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<div class="section" id="surprising-imports">
<h1>Surprising imports<a class="headerlink" href="#surprising-imports" title="Permalink to this headline">¶</a></h1>
<p>If <tt class="docutils literal"><span class="pre">from</span> <span class="pre">a</span> <span class="pre">import</span> <span class="pre">b</span> <span class="pre">;</span> <span class="pre">print</span> <span class="pre">b.c</span></tt> works, shouldn’t
<tt class="docutils literal"><span class="pre">import</span> <span class="pre">a</span> <span class="pre">;</span> <span class="pre">print</span> <span class="pre">a.b.c</span></tt> work too? Not necessarily, here is why.</p>
<p>To use an object in Python, the objects needs to be present in the
namespace. To create a new entry in a module namespace, it is possible
to either define the functions and classes <em>or import</em> them from a
package or a module. Packages are also namespaces, <strong>the objects which
are included in the current namespace when the package is imported are
the objects which are defined and imported by the</strong> <tt class="xref py py-mod docutils literal"><span class="pre">__init__.py</span></tt>
module of the package. If <tt class="xref py py-mod docutils literal"><span class="pre">__init__.py</span></tt> is empty for a
package, importing the package does not make the modules in the
package available.</p>
<p>Rule of thumb, if you don’t want surprises always import modules
instead of packages, because with packages, what is imported depends
on the <tt class="xref py py-mod docutils literal"><span class="pre">__init__</span></tt> package and changes.</p>
<p>Here is an example</p>
<div class="highlight-sh"><div class="highlight"><pre>~<span class="nv">$ </span>tree package
package
|-- blabi.py
|-- __init__.py
<span class="sb">`</span>-- shbonz
|-- __init__.py
<span class="sb">`</span>-- shekel.py
</pre></div>
</div>
<p><tt class="xref py py-mod docutils literal"><span class="pre">package</span></tt> and <tt class="xref py py-mod docutils literal"><span class="pre">shbonz</span></tt> are packages (<tt class="xref py py-mod docutils literal"><span class="pre">shbonz</span></tt> is a
<em>subpackage</em>), which means they are filesystem directories with a file
named <em>__init__.py</em> in it. Those can be empty files.</p>
<div class="highlight-sh"><pre>~$ cat package/__init__.py package/shbonz/__init__.py
~$
~$ # the __init__.py are just empty files</pre>
</div>
<p><tt class="xref py py-mod docutils literal"><span class="pre">blabi</span></tt> et <tt class="xref py py-mod docutils literal"><span class="pre">shekel</span></tt> are modules, they contain classes,
functions and constants. Fore example, <tt class="xref py py-mod docutils literal"><span class="pre">shekel</span></tt> defines a
constant <tt class="xref py py-obj docutils literal"><span class="pre">pi</span></tt></p>
<div class="highlight-sh"><div class="highlight"><pre>~<span class="nv">$ </span>cat package/shbonz/shekel.py
<span class="nv">pi</span><span class="o">=</span>3.14
</pre></div>
</div>
<p>As the <em>__init__.py</em> files are empty, they do not import <tt class="xref py py-obj docutils literal"><span class="pre">pi</span></tt>, it
is not available when importing the packages:</p>
<div class="highlight-sh"><div class="highlight"><pre>~<span class="nv">$ </span>python -c <span class="s1">'import package; print package.shbonz.shekel.pi'</span>
Traceback <span class="o">(</span>most recent call last<span class="o">)</span>:
AttributeError: <span class="s1">'module'</span> object has no attribute <span class="s1">'shbonz'</span>
~<span class="nv">$ </span>python -c <span class="s1">'from package import shbonz; print shbonz.shekel.pi'</span>
Traceback <span class="o">(</span>most recent call last<span class="o">)</span>:
AttributeError: <span class="s1">'module'</span> object has no attribute <span class="s1">'shekel'</span>
</pre></div>
</div>
<p>The <em>module</em>, and not its parent packages must be imported, for the
constant to be available:</p>
<blockquote>
~$ python -c ‘from package.shbonz import shekel; print shekel.pi’
3.14</blockquote>
<p>Now if the package’s <tt class="xref py py-mod docutils literal"><span class="pre">__init__</span></tt> module import the constant, it
will be available:</p>
<div class="highlight-sh"><div class="highlight"><pre>~<span class="nv">$ </span><span class="nb">echo</span> <span class="s2">"import shbonz"</span> > package/__init__.py
~<span class="nv">$ </span><span class="nb">echo</span> <span class="s2">"import shekel"</span> > package/shbonz/__init__.py
~<span class="nv">$ </span>python -c <span class="s1">'import package;print package.shbonz.shekel.pi'</span>
3.14
</pre></div>
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<p><a class="reference external" href="http://effbot.org/zone/import-confusion.htm">http://effbot.org/zone/import-confusion.htm</a></p>
<p><em>5 May 2010</em></p>
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