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difference-between-and-is-in-python-and-when-to-use-each-of-them.html
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<h1 id="difference-between-and-is-in-python-and-when-to-use-each-of-them">Difference between `==` and `is` in Python, and when to use each of them</h1>
<p>Posted on Sat 26 January 2019 in <a href="http://www.emadmokhtar.com/category/python.html">Python</a> <b>Read in 3 min.</b></p>
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<p><img alt="Difference" src="http://www.emadmokhtar.com/images/will-francis-1071726-unsplash.jpg"></p>
<p>In Python there are many <a href="https://realpython.com/python-operators-expressions/#comparison-operators">comparison operators</a>; you always use them to check something in your code and let your code take decision according to the comparison</p>
<p>In this article, I want to go deep with two operators <code>==</code> to check equality, and <code>is</code> to check identity.</p>
<h2>The <code>==</code> operator</h2>
<p>The equality comparison operator. </p>
<div class="highlight"><pre><span></span><span class="o">>>></span> <span class="mi">1</span> <span class="o">==</span> <span class="mi">1</span>
<span class="bp">True</span>
</pre></div>
<p>In the above code, we are checking whether the value of int <strong>1</strong> is an equal value of int <strong>1</strong>, in other words, we are checking the values equality here. </p>
<div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Number</span><span class="p">:</span>
<span class="k">def</span> <span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">number</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">number</span> <span class="o">=</span> <span class="n">number</span>
<span class="o">>>></span> <span class="n">Number</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span> <span class="o">==</span> <span class="n">Number</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span>
<span class="bp">False</span> <span class="c1"># Wooot</span>
</pre></div>
<p>In the above code, we are checking if the values of 2 objects are equal. They are not equal. Classes in Python 🐍 have <a href="https://wiki.python.org/moin/DunderAlias">dunder methods</a> to implement special logic of magic.</p>
<iframe src="https://giphy.com/embed/12NUbkX6p4xOO4" width="480" height="440" frameBorder="0" class="giphy-embed" allowFullScreen></iframe>
<p><a href="https://giphy.com/gifs/shia-labeouf-12NUbkX6p4xOO4">via GIPHY</a></p>
<p>For example, <code>__init__()</code> is dunder method to initiate a new object or the class constructor. For our example above, there is a dunder method used to check the equality which is <code>__eq__()</code> , if we implement it correctly, the expression in the code about should work.</p>
<div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Number</span><span class="p">:</span>
<span class="k">def</span> <span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">number</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">number</span> <span class="o">=</span> <span class="n">number</span>
<span class="k">def</span> <span class="fm">__eq__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">another_number</span><span class="p">):</span>
<span class="k">if</span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">another_number</span><span class="p">,</span> <span class="n">Number</span><span class="p">):</span>
<span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">number</span> <span class="o">==</span> <span class="n">another_number</span><span class="o">.</span><span class="n">number</span>
<span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">number</span> <span class="o">==</span> <span class="n">another_number</span>
<span class="o">>>></span> <span class="n">Number</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span> <span class="o">==</span> <span class="n">Number</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span>
<span class="bp">True</span>
<span class="o">>>></span> <span class="n">Number</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span> <span class="o">==</span> <span class="mi">1</span>
<span class="bp">True</span>
</pre></div>
<p>All right, now our class has a correct equality logic. The equality logic is implemented in <code>__eq__</code> method. It is also implemented in Python built-in types like Integers.</p>
<div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">inspect</span>
<span class="kn">from</span> <span class="nn">pprint</span> <span class="kn">import</span> <span class="n">pprint</span>
<span class="c1"># 1 is object of int class</span>
<span class="n">pprint</span><span class="p">(</span><span class="n">inspect</span><span class="o">.</span><span class="n">getmembers</span><span class="p">(</span><span class="mi">1</span><span class="p">))</span>
<span class="n">pprint</span><span class="p">(</span><span class="n">inspect</span><span class="o">.</span><span class="n">getmembers</span><span class="p">(</span><span class="nb">int</span><span class="p">))</span>
</pre></div>
<h2>The <code>is</code> operator</h2>
<p>The identity check operator</p>
<div class="highlight"><pre><span></span><span class="o">>>></span> <span class="n">none_obj</span> <span class="o">=</span> <span class="bp">None</span>
<span class="o">>>></span> <span class="n">none_obj</span> <span class="ow">is</span> <span class="bp">None</span>
<span class="bp">True</span>
<span class="o">>>></span> <span class="n">number_one</span> <span class="o">=</span> <span class="mi">1</span>
<span class="o">>>></span> <span class="n">number_one</span> <span class="ow">is</span> <span class="mi">1</span>
</pre></div>
<p>In Python, every object created will store a reference to it, like in the above code <strong>none_obj</strong> which is storing a reference to <strong>None</strong> , and <strong>number_one</strong> object is storing a reference to <strong>1</strong>. Let’s examine how this is done by Python.</p>
<div class="highlight"><pre><span></span><span class="o">>>></span> <span class="n">none_obj</span> <span class="o">=</span> <span class="bp">None</span>
<span class="o">>>></span> <span class="nb">id</span><span class="p">(</span><span class="n">none_obj</span><span class="p">)</span>
<span class="mi">4304631824</span>
<span class="o">>>></span> <span class="nb">id</span><span class="p">(</span><span class="bp">None</span><span class="p">)</span>
<span class="mi">4304631824</span>
<span class="o">>>></span> <span class="nb">id</span><span class="p">(</span><span class="n">number_one</span><span class="p">)</span>
<span class="mi">4304948352</span>
<span class="o">>>></span> <span class="nb">id</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span>
<span class="mi">4304948352</span>
</pre></div>
<p>When you create a new variable that stores an object or value, this variable name stores a reference to it, so if you create another variable to store the same object, Python stores a pointer to this object instead of creating a new one.</p>
<div class="highlight"><pre><span></span><span class="n">a</span> <span class="o">=</span> <span class="n">Number</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span>
<span class="n">b</span> <span class="o">=</span> <span class="n">a</span>
<span class="o">>>></span> <span class="nb">id</span><span class="p">(</span><span class="n">a</span><span class="p">),</span> <span class="nb">id</span><span class="p">(</span><span class="n">b</span><span class="p">)</span>
<span class="p">(</span><span class="mi">4520963656</span><span class="p">,</span> <span class="mi">4520963656</span><span class="p">)</span>
<span class="o">>>></span> <span class="n">a</span> <span class="ow">is</span> <span class="n">b</span>
<span class="bp">True</span>
</pre></div>
<p>Technically speaking, when you check the identity, Python checks <code>id(a) == id(b)</code>, which means:
Do objects <strong>a</strong> and <strong>b</strong> refer to the same object? So the answer will be <strong>True</strong> if they refer to the same object.</p>
<h2>Conclusion</h2>
<p>If you need to check the value equality, use the <code>==</code> operator and if you need to check whether 2 objects are the same, use <code>is</code>.</p>
<p>Happy Pythoning 😉</p>
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