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<div class="section" id="support-for-qsettings">
<h1>Support for QSettings<a class="headerlink" href="#support-for-qsettings" title="Permalink to this headline">¶</a></h1>
<div class="versionadded">
<p><span class="versionmodified">New in version v4.8.3.</span></p>
</div>
<p>Qt provies the <code class="docutils literal"><span class="pre">QSettings</span></code> class as a platform independent API for the
persistent storage and retrieval of application settings. Settings are
retrieved using the <code class="docutils literal"><span class="pre">QSettings.value()</span></code> method which, when using v1 of
PyQt4’s <code class="docutils literal"><span class="pre">QVariant</span></code> API returns a <code class="docutils literal"><span class="pre">QVariant</span></code> object. An application will
then typically convert the <code class="docutils literal"><span class="pre">QVariant</span></code> to the appropriate fundamental type
using <code class="docutils literal"><span class="pre">QVariant.toBool()</span></code>, <code class="docutils literal"><span class="pre">QVariant.toInt()</span></code>.</p>
<p>The explicit use of these conversion methods works around a problem with the
way different platforms implement the storage of settings. Some platforms only
ever store string values which means that the type of the original value is
lost. Explicitly calling a conversion method provides the missing type
information about the value being retrieved.</p>
<p>A problem then arises when v2 of the <code class="docutils literal"><span class="pre">QVariant</span></code> API is used (i.e. the default
for Python v3) because those explicit conversion methods are not available.
Therefore a setting with an integer value of <code class="docutils literal"><span class="pre">42</span></code> may be retrieved as a
string value of <code class="docutils literal"><span class="pre">'42'</span></code>. This inconsistency is made worse by being platform
specific.</p>
<p>As a solution to the problem PyQt4’s implementation of <code class="docutils literal"><span class="pre">QSettings.value()</span></code>
takes an optional third argument called <code class="docutils literal"><span class="pre">type</span></code>. This is either a Python type
object, e.g. <code class="docutils literal"><span class="pre">int</span></code>, or a string that is the name of a C++ type, e.g.
<code class="docutils literal"><span class="pre">'QStringList'</span></code>. The value returned will be an object of the requested type.</p>
<p>For example:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">PyQt4.QtCore</span> <span class="k">import</span> <span class="n">QSettings</span><span class="p">,</span> <span class="n">QPoint</span>
<span class="n">settings</span> <span class="o">=</span> <span class="n">QSettings</span><span class="p">(</span><span class="s1">'foo'</span><span class="p">,</span> <span class="s1">'foo'</span><span class="p">)</span>
<span class="n">settings</span><span class="o">.</span><span class="n">setValue</span><span class="p">(</span><span class="s1">'int_value'</span><span class="p">,</span> <span class="mi">42</span><span class="p">)</span>
<span class="n">settings</span><span class="o">.</span><span class="n">setValue</span><span class="p">(</span><span class="s1">'point_value'</span><span class="p">,</span> <span class="n">QPoint</span><span class="p">(</span><span class="mi">10</span><span class="p">,</span> <span class="mi">12</span><span class="p">))</span>
<span class="c1"># This will write the setting to the platform specific storage.</span>
<span class="k">del</span> <span class="n">settings</span>
<span class="n">settings</span> <span class="o">=</span> <span class="n">QSettings</span><span class="p">(</span><span class="s1">'foo'</span><span class="p">,</span> <span class="s1">'foo'</span><span class="p">)</span>
<span class="n">int_value</span> <span class="o">=</span> <span class="n">settings</span><span class="o">.</span><span class="n">value</span><span class="p">(</span><span class="s1">'int_value'</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="nb">int</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="s2">"int_value: </span><span class="si">%s</span><span class="s2">"</span> <span class="o">%</span> <span class="nb">repr</span><span class="p">(</span><span class="n">int_value</span><span class="p">))</span>
<span class="n">point_value</span> <span class="o">=</span> <span class="n">settings</span><span class="o">.</span><span class="n">value</span><span class="p">(</span><span class="s1">'point_value'</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="n">QPoint</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="s2">"point_value: </span><span class="si">%s</span><span class="s2">"</span> <span class="o">%</span> <span class="nb">repr</span><span class="p">(</span><span class="n">point_value</span><span class="p">))</span>
</pre></div>
</div>
<p>When this is executed then the following will be displayed for all platforms:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">int_value</span><span class="p">:</span> <span class="mi">42</span>
<span class="n">point_value</span><span class="p">:</span> <span class="n">PyQt4</span><span class="o">.</span><span class="n">QtCore</span><span class="o">.</span><span class="n">QPoint</span><span class="p">(</span><span class="mi">10</span><span class="p">,</span> <span class="mi">20</span><span class="p">)</span>
</pre></div>
</div>
<p>If the value of the setting is a container (corresponding to either
<code class="docutils literal"><span class="pre">QVariantList</span></code>, <code class="docutils literal"><span class="pre">QVariantMap</span></code> or <code class="docutils literal"><span class="pre">QVariantHash</span></code>) then the type is applied
to the contents of the container.</p>
<p>For example:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">PyQt4.QtCore</span> <span class="k">import</span> <span class="n">QSettings</span>
<span class="n">settings</span> <span class="o">=</span> <span class="n">QSettings</span><span class="p">(</span><span class="s1">'foo'</span><span class="p">,</span> <span class="s1">'foo'</span><span class="p">)</span>
<span class="n">settings</span><span class="o">.</span><span class="n">setValue</span><span class="p">(</span><span class="s1">'list_value'</span><span class="p">,</span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">])</span>
<span class="n">settings</span><span class="o">.</span><span class="n">setValue</span><span class="p">(</span><span class="s1">'dict_value'</span><span class="p">,</span> <span class="p">{</span><span class="s1">'one'</span><span class="p">:</span> <span class="mi">1</span><span class="p">,</span> <span class="s1">'two'</span><span class="p">:</span> <span class="mi">2</span><span class="p">})</span>
<span class="c1"># This will write the setting to the platform specific storage.</span>
<span class="k">del</span> <span class="n">settings</span>
<span class="n">settings</span> <span class="o">=</span> <span class="n">QSettings</span><span class="p">(</span><span class="s1">'foo'</span><span class="p">,</span> <span class="s1">'foo'</span><span class="p">)</span>
<span class="n">list_value</span> <span class="o">=</span> <span class="n">settings</span><span class="o">.</span><span class="n">value</span><span class="p">(</span><span class="s1">'list_value'</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="nb">int</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="s2">"list_value: </span><span class="si">%s</span><span class="s2">"</span> <span class="o">%</span> <span class="nb">repr</span><span class="p">(</span><span class="n">list_value</span><span class="p">))</span>
<span class="n">dict_value</span> <span class="o">=</span> <span class="n">settings</span><span class="o">.</span><span class="n">value</span><span class="p">(</span><span class="s1">'dict_value'</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="nb">int</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="s2">"dict_value: </span><span class="si">%s</span><span class="s2">"</span> <span class="o">%</span> <span class="nb">repr</span><span class="p">(</span><span class="n">dict_value</span><span class="p">))</span>
</pre></div>
</div>
<p>When this is executed then the following will be displayed for all platforms:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">list_value</span><span class="p">:</span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">]</span>
<span class="n">dict_value</span><span class="p">:</span> <span class="p">{</span><span class="s1">'one'</span><span class="p">:</span> <span class="mi">1</span><span class="p">,</span> <span class="s1">'two'</span><span class="p">:</span> <span class="mi">2</span><span class="p">}</span>
</pre></div>
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