<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <title>Things to be Aware Of — PyQt 4.10.3 Reference Guide</title> <link rel="stylesheet" href="_static/default.css" type="text/css" /> <link rel="stylesheet" href="_static/pygments.css" type="text/css" /> <script type="text/javascript"> var DOCUMENTATION_OPTIONS = { URL_ROOT: '', VERSION: '4.10.3', COLLAPSE_INDEX: false, FILE_SUFFIX: '.html', HAS_SOURCE: true }; </script> <script type="text/javascript" src="_static/jquery.js"></script> <script type="text/javascript" src="_static/underscore.js"></script> <script type="text/javascript" src="_static/doctools.js"></script> <link rel="shortcut icon" href="_static/logo_tn.ico"/> <link rel="top" title="PyQt 4.10.3 Reference Guide" href="index.html" /> <link rel="next" title="Using Qt Designer" href="designer.html" /> <link rel="prev" title="Old-style Signal and Slot Support" href="old_style_signals_slots.html" /> </head> <body> <div class="related"> <h3>Navigation</h3> <ul> <li class="right" style="margin-right: 10px"> <a href="genindex.html" title="General Index" accesskey="I">index</a></li> <li class="right" > <a href="py-modindex.html" title="Python Module Index" >modules</a> |</li> <li class="right" > <a href="designer.html" title="Using Qt Designer" accesskey="N">next</a> |</li> <li class="right" > <a href="old_style_signals_slots.html" title="Old-style Signal and Slot Support" accesskey="P">previous</a> |</li> <li><a href="index.html">PyQt 4.10.3 Reference Guide</a> »</li> </ul> </div> <div class="document"> <div class="documentwrapper"> <div class="bodywrapper"> <div class="body"> <div class="section" id="things-to-be-aware-of"> <h1>Things to be Aware Of<a class="headerlink" href="#things-to-be-aware-of" title="Permalink to this headline">¶</a></h1> <div class="section" id="python-strings-qt-strings-and-unicode"> <h2>Python Strings, Qt Strings and Unicode<a class="headerlink" href="#python-strings-qt-strings-and-unicode" title="Permalink to this headline">¶</a></h2> <p>PyQt4 uses the <tt class="docutils literal"><span class="pre">QString</span></tt> class to represent Unicode strings, and the <tt class="docutils literal"><span class="pre">QByteArray</span></tt> to represent byte arrays or strings. In Python v3 the corresponding native object types are <tt class="docutils literal"><span class="pre">str</span></tt> and <tt class="docutils literal"><span class="pre">bytes</span></tt>. In Python v2 the corresponding native object types are <tt class="docutils literal"><span class="pre">unicode</span></tt> and <tt class="docutils literal"><span class="pre">str</span></tt>.</p> <p>PyQt4 does its best to automatically convert between objects of the various types. Explicit conversions can be easily made where necessary.</p> <p>In some cases PyQt4 will not perform automatic conversions where it is necessary to distinguish between different overloaded methods.</p> <p>For Python v3 the following conversions are done by default.</p> <ul class="simple"> <li>If Qt expects a <tt class="docutils literal"><span class="pre">char</span> <span class="pre">*</span></tt> (or a <tt class="docutils literal"><span class="pre">const</span></tt> version) then PyQt4 will accept a <tt class="docutils literal"><span class="pre">str</span></tt> or <tt class="docutils literal"><span class="pre">QString</span></tt> that contains only ASCII characters, a <tt class="docutils literal"><span class="pre">bytes</span></tt>, a <tt class="docutils literal"><span class="pre">QByteArray</span></tt>, or a Python object that implements the buffer protocol.</li> <li>If Qt expects a <tt class="docutils literal"><span class="pre">char</span></tt> (or a <tt class="docutils literal"><span class="pre">const</span></tt> version) then PyQt4 will accept the same types as for <tt class="docutils literal"><span class="pre">char</span> <span class="pre">*</span></tt> and also require that a single character is provided.</li> <li>If Qt expects a <tt class="docutils literal"><span class="pre">signed</span> <span class="pre">char</span> <span class="pre">*</span></tt> or an <tt class="docutils literal"><span class="pre">unsigned</span> <span class="pre">char</span> <span class="pre">*</span></tt> (or a <tt class="docutils literal"><span class="pre">const</span></tt> version) then PyQt4 will accept a <tt class="docutils literal"><span class="pre">bytes</span></tt>.</li> <li>If Qt expects a <tt class="docutils literal"><span class="pre">signed</span> <span class="pre">char</span></tt> or an <tt class="docutils literal"><span class="pre">unsigned</span> <span class="pre">char</span></tt> (or a <tt class="docutils literal"><span class="pre">const</span></tt> version) then PyQt4 will accept a <tt class="docutils literal"><span class="pre">bytes</span></tt> of length 1.</li> <li>If Qt expects a <tt class="docutils literal"><span class="pre">QString</span></tt> then PyQt4 will accept a <tt class="docutils literal"><span class="pre">str</span></tt>, a <tt class="docutils literal"><span class="pre">bytes</span></tt> that contains only ASCII characters, a <tt class="docutils literal"><span class="pre">QChar</span></tt> or a <tt class="docutils literal"><span class="pre">QByteArray</span></tt>.</li> <li>If Qt expects a <tt class="docutils literal"><span class="pre">QByteArray</span></tt> then PyQt4 will also accept a <tt class="docutils literal"><span class="pre">str</span></tt> that contains only Latin-1 characters, or a <tt class="docutils literal"><span class="pre">bytes</span></tt>.</li> </ul> <p>For Python v2 the following conversions are done by default.</p> <ul class="simple"> <li>If Qt expects a <tt class="docutils literal"><span class="pre">char</span> <span class="pre">*</span></tt>, <tt class="docutils literal"><span class="pre">signed</span> <span class="pre">char</span> <span class="pre">*</span></tt> or an <tt class="docutils literal"><span class="pre">unsigned</span> <span class="pre">char</span> <span class="pre">*</span></tt> (or a <tt class="docutils literal"><span class="pre">const</span></tt> version) then PyQt4 will accept a <tt class="docutils literal"><span class="pre">unicode</span></tt> or <tt class="docutils literal"><span class="pre">QString</span></tt> that contains only ASCII characters, a <tt class="docutils literal"><span class="pre">str</span></tt>, a <tt class="docutils literal"><span class="pre">QByteArray</span></tt>, or a Python object that implements the buffer protocol.</li> <li>If Qt expects a <tt class="docutils literal"><span class="pre">char</span></tt>, <tt class="docutils literal"><span class="pre">signed</span> <span class="pre">char</span></tt> or an <tt class="docutils literal"><span class="pre">unsigned</span> <span class="pre">char</span></tt> (or a <tt class="docutils literal"><span class="pre">const</span></tt> version) then PyQt4 will accept the same types as for <tt class="docutils literal"><span class="pre">char</span> <span class="pre">*</span></tt>, <tt class="docutils literal"><span class="pre">signed</span> <span class="pre">char</span> <span class="pre">*</span></tt> and <tt class="docutils literal"><span class="pre">unsigned</span> <span class="pre">char</span> <span class="pre">*</span></tt> and also require that a single character is provided.</li> <li>If Qt expects a <tt class="docutils literal"><span class="pre">QString</span></tt> then PyQt4 will accept a <tt class="docutils literal"><span class="pre">unicode</span></tt>, a <tt class="docutils literal"><span class="pre">str</span></tt> that contains only ASCII characters, a <tt class="docutils literal"><span class="pre">QChar</span></tt> or a <tt class="docutils literal"><span class="pre">QByteArray</span></tt>.</li> <li>If Qt expects a <tt class="docutils literal"><span class="pre">QByteArray</span></tt> then PyQt4 will accept a <tt class="docutils literal"><span class="pre">unicode</span></tt> that contains only Latin-1 characters, or a <tt class="docutils literal"><span class="pre">str</span></tt>.</li> </ul> <p>Note that the different behaviour between Python v2 and v3 is due to v3’s reduced support for the buffer protocol.</p> </div> <div class="section" id="garbage-collection"> <h2>Garbage Collection<a class="headerlink" href="#garbage-collection" title="Permalink to this headline">¶</a></h2> <p>C++ does not garbage collect unreferenced class instances, whereas Python does. In the following C++ fragment both colours exist even though the first can no longer be referenced from within the program:</p> <div class="highlight-python"><pre>col = new QColor(); col = new QColor();</pre> </div> <p>In the corresponding Python fragment, the first colour is destroyed when the second is assigned to <tt class="docutils literal"><span class="pre">col</span></tt>:</p> <div class="highlight-python"><div class="highlight"><pre><span class="n">col</span> <span class="o">=</span> <span class="n">QtGui</span><span class="o">.</span><span class="n">QColor</span><span class="p">()</span> <span class="n">col</span> <span class="o">=</span> <span class="n">QtGui</span><span class="o">.</span><span class="n">QColor</span><span class="p">()</span> </pre></div> </div> <p>In Python, each colour must be assigned to different names. Typically this is done within class definitions, so the code fragment would be something like:</p> <div class="highlight-python"><div class="highlight"><pre><span class="bp">self</span><span class="o">.</span><span class="n">col1</span> <span class="o">=</span> <span class="n">QtGui</span><span class="o">.</span><span class="n">QColor</span><span class="p">()</span> <span class="bp">self</span><span class="o">.</span><span class="n">col2</span> <span class="o">=</span> <span class="n">QtGui</span><span class="o">.</span><span class="n">QColor</span><span class="p">()</span> </pre></div> </div> <p>Sometimes a Qt class instance will maintain a pointer to another instance and will eventually call the destructor of that second instance. The most common example is that a <tt class="docutils literal"><span class="pre">QObject</span></tt> (and any of its sub-classes) keeps pointers to its children and will automatically call their destructors. In these cases, the corresponding Python object will also keep a reference to the corresponding child objects.</p> <p>So, in the following Python fragment, the first <tt class="docutils literal"><span class="pre">QLabel</span></tt> is not destroyed when the second is assigned to <tt class="docutils literal"><span class="pre">lab</span></tt> because the parent <tt class="docutils literal"><span class="pre">QWidget</span></tt> still has a reference to it:</p> <div class="highlight-python"><div class="highlight"><pre><span class="n">parent</span> <span class="o">=</span> <span class="n">QtGui</span><span class="o">.</span><span class="n">QWidget</span><span class="p">()</span> <span class="n">lab</span> <span class="o">=</span> <span class="n">QtGui</span><span class="o">.</span><span class="n">QLabel</span><span class="p">(</span><span class="s">"First label"</span><span class="p">,</span> <span class="n">parent</span><span class="p">)</span> <span class="n">lab</span> <span class="o">=</span> <span class="n">QtGui</span><span class="o">.</span><span class="n">QLabel</span><span class="p">(</span><span class="s">"Second label"</span><span class="p">,</span> <span class="n">parent</span><span class="p">)</span> </pre></div> </div> </div> <div class="section" id="multiple-inheritance"> <h2>Multiple Inheritance<a class="headerlink" href="#multiple-inheritance" title="Permalink to this headline">¶</a></h2> <p>It is not possible to define a new Python class that sub-classes from more than one Qt class.</p> </div> <div class="section" id="access-to-protected-member-functions"> <h2>Access to Protected Member Functions<a class="headerlink" href="#access-to-protected-member-functions" title="Permalink to this headline">¶</a></h2> <p>When an instance of a C++ class is not created from Python it is not possible to access the protected member functions, or emit any signals, of that instance. Attempts to do so will raise a Python exception. Also, any Python methods corresponding to the instance’s virtual member functions will never be called.</p> </div> <div class="section" id="none-and-null"> <h2><tt class="docutils literal"><span class="pre">None</span></tt> and <tt class="docutils literal"><span class="pre">NULL</span></tt><a class="headerlink" href="#none-and-null" title="Permalink to this headline">¶</a></h2> <p>Throughout PyQt4, the <tt class="docutils literal"><span class="pre">None</span></tt> value can be specified wherever <tt class="docutils literal"><span class="pre">NULL</span></tt> is acceptable to the underlying C++ code.</p> <p>Equally, <tt class="docutils literal"><span class="pre">NULL</span></tt> is converted to <tt class="docutils literal"><span class="pre">None</span></tt> whenever it is returned by the underlying C++ code.</p> </div> <div class="section" id="support-for-void"> <h2>Support for <tt class="docutils literal"><span class="pre">void</span> <span class="pre">*</span></tt><a class="headerlink" href="#support-for-void" title="Permalink to this headline">¶</a></h2> <p>PyQt4 (actually SIP) represents <tt class="docutils literal"><span class="pre">void</span> <span class="pre">*</span></tt> values as objects of type <tt class="docutils literal"><span class="pre">sip.voidptr</span></tt>. Such values are often used to pass the addresses of external objects between different Python modules. To make this easier, a Python integer (or anything that Python can convert to an integer) can be used whenever a <tt class="docutils literal"><span class="pre">sip.voidptr</span></tt> is expected.</p> <p>A <tt class="docutils literal"><span class="pre">sip.voidptr</span></tt> may be converted to a Python integer by using the <tt class="docutils literal"><span class="pre">int()</span></tt> builtin function.</p> <p>A <tt class="docutils literal"><span class="pre">sip.voidptr</span></tt> may be converted to a Python string by using its <tt class="docutils literal"><span class="pre">asstring()</span></tt> method. The <tt class="docutils literal"><span class="pre">asstring()</span></tt> method takes an optional integer argument which is the length of the data in bytes.</p> <p>A <tt class="docutils literal"><span class="pre">sip.voidptr</span></tt> may also be given a size (ie. the size of the block of memory that is pointed to) by calling its <tt class="docutils literal"><span class="pre">setsize()</span></tt> method. If it has a size then it is also able to support Python’s buffer protocol and behaves like a Python <tt class="docutils literal"><span class="pre">memoryview</span></tt> object so that the block of memory can be treated as a mutable list of bytes. It also means that the Python <tt class="xref py py-mod docutils literal"><span class="pre">struct</span></tt> module can be used to unpack and pack binary data structures in memory, memory mapped files or shared memory.</p> </div> <div class="section" id="super-and-pyqt4-classes"> <h2><tt class="docutils literal"><span class="pre">super</span></tt> and PyQt4 Classes<a class="headerlink" href="#super-and-pyqt4-classes" title="Permalink to this headline">¶</a></h2> <p>In versions of PyQt4 earlier than v4.5 there were restrictions on the use of <tt class="docutils literal"><span class="pre">super</span></tt> with PyQt4 classes. These restrictions no longer apply with v4.5 and later.</p> </div> </div> </div> </div> </div> <div class="sphinxsidebar"> <div class="sphinxsidebarwrapper"> <p class="logo"><a href="index.html"> <img class="logo" src="_static/logo.png" alt="Logo"/> </a></p> <h3><a href="index.html">Table Of Contents</a></h3> <ul> <li><a class="reference internal" href="#">Things to be Aware Of</a><ul> <li><a class="reference internal" href="#python-strings-qt-strings-and-unicode">Python Strings, Qt Strings and Unicode</a></li> <li><a class="reference internal" href="#garbage-collection">Garbage Collection</a></li> <li><a class="reference internal" href="#multiple-inheritance">Multiple Inheritance</a></li> <li><a class="reference internal" href="#access-to-protected-member-functions">Access to Protected Member Functions</a></li> <li><a class="reference internal" href="#none-and-null"><tt class="docutils literal"><span class="pre">None</span></tt> and <tt class="docutils literal"><span class="pre">NULL</span></tt></a></li> <li><a class="reference internal" href="#support-for-void">Support for <tt class="docutils literal"><span class="pre">void</span> <span class="pre">*</span></tt></a></li> <li><a class="reference internal" href="#super-and-pyqt4-classes"><tt class="docutils literal"><span class="pre">super</span></tt> and PyQt4 Classes</a></li> </ul> </li> </ul> <h4>Previous topic</h4> <p class="topless"><a href="old_style_signals_slots.html" title="previous chapter">Old-style Signal and Slot Support</a></p> <h4>Next topic</h4> <p class="topless"><a href="designer.html" title="next chapter">Using Qt Designer</a></p> <div id="searchbox" style="display: none"> <h3>Quick search</h3> <form class="search" action="search.html" method="get"> <input type="text" name="q" /> <input type="submit" value="Go" /> <input type="hidden" name="check_keywords" value="yes" /> <input type="hidden" name="area" value="default" /> </form> <p class="searchtip" style="font-size: 90%"> Enter search terms or a module, class or function name. </p> </div> <script type="text/javascript">$('#searchbox').show(0);</script> </div> </div> <div class="clearer"></div> </div> <div class="related"> <h3>Navigation</h3> <ul> <li class="right" style="margin-right: 10px"> <a href="genindex.html" title="General Index" >index</a></li> <li class="right" > <a href="py-modindex.html" title="Python Module Index" >modules</a> |</li> <li class="right" > <a href="designer.html" title="Using Qt Designer" >next</a> |</li> <li class="right" > <a href="old_style_signals_slots.html" title="Old-style Signal and Slot Support" >previous</a> |</li> <li><a href="index.html">PyQt 4.10.3 Reference Guide</a> »</li> </ul> </div> <div class="footer"> © Copyright 2013 Riverbank Computing Limited. 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