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<div class="section" id="a-bit-more-on-mutability-and-copies">
<span id="a-bit-on-mutability"></span><h1>A bit more on mutability (and copies)<a class="headerlink" href="#a-bit-more-on-mutability-and-copies" title="Permalink to this headline"></a></h1>
<div class="section" id="mutable-objects">
<h2>mutable objects<a class="headerlink" href="#mutable-objects" title="Permalink to this headline"></a></h2>
<p>We’ve talked about this: mutable objects can have their contents changed in place.</p>
<p>Immutable objects can not.</p>
<p>This has implications when you have a container with mutable objects in it:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [28]: </span><span class="n">list1</span> <span class="o">=</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="p">[</span><span class="s1">'a'</span><span class="p">,</span><span class="s1">'b'</span><span class="p">]</span> <span class="p">]</span>
</pre></div>
</div>
<p>one way to make a copy of a list:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [29]: </span><span class="n">list2</span> <span class="o">=</span> <span class="n">list1</span><span class="p">[:]</span>
<span class="gp">In [30]: </span><span class="n">list2</span> <span class="ow">is</span> <span class="n">list1</span>
<span class="gh">Out[30]: </span><span class="go">False</span>
</pre></div>
</div>
<p>they are different lists.</p>
<p>What if we set an element to a new value?</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [31]: </span><span class="n">list1</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span> <span class="o">=</span> <span class="p">[</span><span class="mi">5</span><span class="p">,</span><span class="mi">6</span><span class="p">,</span><span class="mi">7</span><span class="p">]</span>
<span class="gp">In [32]: </span><span class="n">list1</span>
<span class="gh">Out[32]: </span><span class="go">[[5, 6, 7], ['a', 'b']]</span>
<span class="gp">In [33]: </span><span class="n">list2</span>
<span class="gh">Out[33]: </span><span class="go">[[1, 2, 3], ['a', 'b']]</span>
</pre></div>
</div>
<p>So they are independent.</p>
<p>But what if we mutate an element?</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [34]: </span><span class="n">list1</span><span class="p">[</span><span class="mi">1</span><span class="p">]</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="s1">'c'</span><span class="p">)</span>
<span class="gp">In [35]: </span><span class="n">list1</span>
<span class="gh">Out[35]: </span><span class="go">[[5, 6, 7], ['a', 'b', 'c']]</span>
<span class="gp">In [36]: </span><span class="n">list2</span>
<span class="gh">Out[36]: </span><span class="go">[[1, 2, 3], ['a', 'b', 'c']]</span>
</pre></div>
</div>
<p>uuh oh! mutating an element in one list mutated the one in the other list.</p>
<p>Why is that?</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [38]: </span><span class="n">list1</span><span class="p">[</span><span class="mi">1</span><span class="p">]</span> <span class="ow">is</span> <span class="n">list2</span><span class="p">[</span><span class="mi">1</span><span class="p">]</span>
<span class="gh">Out[38]: </span><span class="go">True</span>
</pre></div>
</div>
<p>The elements are the same object!</p>
<p>This is known as a “shallow” copy – Python doesn’t want to copy more than it needs to, so in this case, it makes a new list, but does not make copies of the contents.</p>
<p>Same for dicts (and any container type – even tuples!)</p>
<p>If the elements are immutable, it doesn’t really make a differnce – but be very careful with mutable elements.</p>
</div>
<div class="section" id="the-copy-module">
<h2>The copy module<a class="headerlink" href="#the-copy-module" title="Permalink to this headline"></a></h2>
<p>most objects have a way to make copies (<code class="docutils literal notranslate"><span class="pre">dict.copy()</span></code> for instance).</p>
<p>but if not, you can use the <code class="docutils literal notranslate"><span class="pre">copy</span></code> module to make a copy:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [39]: </span><span class="kn">import</span> <span class="nn">copy</span>
<span class="gp">In [40]: </span><span class="n">list3</span> <span class="o">=</span> <span class="n">copy</span><span class="o">.</span><span class="n">copy</span><span class="p">(</span><span class="n">list2</span><span class="p">)</span>
<span class="gp">In [41]: </span><span class="n">list3</span>
<span class="gh">Out[41]: </span><span class="go">[[1, 2, 3], ['a', 'b', 'c']]</span>
</pre></div>
</div>
<p>This is also a shallow copy.</p>
<p>But there is another option:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [3]: </span><span class="n">list1</span>
<span class="gh">Out[3]: </span><span class="go">[[1, 2, 3], ['a', 'b', 'c']]</span>
<span class="gp">In [4]: </span><span class="n">list2</span> <span class="o">=</span> <span class="n">copy</span><span class="o">.</span><span class="n">deepcopy</span><span class="p">(</span><span class="n">list1</span><span class="p">)</span>
<span class="gp">In [5]: </span><span class="n">list1</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="mi">4</span><span class="p">)</span>
<span class="gp">In [6]: </span><span class="n">list1</span>
<span class="gh">Out[6]: </span><span class="go">[[1, 2, 3, 4], ['a', 'b', 'c']]</span>
<span class="gp">In [7]: </span><span class="n">list2</span>
<span class="gh">Out[7]: </span><span class="go">[[1, 2, 3], ['a', 'b', 'c']]</span>
</pre></div>
</div>
<p><code class="docutils literal notranslate"><span class="pre">deepcopy</span></code> recurses through the object, making copies of everything as it goes.</p>
<p>I happened on this thread on stack overflow:</p>
<p><a class="reference external" href="http://stackoverflow.com/questions/3975376/understanding-dict-copy-shallow-or-deep">http://stackoverflow.com/questions/3975376/understanding-dict-copy-shallow-or-deep</a></p>
<p>The OP is pretty confused – can you sort it out?</p>
<p>Make sure you understand the difference between a reference, a shallow copy, and a deep copy.</p>
</div>
<div class="section" id="mutables-as-default-arguments">
<h2>Mutables as default arguments:<a class="headerlink" href="#mutables-as-default-arguments" title="Permalink to this headline"></a></h2>
<p>Another “gotcha” is using mutables as default arguments:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [11]: </span><span class="k">def</span> <span class="nf">fun</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="n">a</span><span class="o">=</span><span class="p">[]):</span>
<span class="gp"> ....: </span> <span class="n">a</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">x</span><span class="p">)</span>
<span class="gp"> ....: </span> <span class="nb">print</span><span class="p">(</span><span class="n">a</span><span class="p">)</span>
<span class="gp"> ....:</span>
</pre></div>
</div>
<p>This makes sense: maybe you’d pass in a specific list, but if not, the default is an empty list.</p>
<p>But:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [12]: </span><span class="n">fun</span><span class="p">(</span><span class="mi">3</span><span class="p">)</span>
<span class="go">[3]</span>
<span class="gp">In [13]: </span><span class="n">fun</span><span class="p">(</span><span class="mi">4</span><span class="p">)</span>
<span class="go">[3, 4]</span>
</pre></div>
</div>
<p>Huh?!</p>
<p>Remember that that default argument is defined when the function is created: there will be only one list, and every time the function is called, that same list is used.</p>
<p><strong>The solution:</strong></p>
<p>The standard practice for such a mutable default argument:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [15]: </span><span class="k">def</span> <span class="nf">fun</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="n">a</span><span class="o">=</span><span class="kc">None</span><span class="p">):</span>
<span class="gp"> ....: </span> <span class="k">if</span> <span class="n">a</span> <span class="ow">is</span> <span class="kc">None</span><span class="p">:</span>
<span class="gp"> ....: </span> <span class="n">a</span> <span class="o">=</span> <span class="p">[]</span>
<span class="gp"> ....: </span> <span class="n">a</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">x</span><span class="p">)</span>
<span class="gp"> ....: </span> <span class="nb">print</span><span class="p">(</span><span class="n">a</span><span class="p">)</span>
<span class="gp">In [16]: </span><span class="n">fun</span><span class="p">(</span><span class="mi">3</span><span class="p">)</span>
<span class="go">[3]</span>
<span class="gp">In [17]: </span><span class="n">fun</span><span class="p">(</span><span class="mi">4</span><span class="p">)</span>
<span class="go">[4]</span>
</pre></div>
</div>
<p>You get a new list every time the function is called</p>
<p>For more reading.</p>
<p>This: <a class="reference external" href="http://python.net/crew/mwh/hacks/objectthink.html#question">http://python.net/crew/mwh/hacks/objectthink.html#question</a></p>
<p>Is a link to a discussion on comp.lang.python from over 15 years ago – but the issues are still the same. In particular, Alex Martelli’s answer is brilliant.</p>
<p>Go read it….</p>
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