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<li><a class="reference internal" href="#">namedtuple</a><ul>
<li><a class="reference internal" href="#defining">Defining</a></li>
<li><a class="reference internal" href="#invalid-field-names">Invalid Field Names</a></li>
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<div class="section" id="namedtuple">
<span id="collections-namedtuple"></span><h1>namedtuple<a class="headerlink" href="#namedtuple" title="Permalink to this headline">¶</a></h1>
<p>The standard <tt class="xref py py-class docutils literal"><span class="pre">tuple</span></tt> uses numerical indexes to access its
members.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">bob</span> <span class="o">=</span> <span class="p">(</span><span class="s">'Bob'</span><span class="p">,</span> <span class="mi">30</span><span class="p">,</span> <span class="s">'male'</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'Representation:'</span><span class="p">,</span> <span class="n">bob</span>
<span class="n">jane</span> <span class="o">=</span> <span class="p">(</span><span class="s">'Jane'</span><span class="p">,</span> <span class="mi">29</span><span class="p">,</span> <span class="s">'female'</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'</span><span class="se">\n</span><span class="s">Field by index:'</span><span class="p">,</span> <span class="n">jane</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span>
<span class="k">print</span> <span class="s">'</span><span class="se">\n</span><span class="s">Fields by index:'</span>
<span class="k">for</span> <span class="n">p</span> <span class="ow">in</span> <span class="p">[</span> <span class="n">bob</span><span class="p">,</span> <span class="n">jane</span> <span class="p">]:</span>
<span class="k">print</span> <span class="s">'</span><span class="si">%s</span><span class="s"> is a </span><span class="si">%d</span><span class="s"> year old </span><span class="si">%s</span><span class="s">'</span> <span class="o">%</span> <span class="n">p</span>
</pre></div>
</div>
<p>This makes <tt class="xref py py-class docutils literal"><span class="pre">tuples</span></tt> convenient containers for simple uses.</p>
<div class="highlight-python"><pre>$ python collections_tuple.py
Representation: ('Bob', 30, 'male')
Field by index: Jane
Fields by index:
Bob is a 30 year old male
Jane is a 29 year old female</pre>
</div>
<p>On the other hand, remembering which index should be used for each
value can lead to errors, especially if the <tt class="xref py py-class docutils literal"><span class="pre">tuple</span></tt> has a lot
of fields and is constructed far from where it is used. A
<tt class="xref py py-class docutils literal"><span class="pre">namedtuple</span></tt> assigns names, as well as the numerical index, to
each member.</p>
<div class="section" id="defining">
<h2>Defining<a class="headerlink" href="#defining" title="Permalink to this headline">¶</a></h2>
<p><tt class="xref py py-class docutils literal"><span class="pre">namedtuple</span></tt> instances are just as memory efficient as regular
tuples because they do not have per-instance dictionaries. Each kind
of <tt class="xref py py-class docutils literal"><span class="pre">namedtuple</span></tt> is represented by its own class, created by
using the <tt class="xref py py-func docutils literal"><span class="pre">namedtuple()</span></tt> factory function. The arguments are the
name of the new class and a string containing the names of the
elements.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">collections</span>
<span class="n">Person</span> <span class="o">=</span> <span class="n">collections</span><span class="o">.</span><span class="n">namedtuple</span><span class="p">(</span><span class="s">'Person'</span><span class="p">,</span> <span class="s">'name age gender'</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'Type of Person:'</span><span class="p">,</span> <span class="nb">type</span><span class="p">(</span><span class="n">Person</span><span class="p">)</span>
<span class="n">bob</span> <span class="o">=</span> <span class="n">Person</span><span class="p">(</span><span class="n">name</span><span class="o">=</span><span class="s">'Bob'</span><span class="p">,</span> <span class="n">age</span><span class="o">=</span><span class="mi">30</span><span class="p">,</span> <span class="n">gender</span><span class="o">=</span><span class="s">'male'</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'</span><span class="se">\n</span><span class="s">Representation:'</span><span class="p">,</span> <span class="n">bob</span>
<span class="n">jane</span> <span class="o">=</span> <span class="n">Person</span><span class="p">(</span><span class="n">name</span><span class="o">=</span><span class="s">'Jane'</span><span class="p">,</span> <span class="n">age</span><span class="o">=</span><span class="mi">29</span><span class="p">,</span> <span class="n">gender</span><span class="o">=</span><span class="s">'female'</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'</span><span class="se">\n</span><span class="s">Field by name:'</span><span class="p">,</span> <span class="n">jane</span><span class="o">.</span><span class="n">name</span>
<span class="k">print</span> <span class="s">'</span><span class="se">\n</span><span class="s">Fields by index:'</span>
<span class="k">for</span> <span class="n">p</span> <span class="ow">in</span> <span class="p">[</span> <span class="n">bob</span><span class="p">,</span> <span class="n">jane</span> <span class="p">]:</span>
<span class="k">print</span> <span class="s">'</span><span class="si">%s</span><span class="s"> is a </span><span class="si">%d</span><span class="s"> year old </span><span class="si">%s</span><span class="s">'</span> <span class="o">%</span> <span class="n">p</span>
</pre></div>
</div>
<p>As the example illustrates, it is possible to access the fields of the
<tt class="xref py py-class docutils literal"><span class="pre">namedtuple</span></tt> by name using dotted notation (<tt class="docutils literal"><span class="pre">obj.attr</span></tt>) as
well as using the positional indexes of standard tuples.</p>
<div class="highlight-python"><pre>$ python collections_namedtuple_person.py
Type of Person: <type 'type'>
Representation: Person(name='Bob', age=30, gender='male')
Field by name: Jane
Fields by index:
Bob is a 30 year old male
Jane is a 29 year old female</pre>
</div>
</div>
<div class="section" id="invalid-field-names">
<h2>Invalid Field Names<a class="headerlink" href="#invalid-field-names" title="Permalink to this headline">¶</a></h2>
<p>As the field names are parsed, invalid values cause <a class="reference internal" href="../exceptions/index.html#exceptions-valueerror"><em>ValueError</em></a> exceptions.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">collections</span>
<span class="k">try</span><span class="p">:</span>
<span class="n">collections</span><span class="o">.</span><span class="n">namedtuple</span><span class="p">(</span><span class="s">'Person'</span><span class="p">,</span> <span class="s">'name class age gender'</span><span class="p">)</span>
<span class="k">except</span> <span class="ne">ValueError</span><span class="p">,</span> <span class="n">err</span><span class="p">:</span>
<span class="k">print</span> <span class="n">err</span>
<span class="k">try</span><span class="p">:</span>
<span class="n">collections</span><span class="o">.</span><span class="n">namedtuple</span><span class="p">(</span><span class="s">'Person'</span><span class="p">,</span> <span class="s">'name age gender age'</span><span class="p">)</span>
<span class="k">except</span> <span class="ne">ValueError</span><span class="p">,</span> <span class="n">err</span><span class="p">:</span>
<span class="k">print</span> <span class="n">err</span>
</pre></div>
</div>
<p>Names are invalid if they are repeated or conflict with Python
keywords.</p>
<div class="highlight-python"><pre>$ python collections_namedtuple_bad_fields.py
Type names and field names cannot be a keyword: 'class'
Encountered duplicate field name: 'age'</pre>
</div>
<p>In situations where a <tt class="xref py py-class docutils literal"><span class="pre">namedtuple</span></tt> is being created based on
values outside of the control of the programm (such as to represent
the rows returned by a database query, where the schema is not known
in advance), set the <em>rename</em> option to <tt class="xref docutils literal"><span class="pre">True</span></tt> so the fields are
renamed.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">collections</span>
<span class="n">with_class</span> <span class="o">=</span> <span class="n">collections</span><span class="o">.</span><span class="n">namedtuple</span><span class="p">(</span><span class="s">'Person'</span><span class="p">,</span> <span class="s">'name class age gender'</span><span class="p">,</span> <span class="n">rename</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span>
<span class="k">print</span> <span class="n">with_class</span><span class="o">.</span><span class="n">_fields</span>
<span class="n">two_ages</span> <span class="o">=</span> <span class="n">collections</span><span class="o">.</span><span class="n">namedtuple</span><span class="p">(</span><span class="s">'Person'</span><span class="p">,</span> <span class="s">'name age gender age'</span><span class="p">,</span> <span class="n">rename</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span>
<span class="k">print</span> <span class="n">two_ages</span><span class="o">.</span><span class="n">_fields</span>
</pre></div>
</div>
<p>The field with name <tt class="docutils literal"><span class="pre">class</span></tt> becomes <tt class="docutils literal"><span class="pre">_1</span></tt> and the duplicate <tt class="docutils literal"><span class="pre">age</span></tt>
field is changed to <tt class="docutils literal"><span class="pre">_3</span></tt>.</p>
<div class="highlight-python"><pre>$ python collections_namedtuple_rename.py
('name', '_1', 'age', 'gender')
('name', 'age', 'gender', '_3')</pre>
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