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<h1>No Pointers in Python?</h1>
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<section class="tex2jax_ignore mathjax_ignore" id="no-pointers-in-python">
<h1>No Pointers in Python?<a class="headerlink" href="#no-pointers-in-python" title="Permalink to this heading">#</a></h1>
<hr><p>To run your code in Colab, click on the rocket icon at the top of this page. <img alt="Colab" src="https://colab.research.google.com/assets/colab-badge.svg" />
<br></p>
<p>I’ve come across numerous computer science and engineering students who mistakenly believe that languages such as Java, Python, C#, and Objective-C don’t incorporate the concept of pointers. Many of them share a common sentiment, saying, “I prefer Python/Java/C# because I don’t have to deal with those <em>tricky</em> pointers.”</p>
<p>Personally, I’d hesitate to collaborate with individuals who hold this misconception, as their approach could potentially introduce a slew of bugs into the project.</p>
<p>Now, let’s delve into an example involving a list in Python.</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">x</span> <span class="o">=</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="mi">4</span><span class="p">,</span> <span class="mi">5</span><span class="p">]</span>
<span class="nb">print</span><span class="p">(</span><span class="n">x</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="cell_output docutils container">
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>[1, 2, 3, 4, 5]
</pre></div>
</div>
</div>
</div>
<p>Let’s assign it to a new variable <code class="docutils literal notranslate"><span class="pre">y</span></code> and change one of the elements in the list.</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">y</span> <span class="o">=</span> <span class="n">x</span>
<span class="n">y</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span> <span class="o">=</span> <span class="mi">0</span>
<span class="nb">print</span><span class="p">(</span><span class="n">y</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="cell_output docutils container">
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>[0, 2, 3, 4, 5]
</pre></div>
</div>
</div>
</div>
<p>Let’s see what happened to <code class="docutils literal notranslate"><span class="pre">x</span></code>.</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="nb">print</span><span class="p">(</span><span class="n">x</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="cell_output docutils container">
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>[0, 2, 3, 4, 5]
</pre></div>
</div>
</div>
</div>
<p>We modified only <code class="docutils literal notranslate"><span class="pre">y</span></code>, not <code class="docutils literal notranslate"><span class="pre">x</span></code>. Why is <code class="docutils literal notranslate"><span class="pre">x</span></code> also changed? Because <code class="docutils literal notranslate"><span class="pre">x</span></code> and <code class="docutils literal notranslate"><span class="pre">y</span></code> are internally pointers.</p>
<p>When we created the list, <code class="docutils literal notranslate"><span class="pre">x</span></code>, a new list object was created and the address of the object was assigned to <code class="docutils literal notranslate"><span class="pre">x</span></code> as shown below.</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span> <span class="n">x</span>
<span class="o">+---+</span>
<span class="o">|</span> <span class="o">-|-------></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="mi">4</span><span class="p">,</span> <span class="mi">5</span><span class="p">]</span>
<span class="o">+---+</span>
</pre></div>
</div>
<p>The variable <code class="docutils literal notranslate"><span class="pre">y</span></code> is not a new copy of the list. Instead, it is a copy of the value of <code class="docutils literal notranslate"><span class="pre">x</span></code>, the <strong>address</strong> of the list.</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span> <span class="n">x</span>
<span class="o">+---+</span>
<span class="o">|</span> <span class="o">-|-------></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="mi">4</span><span class="p">,</span> <span class="mi">5</span><span class="p">]</span>
<span class="o">+---+</span> <span class="o">^</span>
<span class="o">|</span>
<span class="n">y</span> <span class="o">|</span>
<span class="o">+---+</span> <span class="o">|</span>
<span class="o">|</span> <span class="o">-|---------+</span>
<span class="o">+---+</span>
</pre></div>
</div>
<p>So, when you modify <code class="docutils literal notranslate"><span class="pre">y</span></code> to <code class="docutils literal notranslate"><span class="pre">[0,</span> <span class="pre">2,</span> <span class="pre">3,</span> <span class="pre">4,</span> <span class="pre">5]</span></code>, the list becomes</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span> <span class="n">x</span>
<span class="o">+---+</span>
<span class="o">|</span> <span class="o">-|-------></span> <span class="p">[</span><span class="mi">0</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="mi">4</span><span class="p">,</span> <span class="mi">5</span><span class="p">]</span>
<span class="o">+---+</span> <span class="o">^</span>
<span class="o">|</span>
<span class="n">y</span> <span class="o">|</span>
<span class="o">+---+</span> <span class="o">|</span>
<span class="o">|</span> <span class="o">-|---------+</span>
<span class="o">+---+</span>
</pre></div>
</div>
<p>Then, how can we create a new copy of the list for <code class="docutils literal notranslate"><span class="pre">y</span></code>? We need to use <code class="docutils literal notranslate"><span class="pre">copy</span></code> in Python.</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">copy</span>
<span class="n">x</span> <span class="o">=</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="mi">4</span><span class="p">,</span> <span class="mi">5</span><span class="p">]</span>
<span class="n">y</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">x</span><span class="p">)</span>
</pre></div>
</div>
</div>
</div>
<p>This will create a new copy of the list as shown below</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span> <span class="n">x</span>
<span class="o">+---+</span>
<span class="o">|</span> <span class="o">-|-------></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="mi">4</span><span class="p">,</span> <span class="mi">5</span><span class="p">]</span>
<span class="o">+---+</span>
<span class="n">y</span>
<span class="o">+---+</span>
<span class="o">|</span> <span class="o">-|-------></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="mi">4</span><span class="p">,</span> <span class="mi">5</span><span class="p">]</span>
<span class="o">+---+</span>
</pre></div>
</div>
<p>What would happen if you modify <code class="docutils literal notranslate"><span class="pre">y</span></code>?</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">y</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span> <span class="o">=</span> <span class="mi">0</span>
<span class="nb">print</span><span class="p">(</span><span class="n">x</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="n">y</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="cell_output docutils container">
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>[1, 2, 3, 4, 5]
[0, 2, 3, 4, 5]
</pre></div>
</div>
</div>
</div>
<p>The list <code class="docutils literal notranslate"><span class="pre">x</span></code> remains intact because <code class="docutils literal notranslate"><span class="pre">x</span></code> and <code class="docutils literal notranslate"><span class="pre">y</span></code> are pointing at different objects as shown below.</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span> <span class="n">x</span>
<span class="o">+---+</span>
<span class="o">|</span> <span class="o">-|-------></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="mi">4</span><span class="p">,</span> <span class="mi">5</span><span class="p">]</span>
<span class="o">+---+</span>
<span class="n">y</span>
<span class="o">+---+</span>
<span class="o">|</span> <span class="o">-|-------></span> <span class="p">[</span><span class="mi">0</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="mi">4</span><span class="p">,</span> <span class="mi">5</span><span class="p">]</span>
<span class="o">+---+</span>
</pre></div>
</div>
<p>What if <code class="docutils literal notranslate"><span class="pre">x</span></code> is a list contaning another list? Would the <code class="docutils literal notranslate"><span class="pre">copy</span></code> function create a completely seprate copy of <code class="docutils literal notranslate"><span class="pre">x</span></code>?</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">x</span> <span class="o">=</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="mi">4</span><span class="p">,</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="nb">print</span><span class="p">(</span><span class="n">x</span><span class="p">)</span>
<span class="n">y</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">x</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="n">y</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="cell_output docutils container">
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>[1, 2, 3, 4, [5, 6, 7]]
[1, 2, 3, 4, [5, 6, 7]]
</pre></div>
</div>
</div>
</div>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">y</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span> <span class="o">=</span> <span class="mi">0</span>
<span class="nb">print</span><span class="p">(</span><span class="n">x</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="n">y</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="cell_output docutils container">
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>[1, 2, 3, 4, [5, 6, 7]]
[0, 2, 3, 4, [5, 6, 7]]
</pre></div>
</div>
</div>
</div>
<p>Although it creates a copy of <code class="docutils literal notranslate"><span class="pre">x</span></code>, it is not a complete copy of <code class="docutils literal notranslate"><span class="pre">x</span></code>.</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">y</span><span class="p">[</span><span class="mi">4</span><span class="p">][</span><span class="mi">0</span><span class="p">]</span> <span class="o">=</span> <span class="mi">0</span>
<span class="nb">print</span><span class="p">(</span><span class="n">x</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="n">y</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="cell_output docutils container">
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>[1, 2, 3, 4, [0, 6, 7]]
[0, 2, 3, 4, [0, 6, 7]]
</pre></div>
</div>
</div>
</div>
<p>Since the last element of <code class="docutils literal notranslate"><span class="pre">x</span></code> is another list, x[4] is the address (pointer) of the list. So, <code class="docutils literal notranslate"><span class="pre">x</span></code> and <code class="docutils literal notranslate"><span class="pre">y</span></code> can be depicted as shown below.</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span> <span class="n">x</span>
<span class="o">+---+</span>
<span class="o">|</span> <span class="o">-|-------></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="mi">4</span><span class="p">,</span> <span class="p">(</span> <span class="p">)]</span>
<span class="o">+---+</span> <span class="o">|</span>
<span class="o">+-----------></span> <span class="p">[</span><span class="mi">0</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="o">^</span>
<span class="o">|</span>
<span class="n">y</span> <span class="o">+-------------+</span>
<span class="o">+---+</span> <span class="o">|</span>
<span class="o">|</span> <span class="o">-|-------></span> <span class="p">[</span><span class="mi">0</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="mi">4</span><span class="p">,</span> <span class="p">(</span> <span class="p">)]</span>
<span class="o">+---+</span>
</pre></div>
</div>
<p>Since the Python <code class="docutils literal notranslate"><span class="pre">copy</span></code> function basically creates a <em>shallow</em> copy, <code class="docutils literal notranslate"><span class="pre">x</span></code> and <code class="docutils literal notranslate"><span class="pre">y</span></code> are not completely different objects.</p>
<p>Is there a way to create a complete copy of an object? Yes and it is called a <em>deep</em> copy.</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">x</span> <span class="o">=</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="mi">4</span><span class="p">,</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="nb">print</span><span class="p">(</span><span class="n">x</span><span class="p">)</span>
<span class="n">y</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">x</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="n">y</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="cell_output docutils container">
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>[1, 2, 3, 4, [5, 6, 7]]
[1, 2, 3, 4, [5, 6, 7]]
</pre></div>
</div>
</div>
</div>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">y</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span> <span class="o">=</span> <span class="mi">0</span>
<span class="nb">print</span><span class="p">(</span><span class="n">x</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="n">y</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="cell_output docutils container">
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>[1, 2, 3, 4, [5, 6, 7]]
[0, 2, 3, 4, [5, 6, 7]]
</pre></div>
</div>
</div>
</div>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">y</span><span class="p">[</span><span class="mi">4</span><span class="p">][</span><span class="mi">0</span><span class="p">]</span> <span class="o">=</span> <span class="mi">0</span>
<span class="nb">print</span><span class="p">(</span><span class="n">x</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="n">y</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="cell_output docutils container">
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>[1, 2, 3, 4, [5, 6, 7]]
[0, 2, 3, 4, [0, 6, 7]]
</pre></div>
</div>
</div>
</div>
<p>Now, <code class="docutils literal notranslate"><span class="pre">x</span></code> and <code class="docutils literal notranslate"><span class="pre">y</span></code> are compelely different objects and depicted as</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span> <span class="n">x</span>
<span class="o">+---+</span>
<span class="o">|</span> <span class="o">-|-------></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="mi">4</span><span class="p">,</span> <span class="p">(</span> <span class="p">)]</span>
<span class="o">+---+</span> <span class="o">|</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="n">y</span> <span class="o">+-----------></span> <span class="p">[</span><span class="mi">0</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="o">+---+</span> <span class="o">|</span>
<span class="o">|</span> <span class="o">-|-------></span> <span class="p">[</span><span class="mi">0</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="mi">4</span><span class="p">,</span> <span class="p">(</span> <span class="p">)]</span>
<span class="o">+---+</span>
</pre></div>
</div>
<p>OK. Then, how come there is no notion of pointers in Python, Java, C#, and other programming languages? In the world of C/C++, variable <code class="docutils literal notranslate"><span class="pre">x</span></code> can an object (structure in C) itself or a pointer to the object as shown below</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">x</span> <span class="ow">is</span> <span class="n">the</span> <span class="nb">object</span> <span class="n">itself</span>
<span class="n">x</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="mi">4</span><span class="p">,</span> <span class="mi">5</span><span class="p">]</span>
<span class="n">x</span> <span class="ow">is</span> <span class="n">a</span> <span class="n">pointer</span>
<span class="n">x</span>
<span class="o">+---+</span>
<span class="o">|</span> <span class="o">-|-------></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="mi">4</span><span class="p">,</span> <span class="mi">5</span><span class="p">]</span>
<span class="o">+---+</span>
</pre></div>
</div>
<p>For example, in the following code, <code class="docutils literal notranslate"><span class="pre">point</span></code> is a structure and <code class="docutils literal notranslate"><span class="pre">ptr</span></code> is a pointer to the structure.</p>
<div class="highlight-C notranslate"><div class="highlight"><pre><span></span><span class="k">struct</span><span class="w"> </span><span class="nc">Point</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">x</span><span class="p">;</span>
<span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">y</span><span class="p">;</span>
<span class="p">};</span>
<span class="k">struct</span><span class="w"> </span><span class="nc">Point</span><span class="w"> </span><span class="n">point</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">{</span><span class="mi">320</span><span class="p">,</span><span class="w"> </span><span class="mi">200</span><span class="p">};</span>
<span class="k">struct</span><span class="w"> </span><span class="nc">Point</span><span class="w"> </span><span class="o">*</span><span class="n">ptr</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="o">&</span><span class="n">point</span><span class="p">;</span>
</pre></div>
</div>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span> <span class="n">point</span>
<span class="p">(</span><span class="n">x</span> <span class="o">=</span> <span class="mi">1</span><span class="p">,</span> <span class="n">y</span> <span class="o">=</span> <span class="mi">2</span><span class="p">)</span>
<span class="n">ptr</span> <span class="o">^</span>
<span class="o">+---+</span> <span class="o">|</span>
<span class="o">|</span> <span class="o">-|-------></span>
<span class="o">+---+</span>
</pre></div>
</div>
<p>Unlike C and C++, in most other programming languages like Python, Java, and C#, objects are accessed using only pointers (<code class="docutils literal notranslate"><span class="pre">ptr</span></code> in the above example) because there does not exist a varible that is an object itself (<code class="docutils literal notranslate"><span class="pre">point</span></code> in the above example). For example, the declaration in Java</p>
<div class="highlight-Java notranslate"><div class="highlight"><pre><span></span><span class="n">MyClass</span><span class="w"> </span><span class="n">x</span><span class="p">;</span>
</pre></div>
</div>
<p>does not mean that <strong>x contains objects of type MyClass,</strong> but rather that <strong>x contains <em>pointers</em> to objects of type MyClass (or null).</strong> However, because it is extremely burdensome always to be saying “x contains a pointer to MyClass”, the universal practice is just to say “x is a P.”</p>
<p>Do you still think you can write code in Python/Java/C# with no clear understanding of pointers?
Dude, even MATLAB has pointers!</p>
</section>
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