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<h1 class="heading"><a href="index.html"
title="back to the documentation overview"><span>Werkzeug</span></a></h1>
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<div class="section" id="module-werkzeug">
<span id="context-locals"></span><h1>Context Locals<a class="headerlink" href="#module-werkzeug" title="Permalink to this headline">¶</a></h1>
<p>Sooner or later you have some things you want to have in every single view
or helper function or whatever. In PHP the way to go are global
variables. However that is not possible in WSGI applications without a
major drawback: As soon as you operate on the global namespace your
application is not thread safe any longer.</p>
<p>The python standard library comes with a utility called “thread locals”.
A thread local is a global object where you can put stuff in and get back
later in a thread safe way. That means whenever you set or get an object
to / from a thread local object the thread local object checks in which
thread you are and delivers the correct value.</p>
<p>This however has a few disadvantages. For example besides threads there
are other ways to handle concurrency in Python. A very popular approach
are greenlets. Also, whether every request gets its own thread is not
guaranteed in WSGI. It could be that a request is reusing a thread from
before and data is left in the thread local object.</p>
<p>Here a simple example how you can use werkzeug.local:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">werkzeug</span> <span class="kn">import</span> <span class="n">Local</span><span class="p">,</span> <span class="n">LocalManager</span>
<span class="n">local</span> <span class="o">=</span> <span class="n">Local</span><span class="p">()</span>
<span class="n">local_manager</span> <span class="o">=</span> <span class="n">LocalManager</span><span class="p">([</span><span class="n">local</span><span class="p">])</span>
<span class="k">def</span> <span class="nf">application</span><span class="p">(</span><span class="n">environ</span><span class="p">,</span> <span class="n">start_response</span><span class="p">):</span>
<span class="n">local</span><span class="o">.</span><span class="n">request</span> <span class="o">=</span> <span class="n">request</span> <span class="o">=</span> <span class="n">Request</span><span class="p">(</span><span class="n">environ</span><span class="p">)</span>
<span class="o">...</span>
<span class="n">application</span> <span class="o">=</span> <span class="n">local_manager</span><span class="o">.</span><span class="n">make_middleware</span><span class="p">(</span><span class="n">application</span><span class="p">)</span>
</pre></div>
</div>
<p>Now what this code does is binding request to <cite>local.request</cite>. Every
other piece of code executed after this assignment in the same context can
safely access local.request and will get the same request object. The
<cite>make_middleware</cite> method on the local manager ensures that everything is
cleaned up after the request.</p>
<p>The same context means the same greenlet (if you’re using greenlets) in
the same thread and same process.</p>
<p>If a request object is not yet set on the local object and you try to
access it you will get an <cite>AttributeError</cite>. You can use <cite>getattr</cite> to avoid
that:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">def</span> <span class="nf">get_request</span><span class="p">():</span>
<span class="k">return</span> <span class="nb">getattr</span><span class="p">(</span><span class="n">local</span><span class="p">,</span> <span class="s">'request'</span><span class="p">,</span> <span class="bp">None</span><span class="p">)</span>
</pre></div>
</div>
<p>This will try to get the request or return <cite>None</cite> if the request is not
(yet?) available.</p>
<p>Note that local objects cannot manage themselves, for that you need a local
manager. You can pass a local manager multiple locals or add additionals
later by appending them to <cite>manager.locals</cite> and everytime the manager
cleans up it will clean up all the data left in the locals for this
context.</p>
<dl class="function">
<dt id="werkzeug.release_local">
<tt class="descclassname">werkzeug.</tt><tt class="descname">release_local</tt><big>(</big><em>local</em><big>)</big><a class="headerlink" href="#werkzeug.release_local" title="Permalink to this definition">¶</a></dt>
<dd><p>Releases the contents of the local for the current context.
This makes it possible to use locals without a manager.</p>
<p>Example:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">loc</span> <span class="o">=</span> <span class="n">Local</span><span class="p">()</span>
<span class="gp">>>> </span><span class="n">loc</span><span class="o">.</span><span class="n">foo</span> <span class="o">=</span> <span class="mi">42</span>
<span class="gp">>>> </span><span class="n">release_local</span><span class="p">(</span><span class="n">loc</span><span class="p">)</span>
<span class="gp">>>> </span><span class="nb">hasattr</span><span class="p">(</span><span class="n">loc</span><span class="p">,</span> <span class="s">'foo'</span><span class="p">)</span>
<span class="go">False</span>
</pre></div>
</div>
<p>With this function one can release <tt class="xref py py-class docutils literal"><span class="pre">Local</span></tt> objects as well
as <tt class="xref py py-class docutils literal"><span class="pre">StackLocal</span></tt> objects. However it is not possible to
release data held by proxies that way, one always has to retain
a reference to the underlying local object in order to be able
to release it.</p>
<p class="versionadded">
<span class="versionmodified">New in version 0.6.1.</span></p>
</dd></dl>
<dl class="class">
<dt id="werkzeug.LocalManager">
<em class="property">class </em><tt class="descclassname">werkzeug.</tt><tt class="descname">LocalManager</tt><big>(</big><em>locals=None</em><big>)</big><a class="headerlink" href="#werkzeug.LocalManager" title="Permalink to this definition">¶</a></dt>
<dd><p>Local objects cannot manage themselves. For that you need a local
manager. You can pass a local manager multiple locals or add them later
by appending them to <cite>manager.locals</cite>. Everytime the manager cleans up
it, will clean up all the data left in the locals for this context.</p>
<p class="versionchanged">
<span class="versionmodified">Changed in version 0.6.1: </span>Instead of a manager the <a title="werkzeug.release_local" class="reference internal" href="#werkzeug.release_local"><tt class="xref py py-func docutils literal"><span class="pre">release_local()</span></tt></a> function can be used
as well.</p>
<dl class="method">
<dt id="werkzeug.LocalManager.cleanup">
<tt class="descname">cleanup</tt><big>(</big><big>)</big><a class="headerlink" href="#werkzeug.LocalManager.cleanup" title="Permalink to this definition">¶</a></dt>
<dd>Manually clean up the data in the locals for this context. Call
this at the end of the request or use <cite>make_middleware()</cite>.</dd></dl>
<dl class="method">
<dt id="werkzeug.LocalManager.make_middleware">
<tt class="descname">make_middleware</tt><big>(</big><em>app</em><big>)</big><a class="headerlink" href="#werkzeug.LocalManager.make_middleware" title="Permalink to this definition">¶</a></dt>
<dd>Wrap a WSGI application so that cleaning up happens after
request end.</dd></dl>
<dl class="method">
<dt id="werkzeug.LocalManager.middleware">
<tt class="descname">middleware</tt><big>(</big><em>func</em><big>)</big><a class="headerlink" href="#werkzeug.LocalManager.middleware" title="Permalink to this definition">¶</a></dt>
<dd><p>Like <cite>make_middleware</cite> but for decorating functions.</p>
<p>Example usage:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="nd">@manager.middleware</span>
<span class="k">def</span> <span class="nf">application</span><span class="p">(</span><span class="n">environ</span><span class="p">,</span> <span class="n">start_response</span><span class="p">):</span>
<span class="o">...</span>
</pre></div>
</div>
<p>The difference to <cite>make_middleware</cite> is that the function passed
will have all the arguments copied from the inner application
(name, docstring, module).</p>
</dd></dl>
<dl class="method">
<dt id="werkzeug.LocalManager.get_ident">
<tt class="descname">get_ident</tt><big>(</big><big>)</big><a class="headerlink" href="#werkzeug.LocalManager.get_ident" title="Permalink to this definition">¶</a></dt>
<dd>Return the context identifier the local objects use internally for
this context. You cannot override this method to change the behavior
but use it to link other context local objects (such as SQLAlchemy’s
scoped sessions) to the Werkzeug locals.</dd></dl>
</dd></dl>
<dl class="class">
<dt id="werkzeug.LocalStack">
<em class="property">class </em><tt class="descclassname">werkzeug.</tt><tt class="descname">LocalStack</tt><a class="headerlink" href="#werkzeug.LocalStack" title="Permalink to this definition">¶</a></dt>
<dd><p>This class works similar to a <tt class="xref py py-class docutils literal"><span class="pre">Local</span></tt> but keeps a stack
of objects instead. This is best explained with an example:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">ls</span> <span class="o">=</span> <span class="n">LocalStack</span><span class="p">()</span>
<span class="gp">>>> </span><span class="n">ls</span><span class="o">.</span><span class="n">push</span><span class="p">(</span><span class="mi">42</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">ls</span><span class="o">.</span><span class="n">top</span>
<span class="go">42</span>
<span class="gp">>>> </span><span class="n">ls</span><span class="o">.</span><span class="n">push</span><span class="p">(</span><span class="mi">23</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">ls</span><span class="o">.</span><span class="n">top</span>
<span class="go">23</span>
<span class="gp">>>> </span><span class="n">ls</span><span class="o">.</span><span class="n">pop</span><span class="p">()</span>
<span class="go">23</span>
<span class="gp">>>> </span><span class="n">ls</span><span class="o">.</span><span class="n">top</span>
<span class="go">42</span>
</pre></div>
</div>
<p>They can be force released by using a <a title="werkzeug.LocalManager" class="reference internal" href="#werkzeug.LocalManager"><tt class="xref py py-class docutils literal"><span class="pre">LocalManager</span></tt></a> or with
the <a title="werkzeug.release_local" class="reference internal" href="#werkzeug.release_local"><tt class="xref py py-func docutils literal"><span class="pre">release_local()</span></tt></a> function but the correct way is to pop the
item from the stack after using. When the stack is empty it will
no longer be bound to the current context (and as such released).</p>
<p>By calling the stack without arguments it returns a proxy that resolves to
the topmost item on the stack.</p>
<p class="versionadded">
<span class="versionmodified">New in version 0.6.1.</span></p>
<dl class="method">
<dt id="werkzeug.LocalStack.push">
<tt class="descname">push</tt><big>(</big><em>obj</em><big>)</big><a class="headerlink" href="#werkzeug.LocalStack.push" title="Permalink to this definition">¶</a></dt>
<dd>Pushes a new item to the stack</dd></dl>
<dl class="method">
<dt id="werkzeug.LocalStack.pop">
<tt class="descname">pop</tt><big>(</big><big>)</big><a class="headerlink" href="#werkzeug.LocalStack.pop" title="Permalink to this definition">¶</a></dt>
<dd>Removes the topmost item from the stack, will return the
old value or <cite>None</cite> if the stack was already empty.</dd></dl>
<dl class="attribute">
<dt id="werkzeug.LocalStack.top">
<tt class="descname">top</tt><a class="headerlink" href="#werkzeug.LocalStack.top" title="Permalink to this definition">¶</a></dt>
<dd>The topmost item on the stack. If the stack is empty,
<cite>None</cite> is returned.</dd></dl>
</dd></dl>
<dl class="class">
<dt id="werkzeug.LocalProxy">
<em class="property">class </em><tt class="descclassname">werkzeug.</tt><tt class="descname">LocalProxy</tt><big>(</big><em>local</em>, <em>name=None</em><big>)</big><a class="headerlink" href="#werkzeug.LocalProxy" title="Permalink to this definition">¶</a></dt>
<dd><p>Acts as a proxy for a werkzeug local. Forwards all operations to
a proxied object. The only operations not supported for forwarding
are right handed operands and any kind of assignment.</p>
<p>Example usage:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">werkzeug</span> <span class="kn">import</span> <span class="n">Local</span>
<span class="n">l</span> <span class="o">=</span> <span class="n">Local</span><span class="p">()</span>
<span class="c"># these are proxies</span>
<span class="n">request</span> <span class="o">=</span> <span class="n">l</span><span class="p">(</span><span class="s">'request'</span><span class="p">)</span>
<span class="n">user</span> <span class="o">=</span> <span class="n">l</span><span class="p">(</span><span class="s">'user'</span><span class="p">)</span>
<span class="kn">from</span> <span class="nn">werkzeug</span> <span class="kn">import</span> <span class="n">LocalStack</span>
<span class="n">_response_local</span> <span class="o">=</span> <span class="n">LocalStack</span><span class="p">()</span>
<span class="c"># this is a proxy</span>
<span class="n">response</span> <span class="o">=</span> <span class="n">_response_local</span><span class="p">()</span>
</pre></div>
</div>
<p>Whenever something is bound to l.user / l.request the proxy objects
will forward all operations. If no object is bound a <tt class="xref py py-exc docutils literal"><span class="pre">RuntimeError</span></tt>
will be raised.</p>
<p>To create proxies to <tt class="xref py py-class docutils literal"><span class="pre">Local</span></tt> or <a title="werkzeug.LocalStack" class="reference internal" href="#werkzeug.LocalStack"><tt class="xref py py-class docutils literal"><span class="pre">LocalStack</span></tt></a> objects,
call the object as shown above. If you want to have a proxy to an
object looked up by a function, you can (as of Werkzeug 0.6.1) pass
a function to the <a title="werkzeug.LocalProxy" class="reference internal" href="#werkzeug.LocalProxy"><tt class="xref py py-class docutils literal"><span class="pre">LocalProxy</span></tt></a> constructor:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">session</span> <span class="o">=</span> <span class="n">LocalProxy</span><span class="p">(</span><span class="k">lambda</span><span class="p">:</span> <span class="n">get_current_request</span><span class="p">()</span><span class="o">.</span><span class="n">session</span><span class="p">)</span>
</pre></div>
</div>
<p class="versionchanged">
<span class="versionmodified">Changed in version 0.6.1: </span>The class can be instanciated with a callable as well now.</p>
<p>Keep in mind that <tt class="docutils literal"><span class="pre">repr()</span></tt> is also forwarded, so if you want to find
out if you are dealing with a proxy you can do an <tt class="docutils literal"><span class="pre">isinstance()</span></tt> check:</p>
<div class="highlight-pycon"><div class="highlight"><pre><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">werkzeug</span> <span class="kn">import</span> <span class="n">LocalProxy</span>
<span class="gp">>>> </span><span class="nb">isinstance</span><span class="p">(</span><span class="n">request</span><span class="p">,</span> <span class="n">LocalProxy</span><span class="p">)</span>
<span class="go">True</span>
</pre></div>
</div>
<p>You can also create proxy objects by hand:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">werkzeug</span> <span class="kn">import</span> <span class="n">Local</span><span class="p">,</span> <span class="n">LocalProxy</span>
<span class="n">local</span> <span class="o">=</span> <span class="n">Local</span><span class="p">()</span>
<span class="n">request</span> <span class="o">=</span> <span class="n">LocalProxy</span><span class="p">(</span><span class="n">local</span><span class="p">,</span> <span class="s">'request'</span><span class="p">)</span>
</pre></div>
</div>
<dl class="method">
<dt id="werkzeug.LocalProxy._get_current_object">
<tt class="descname">_get_current_object</tt><big>(</big><big>)</big><a class="headerlink" href="#werkzeug.LocalProxy._get_current_object" title="Permalink to this definition">¶</a></dt>
<dd>Return the current object. This is useful if you want the real
object behind the proxy at a time for performance reasons or because
you want to pass the object into a different context.</dd></dl>
</dd></dl>
</div>
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