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<li class="toctree-l2"><a class="reference internal" href="what_else.html#int">int</a></li>
<li class="toctree-l2"><a class="reference internal" href="what_else.html#isinstance">isinstance</a></li>
<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>
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<div class="section" id="str">
<span id="str-object"></span><h1>str<a class="headerlink" href="#str" title="Permalink to this headline">ΒΆ</a></h1>
<p>The <a class="reference external" href="http://docs.python.org/library/functions.html#str" title="(in Python v2.7)"><code class="xref py py-class docutils literal"><span class="pre">str</span></code></a> object in Python 3 is quite similar but not identical to the
Python 2 <code class="xref py py-class docutils literal"><span class="pre">unicode</span></code> object.</p>
<p>The major difference is the stricter type-checking of Py3’s <code class="docutils literal"><span class="pre">str</span></code> that
enforces a distinction between unicode strings and byte-strings, such as when
comparing, concatenating, joining, or replacing parts of strings.</p>
<p>There are also other differences, such as the <code class="docutils literal"><span class="pre">repr</span></code> of unicode strings in
Py2 having a <code class="docutils literal"><span class="pre">u'...'</span></code> prefix, versus simply <code class="docutils literal"><span class="pre">'...'</span></code>, and the removal of
the <code class="xref py py-func docutils literal"><span class="pre">str.decode()</span></code> method in Py3.</p>
<p><code class="xref py py-mod docutils literal"><span class="pre">future</span></code> contains a <code class="xref py py-class docutils literal"><span class="pre">newstr`</span></code> type that is a backport of the
<code class="xref py py-mod docutils literal"><span class="pre">str</span></code> object from Python 3. This inherits from the Python 2
<code class="xref py py-class docutils literal"><span class="pre">unicode</span></code> class but has customizations to improve compatibility with
Python 3’s <a class="reference external" href="http://docs.python.org/library/functions.html#str" title="(in Python v2.7)"><code class="xref py py-class docutils literal"><span class="pre">str</span></code></a> object. You can use it as follows:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">__future__</span> <span class="kn">import</span> <span class="n">unicode_literals</span>
<span class="gp">>>> </span><span class="kn">from</span> <span class="nn">builtins</span> <span class="kn">import</span> <span class="nb">str</span>
</pre></div>
</div>
<p>On Py2, this gives us:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="nb">str</span>
<span class="go">future.types.newstr.newstr</span>
</pre></div>
</div>
<p>(On Py3, it is simply the usual builtin <a class="reference external" href="http://docs.python.org/library/functions.html#str" title="(in Python v2.7)"><code class="xref py py-class docutils literal"><span class="pre">str</span></code></a> object.)</p>
<p>Then, for example, the following code has the same effect on Py2 as on Py3:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">s</span> <span class="o">=</span> <span class="nb">str</span><span class="p">(</span><span class="s">u'ABCD'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="k">assert</span> <span class="n">s</span> <span class="o">!=</span> <span class="n">b</span><span class="s">'ABCD'</span>
<span class="gp">>>> </span><span class="k">assert</span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">s</span><span class="o">.</span><span class="n">encode</span><span class="p">(</span><span class="s">'utf-8'</span><span class="p">),</span> <span class="nb">bytes</span><span class="p">)</span>
<span class="gp">>>> </span><span class="k">assert</span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">b</span><span class="o">.</span><span class="n">decode</span><span class="p">(</span><span class="s">'utf-8'</span><span class="p">),</span> <span class="nb">str</span><span class="p">)</span>
<span class="go">These raise TypeErrors:</span>
<span class="gp">>>> </span><span class="nb">bytes</span><span class="p">(</span><span class="n">b</span><span class="s">'B'</span><span class="p">)</span> <span class="ow">in</span> <span class="n">s</span>
<span class="gt">Traceback (most recent call last):</span>
File <span class="nb">"<stdin>"</span>, line <span class="m">1</span>, in <span class="n"><module></span>
<span class="gr">TypeError</span>: <span class="n">'in <string>' requires string as left operand, not <type 'str'></span>
<span class="gp">>>> </span><span class="n">s</span><span class="o">.</span><span class="n">find</span><span class="p">(</span><span class="nb">bytes</span><span class="p">(</span><span class="n">b</span><span class="s">'A'</span><span class="p">))</span>
<span class="gt">Traceback (most recent call last):</span>
File <span class="nb">"<stdin>"</span>, line <span class="m">1</span>, in <span class="n"><module></span>
<span class="gr">TypeError</span>: <span class="n">argument can't be <type 'str'></span>
</pre></div>
</div>
<p>Various other operations that mix strings and bytes or other types are
permitted on Py2 with the <code class="xref py py-class docutils literal"><span class="pre">newstr</span></code> class even though they
are illegal with Python 3. For example:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">s2</span> <span class="o">=</span> <span class="n">b</span><span class="s">'/'</span> <span class="o">+</span> <span class="nb">str</span><span class="p">(</span><span class="s">'ABCD'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">s2</span>
<span class="go">'/ABCD'</span>
<span class="gp">>>> </span><span class="nb">type</span><span class="p">(</span><span class="n">s2</span><span class="p">)</span>
<span class="go">future.types.newstr.newstr</span>
</pre></div>
</div>
<p>This is allowed for compatibility with parts of the Python 2 standard
library and various third-party libraries that mix byte-strings and unicode
strings loosely. One example is <code class="docutils literal"><span class="pre">os.path.join</span></code> on Python 2, which
attempts to add the byte-string <code class="docutils literal"><span class="pre">b'/'</span></code> to its arguments, whether or not
they are unicode. (See <code class="docutils literal"><span class="pre">posixpath.py</span></code>.) Another example is the
<code class="xref py py-func docutils literal"><span class="pre">escape()</span></code> function in Django 1.4’s <code class="xref py py-mod docutils literal"><span class="pre">django.utils.html</span></code>.</p>
<p>In most other ways, these <code class="xref py py-class docutils literal"><span class="pre">builtins.str</span></code> objects on Py2 have the
same behaviours as Python 3’s <a class="reference external" href="http://docs.python.org/library/functions.html#str" title="(in Python v2.7)"><code class="xref py py-class docutils literal"><span class="pre">str</span></code></a>:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">s</span> <span class="o">=</span> <span class="nb">str</span><span class="p">(</span><span class="s">'ABCD'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="k">assert</span> <span class="nb">repr</span><span class="p">(</span><span class="n">s</span><span class="p">)</span> <span class="o">==</span> <span class="s">'ABCD'</span> <span class="c"># consistent repr with Py3 (no u prefix)</span>
<span class="gp">>>> </span><span class="k">assert</span> <span class="nb">list</span><span class="p">(</span><span class="n">s</span><span class="p">)</span> <span class="o">==</span> <span class="p">[</span><span class="s">'A'</span><span class="p">,</span> <span class="s">'B'</span><span class="p">,</span> <span class="s">'C'</span><span class="p">,</span> <span class="s">'D'</span><span class="p">]</span>
<span class="gp">>>> </span><span class="k">assert</span> <span class="n">s</span><span class="o">.</span><span class="n">split</span><span class="p">(</span><span class="s">'B'</span><span class="p">)</span> <span class="o">==</span> <span class="p">[</span><span class="s">'A'</span><span class="p">,</span> <span class="s">'CD'</span><span class="p">]</span>
</pre></div>
</div>
<p>The <a class="reference external" href="http://docs.python.org/library/functions.html#str" title="(in Python v2.7)"><code class="xref py py-class docutils literal"><span class="pre">str</span></code></a> type from <code class="xref py py-mod docutils literal"><span class="pre">builtins</span></code> also provides support for the
<code class="docutils literal"><span class="pre">surrogateescape</span></code> error handler on Python 2.x. Here is an example that works
identically on Python 2.x and 3.x:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">builtins</span> <span class="kn">import</span> <span class="nb">str</span>
<span class="gp">>>> </span><span class="n">s</span> <span class="o">=</span> <span class="nb">str</span><span class="p">(</span><span class="s">u'</span><span class="se">\udcff</span><span class="s">'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">s</span><span class="o">.</span><span class="n">encode</span><span class="p">(</span><span class="s">'utf-8'</span><span class="p">,</span> <span class="s">'surrogateescape'</span><span class="p">)</span>
<span class="go">b'\xff'</span>
</pre></div>
</div>
<p>This feature is in alpha. Please leave feedback <a class="reference external" href="https://github.com/PythonCharmers/python-future/issues">here</a> about whether this
works for you.</p>
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