c"># We don&#39;t handle this yet (dbm)</span>
<span class="n">lib2to3</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_metaclass</span> <span class="c"># Causes SyntaxError in Py2! Use the one from ``six`` instead</span>
<span class="n">lib2to3</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_unicode</span>   <span class="c"># Strips off the u&#39;&#39; prefix, which removes a potentially</span>
                            <span class="c"># helpful source of information for disambiguating</span>
                            <span class="c"># unicode/byte strings.</span>
<span class="n">lib2to3</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_urllib</span>    <span class="c"># Included in libfuturize.fix_future_standard_library_urllib</span>
<span class="n">lib2to3</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_xrange</span>    <span class="c"># Custom one because of a bug with Py3.3&#39;s lib2to3</span>
</pre></div>
</div>
<p>Fixes applied with the <code class="docutils literal"><span class="pre">futurize</span> <span class="pre">--conservative</span></code> option:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">libfuturize</span><span class="o">.</span><span class="n">fixes</span><span class="o">.</span><span class="n">fix_division_safe</span>    <span class="c"># instead of libfuturize.fixes.fix_division.</span>
</pre></div>
</div>
</div>
<div class="section" id="separating-text-from-bytes">
<span id="forwards-conversion-text"></span><h3>Separating text from bytes<a class="headerlink" href="#separating-text-from-bytes" title="Permalink to this headline">¶</a></h3>
<p>After applying stage 2, the recommended step is to decide which of your Python
2 strings represent text and which represent binary data and to prefix all
string literals with either <code class="docutils literal"><span class="pre">b</span></code> or <code class="docutils literal"><span class="pre">u</span></code> accordingly. Furthermore, to ensure
that these types behave similarly on Python 2 as on Python 3, also wrap
byte-strings or text in the <code class="docutils literal"><span class="pre">bytes</span></code> and <code class="docutils literal"><span class="pre">str</span></code> types from <code class="docutils literal"><span class="pre">future</span></code>. For
example:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">builtins</span> <span class="kn">import</span> <span class="nb">bytes</span><span class="p">,</span> <span class="nb">str</span>
<span class="n">b</span> <span class="o">=</span> <span class="nb">bytes</span><span class="p">(</span><span class="n">b</span><span class="s">&#39;</span><span class="se">\x00</span><span class="s">ABCD&#39;</span><span class="p">)</span>
<span class="n">s</span> <span class="o">=</span> <span class="nb">str</span><span class="p">(</span><span class="s">u&#39;This is normal text&#39;</span><span class="p">)</span>
</pre></div>
</div>
<p>Any unadorned string literals will then represent native platform strings
(byte-strings on Py2, unicode strings on Py3).</p>
<p>An alternative is to pass the <code class="docutils literal"><span class="pre">--unicode-literals</span></code> flag:</p>
<div class="highlight-python"><div class="highlight"><pre>$ futurize --unicode-literals mypython2script.py
</pre></div>
</div>
<p>After running this, all string literals that were not explicitly marked up as
<code class="docutils literal"><span class="pre">b''</span></code> will mean text (Python 3 <code class="docutils literal"><span class="pre">str</span></code> or Python 2 <code class="docutils literal"><span class="pre">unicode</span></code>).</p>
</div>
<div class="section" id="post-conversion">
<span id="forwards-conversion-stage3"></span><h3>Post-conversion<a class="headerlink" href="#post-conversion" title="Permalink to this headline">¶</a></h3>
<p>After running <code class="docutils literal"><span class="pre">futurize</span></code>, we recommend first running your tests on Python 3 and making further code changes until they pass on Python 3.</p>
<p>The next step would be manually tweaking the code to re-enable Python 2
compatibility with the help of the <code class="docutils literal"><span class="pre">future</span></code> package. For example, you can add
the <code class="docutils literal"><span class="pre">&#64;python_2_unicode_compatible</span></code> decorator to any classes that define custom
<code class="docutils literal"><span class="pre">__str__</span></code> methods. See <a class="reference internal" href="what_else.html#what-else"><span>What else you need to know</span></a> for more info.</p>
</div>
</div>
<div class="section" id="futurize-quick-start-guide">
<span id="futurize-cheatsheet"></span><h2><code class="docutils literal"><span class="pre">futurize</span></code> quick-start guide<a class="headerlink" href="#futurize-quick-start-guide" title="Permalink to this headline">¶</a></h2>
<p>How to convert Py2 code to Py2/3 code using <code class="docutils literal"><span class="pre">futurize</span></code>:</p>
<div class="section" id="step-0-setup">
<span id="porting-setup"></span><h3>Step 0: setup<a class="headerlink" href="#step-0-setup" title="Permalink to this headline">¶</a></h3>
<p>Step 0 goal: set up and see the tests passing on Python 2 and failing on Python 3.</p>
<ol class="loweralpha">
<li><p class="first">Clone the package from github/bitbucket. Optionally rename your repo to <code class="docutils literal"><span class="pre">package-future</span></code>. Examples: <code class="docutils literal"><span class="pre">reportlab-future</span></code>, <code class="docutils literal"><span class="pre">paramiko-future</span></code>, <code class="docutils literal"><span class="pre">mezzanine-future</span></code>.</p>
</li>
<li><p class="first">Create and activate a Python 2 conda environment or virtualenv. Install the package with <code class="docutils literal"><span class="pre">python</span> <span class="pre">setup.py</span> <span class="pre">install</span></code> and run its test suite on Py2.7 or Py2.6 (e.g. <code class="docutils literal"><span class="pre">python</span> <span class="pre">setup.py</span> <span class="pre">test</span></code> or <code class="docutils literal"><span class="pre">py.test</span></code> or <code class="docutils literal"><span class="pre">nosetests</span></code>)</p>
</li>
<li><p class="first">Optionally: if there is a <code class="docutils literal"><span class="pre">.travis.yml</span></code> file, add Python version 3.3 and remove any versions &lt; 2.6.</p>
</li>
<li><p class="first">Install Python 3.3 with e.g. <code class="docutils literal"><span class="pre">sudo</span> <span class="pre">apt-get</span> <span class="pre">install</span> <span class="pre">python3</span></code>. On other platforms, an easy way is to use <a class="reference external" href="http://repo.continuum.io/miniconda/index.html">Miniconda</a>. Then e.g.:</p>
<div class="highlight-python"><div class="highlight"><pre>conda create -n py33 python=3.3 pip
</pre></div>
</div>
</li>
</ol>
</div>
<div class="section" id="step-1-modern-py2-code">
<span id="porting-step1"></span><h3>Step 1: modern Py2 code<a class="headerlink" href="#step-1-modern-py2-code" title="Permalink to this headline">¶</a></h3>
<p>The goal for this step is to modernize the Python 2 code without introducing any dependencies (on <code class="docutils literal"><span class="pre">future</span></code> or e.g. <code class="docutils literal"><span class="pre">six</span></code>) at this stage.</p>
<p><strong>1a</strong>. Install <code class="docutils literal"><span class="pre">future</span></code> into the virtualenv using:</p>
<div class="highlight-python"><div class="highlight"><pre>pip install future
</pre></div>
</div>
<p><strong>1b</strong>. Run <code class="docutils literal"><span class="pre">futurize</span> <span class="pre">--stage1</span> <span class="pre">-w</span> <span class="pre">*.py</span> <span class="pre">subdir1/*.py</span> <span class="pre">subdir2/*.py</span></code>. Note that with
recursive globbing in <code class="docutils literal"><span class="pre">bash</span></code> or <code class="docutils literal"><span class="pre">zsh</span></code>, you can apply stage 1 to all Python
source files recursively with:</p>
<div class="highlight-python"><div class="highlight"><pre>futurize --stage1 -w **/*.py
</pre></div>
</div>
<p><strong>1c</strong>. Commit all changes</p>
<p><strong>1d</strong>. Re-run the test suite on Py2 and fix any errors.</p>
<p>See <a class="reference internal" href="futurize.html#forwards-conversion-stage1"><span>Stage 1: &#8220;safe&#8221; fixes</span></a> for more info.</p>
<div class="section" id="example-error">
<h4>Example error<a class="headerlink" href="#example-error" title="Permalink to this headline">¶</a></h4>
<p>One relatively common error after conversion is:</p>
<div class="highlight-python"><div class="highlight"><pre>Traceback (most recent call last):
  ...
  File &quot;/home/user/Install/BleedingEdge/reportlab/tests/test_encrypt.py&quot;, line 19, in &lt;module&gt;
    from .test_pdfencryption import parsedoc
ValueError: Attempted relative import in non-package
</pre></div>
</div>
<p>If you get this error, try adding an empty <code class="docutils literal"><span class="pre">__init__.py</span></code> file in the package
directory. (In this example, in the tests/ directory.) If this doesn’t help,
and if this message appears for all tests, they must be invoked differently
(from the cmd line or e.g. <code class="docutils literal"><span class="pre">setup.py</span></code>). The way to run a module inside a
package on Python 3, or on Python 2 with <code class="docutils literal"><span class="pre">absolute_import</span></code> in effect, is:</p>
<div class="highlight-python"><div class="highlight"><pre>python -m tests.test_platypus_xref
</pre></div>
</div>
<p>(For more info, see <a class="reference external" href="http://www.python.org/dev/peps/pep-0328/">PEP 328</a> and
the <a class="reference external" href="http://www.python.org/dev/peps/pep-0008/">PEP 8</a> section on absolute
imports.)</p>
</div>
</div>
<div class="section" id="step-2-working-py3-code-that-still-supports-py2">
<span id="porting-step2"></span><h3>Step 2: working Py3 code that still supports Py2<a class="headerlink" href="#step-2-working-py3-code-that-still-supports-py2" title="Permalink to this headline">¶</a></h3>
<p>The goal for this step is to get the tests passing first on Py3 and then on Py2
again with the help of the <code class="docutils literal"><span class="pre">future</span></code> package.</p>
<p><strong>2a</strong>. Run:</p>
<div class="highlight-python"><div class="highlight"><pre>futurize --stage2 myfolder1/*.py myfolder2/*.py
</pre></div>
</div>
<p>Or, using recursive globbing with <code class="docutils literal"><span class="pre">bash</span></code> or <code class="docutils literal"><span class="pre">zsh</span></code>, you can view the stage 2
changes to all Python source files recursively with:</p>
<div class="highlight-python"><div class="highlight"><pre>futurize --stage2 **/*.py
</pre></div>
</div>
<p>To apply the changes, add the <code class="docutils literal"><span class="pre">-w</span></code> argument.</p>
<p>This stage makes further conversions needed to support both Python 2 and 3.
These will likely require imports from <code class="docutils literal"><span class="pre">future</span></code> on Py2 (and sometimes on Py3),
such as:</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">standard_library</span>
<span class="n">standard_library</span><span class="o">.</span><span class="n">install_aliases</span><span class="p">()</span>
<span class="c"># ...</span>
<span class="kn">from</span> <span class="nn">builtins</span> <span class="kn">import</span> <span class="nb">bytes</span>
<span class="kn">from</span> <span class="nn">builtins</span> <span class="kn">import</span> <span class="nb">open</span>
<span class="kn">from</span> <span class="nn">future.utils</span> <span class="kn">import</span> <span class="n">with_metaclass</span>
</pre></div>
</div>
<p>Optionally, you can use the <code class="docutils literal"><span class="pre">--unicode-literals</span></code> flag to add this import to
the top of each module:</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>
</pre></div>
</div>
<p>All strings in the module would then be unicode on Py2 (as on Py3) unless
explicitly marked with a <code class="docutils literal"><span class="pre">b''</span></code> prefix.</p>
<p>If you would like <code class="docutils literal"><span class="pre">futurize</span></code> to import all the changed builtins to have their
Python 3 semantics on Python 2, invoke it like this:</p>
<div class="highlight-python"><div class="highlight"><pre>futurize --stage2 --all-imports myfolder/*.py
</pre></div>
</div>
<p><strong>2b</strong>. Re-run your tests on Py3 now. Make changes until your tests pass on Python 3.</p>
<p><strong>2c</strong>. Commit your changes! :)</p>
<p><strong>2d</strong>. Now run your tests on Python 2 and notice the errors. Add wrappers from
<code class="docutils literal"><span class="pre">future</span></code> to re-enable Python 2 compatibility. See the
<a class="reference internal" href="compatible_idioms.html#compatible-idioms"><span>Cheat Sheet: Writing Python 2-3 compatible code</span></a> cheat sheet and <a class="reference internal" href="what_else.html#what-else"><span>What else you need to know</span></a> for more info.</p>
<p>After each change, re-run the tests on Py3 and Py2 to ensure they pass on both.</p>
<p><strong>2e</strong>. You&#8217;re done! Celebrate! Push your code and announce to the world! Hashtags
#python3 #python-future.</p>
</div>
</div>
<div class="section" id="pasteurize-py3-to-py2-3">
<span id="backwards-conversion"></span><h2><code class="docutils literal"><span class="pre">pasteurize</span></code>: Py3 to Py2/3<a class="headerlink" href="#pasteurize-py3-to-py2-3" title="Permalink to this headline">¶</a></h2>
<p>Running <code class="docutils literal"><span class="pre">pasteurize</span> <span class="pre">-w</span> <span class="pre">mypy3module.py</span></code> turns this Python 3 code:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">configparser</span>

<span class="k">class</span> <span class="nc">Blah</span><span class="p">:</span>
    <span class="k">pass</span>
<span class="k">print</span><span class="p">(</span><span class="s">&#39;Hello&#39;</span><span class="p">,</span> <span class="n">end</span><span class="o">=</span><span class="bp">None</span><span class="p">)</span>
</pre></div>
</div>
<p>into this code which runs on both Py2 and Py3:</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">print_function</span>
<span class="kn">from</span> <span class="nn">future</span> <span class="kn">import</span> <span class="n">standard_library</span>
<span class="n">standard_library</span><span class="o">.</span><span class="n">install_hooks</span><span class="p">()</span>

<span class="kn">import</span> <span class="nn">configparser</span>

<span class="k">class</span> <span class="nc">Blah</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
    <span class="k">pass</span>
<span class="k">print</span><span class="p">(</span><span class="s">&#39;Hello&#39;</span><span class="p">,</span> <span class="n">end</span><span class="o">=</span><span class="bp">None</span><span class="p">)</span>
</pre></div>
</div>
<p>Notice that both <code class="docutils literal"><span class="pre">futurize</span></code> and <code class="docutils literal"><span class="pre">pasteurize</span></code> create explicit new-style
classes that inherit from <code class="docutils literal"><span class="pre">object</span></code> on both Python versions, and both
refer to stdlib modules (as well as builtins) under their Py3 names.</p>
<p><code class="docutils literal"><span class="pre">pasteurize</span></code> also handles the following Python 3 features:</p>
<ul class="simple">
<li>keyword-only arguments</li>
<li>metaclasses (using <a class="reference internal" href="reference.html#future.utils.with_metaclass" title="future.utils.with_metaclass"><code class="xref py py-func docutils literal"><span class="pre">with_metaclass()</span></code></a>)</li>
<li>extended tuple unpacking (PEP 3132)</li>
</ul>
<p>To handle function annotations (PEP 3107), see <a class="reference internal" href="func_annotations.html#func-annotations"><span>Function annotations</span></a>.</p>
</div>
<div class="section" id="known-limitations">
<span id="futurize-limitations"></span><h2>Known limitations<a class="headerlink" href="#known-limitations" title="Permalink to this headline">¶</a></h2>
<p><code class="docutils literal"><span class="pre">futurize</span></code> and <code class="docutils literal"><span class="pre">pasteurize</span></code> are useful to automate much of the
work of porting, particularly the boring repetitive text substitutions. They also
help to flag which parts of the code require attention.</p>
<p>Nevertheless, <code class="docutils literal"><span class="pre">futurize</span></code> and <code class="docutils literal"><span class="pre">pasteurize</span></code> are still incomplete and make
some mistakes, like 2to3, on which they are based. Please report bugs on
<a class="reference external" href="https://github.com/PythonCharmers/python-future/">GitHub</a>. Contributions to
the <code class="docutils literal"><span class="pre">lib2to3</span></code>-based fixers for <code class="docutils literal"><span class="pre">futurize</span></code> and <code class="docutils literal"><span class="pre">pasteurize</span></code> are
particularly welcome! Please see <a class="reference internal" href="faq.html#contributing"><span>Contributing</span></a>.</p>
<p><code class="docutils literal"><span class="pre">futurize</span></code> doesn&#8217;t currently make the following change automatically:</p>
<ol class="arabic">
<li><p class="first">Strings containing <code class="docutils literal"><span class="pre">\U</span></code> produce a <code class="docutils literal"><span class="pre">SyntaxError</span></code> on Python 3. An example is:</p>
<div class="highlight-python"><div class="highlight"><pre>s = &#39;C:\Users&#39;.
</pre></div>
</div>
<p>Python 2 expands this to <code class="docutils literal"><span class="pre">s</span> <span class="pre">=</span> <span class="pre">'C:\\Users'</span></code>, but Python 3 requires a raw
prefix (<code class="docutils literal"><span class="pre">r'...'</span></code>). This also applies to multi-line strings (including
multi-line docstrings).</p>
</li>
</ol>
<p>Also see the tests in <code class="docutils literal"><span class="pre">future/tests/test_futurize.py</span></code> marked
<code class="docutils literal"><span class="pre">&#64;expectedFailure</span></code> or <code class="docutils literal"><span class="pre">&#64;skip</span></code> for known limitations.</p>
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