Hykilpikonna
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<header id="title-block-header">
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<h1 class="title">5.2 Operations on Mutable Data Types</h1>
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<section>
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<p>In the last section, we introduced the concept of mutable data types, and saw how we could mutate Python lists with the <code>list.append</code> method. In this section, we’ll survey some of the other ways of mutating lists and other mutable Python data types. For a full reference of Python’s mutating methods on these data types, please see <a href="../A-python-builtins/02-types.html">Appendix A.2 Python Built-In Data Types Reference</a>.</p>
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<h2 id="list.append-list.insert-and-list.extend"><code>list.append</code>, <code>list.insert</code>, and <code>list.extend</code></h2>
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<p>In addition to <code>list.append</code>, there are two other ways of adding new items to a Python list. The first is <code>list.insert</code>, which takes a list, an <em>index</em> and an object, and inserts the object at the given index into the list at the given index.</p>
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<div class="sourceCode" id="cb1"><pre class="sourceCode python"><code class="sourceCode python"><span id="cb1-1"><a href="#cb1-1"></a><span class="op">>>></span> strings <span class="op">=</span> [<span class="st">'a'</span>, <span class="st">'b'</span>, <span class="st">'c'</span>, <span class="st">'d'</span>]</span>
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<span id="cb1-2"><a href="#cb1-2"></a><span class="op">>>></span> <span class="bu">list</span>.insert(strings, <span class="dv">2</span>, <span class="st">'hello'</span>) <span class="co"># Insert 'hello' into strings at index 2</span></span>
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<span id="cb1-3"><a href="#cb1-3"></a><span class="op">>>></span> strings</span>
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<span id="cb1-4"><a href="#cb1-4"></a>[<span class="st">'a'</span>, <span class="st">'b'</span>, <span class="st">'hello'</span>, <span class="st">'c'</span>, <span class="st">'d'</span>]</span></code></pre></div>
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<p>The second is <code>list.extend</code>, which takes two lists and adds all items from the second list at the end of the first list, as if <code>append</code> were called once per element of the second list.</p>
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<div class="sourceCode" id="cb2"><pre class="sourceCode python"><code class="sourceCode python"><span id="cb2-1"><a href="#cb2-1"></a><span class="op">>>></span> strings <span class="op">=</span> [<span class="st">'a'</span>, <span class="st">'b'</span>, <span class="st">'c'</span>, <span class="st">'d'</span>]</span>
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<span id="cb2-2"><a href="#cb2-2"></a><span class="op">>>></span> <span class="bu">list</span>.extend(strings, [<span class="st">'CSC110'</span>, <span class="st">'CSC111'</span>])</span>
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<span id="cb2-3"><a href="#cb2-3"></a><span class="op">>>></span> strings</span>
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<span id="cb2-4"><a href="#cb2-4"></a>[<span class="st">'a'</span>, <span class="st">'b'</span>, <span class="st">'c'</span>, <span class="st">'d'</span>, <span class="st">'CSC110'</span>, <span class="st">'CSC111'</span>]</span></code></pre></div>
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<h2 id="assigning-to-a-specific-list-index">Assigning to a specific list index</h2>
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<p>There is one more way to put a value into a list: by overwriting the element stored at a specific index. Given a list <code>lst</code>, we’ve seen that we can access specific elements using indexing syntax <code>lst[0]</code>, <code>lst[1]</code>, <code>lst[2]</code>, etc. We can also use this kind of expression as the <em>left side</em> of an assignment statement to mutate the list by modifying a specific index.</p>
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<div class="sourceCode" id="cb3"><pre class="sourceCode python"><code class="sourceCode python"><span id="cb3-1"><a href="#cb3-1"></a><span class="op">>>></span> strings <span class="op">=</span> [<span class="st">'a'</span>, <span class="st">'b'</span>, <span class="st">'c'</span>, <span class="st">'d'</span>]</span>
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<span id="cb3-2"><a href="#cb3-2"></a><span class="op">>>></span> strings[<span class="dv">2</span>] <span class="op">=</span> <span class="st">'Hello'</span></span>
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<span id="cb3-3"><a href="#cb3-3"></a><span class="op">>>></span> strings</span>
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<span id="cb3-4"><a href="#cb3-4"></a>[<span class="st">'a'</span>, <span class="st">'b'</span>, <span class="st">'Hello'</span>, <span class="st">'d'</span>]</span></code></pre></div>
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<p>Note that unlike <code>list.insert</code>, assigning to an index removes the element previously stored at that index from the list!</p>
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<h2 id="mutating-sets">Mutating sets</h2>
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<p>Python <code>set</code>s are mutable. Because they are unordered, they are simpler than <code>list</code>s, and offer just two main mutating methods: <code>set.add</code> and <code>set.remove</code>, which (as you can probably guess) add and remove an element from a set, respectively.<label for="sn-0" class="margin-toggle sidenote-number"></label><input type="checkbox" id="sn-0" class="margin-toggle"/><span class="sidenote"> <code>list</code> also provides a few mutating methods that remove elements, though we did not cover them in this section.</span> We’ll illustrate <code>set.add</code> by showing how to re-implement our <code>squares</code> function from the previous section with <code>set</code> instead of <code>list</code>:</p>
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<div class="sourceCode" id="cb4"><pre class="sourceCode python"><code class="sourceCode python"><span id="cb4-1"><a href="#cb4-1"></a><span class="kw">def</span> squares(numbers: <span class="bu">set</span>[<span class="bu">int</span>]) <span class="op">-></span> <span class="bu">set</span>[<span class="bu">int</span>]:</span>
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<span id="cb4-2"><a href="#cb4-2"></a> <span class="co">"""Return a set containing the squares of all the given numbers.</span></span>
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<span id="cb4-3"><a href="#cb4-3"></a></span>
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<span id="cb4-4"><a href="#cb4-4"></a><span class="co"> ...</span></span>
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<span id="cb4-5"><a href="#cb4-5"></a><span class="co"> """</span></span>
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<span id="cb4-6"><a href="#cb4-6"></a> squares_so_far <span class="op">=</span> <span class="bu">set</span>()</span>
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<span id="cb4-7"><a href="#cb4-7"></a> <span class="cf">for</span> n <span class="kw">in</span> numbers:</span>
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<span id="cb4-8"><a href="#cb4-8"></a> <span class="bu">set</span>.add(squares_so_far, n <span class="op">*</span> n)</span>
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<span id="cb4-9"><a href="#cb4-9"></a></span>
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<span id="cb4-10"><a href="#cb4-10"></a> <span class="cf">return</span> squares_so_far</span></code></pre></div>
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<p>Note that <code>set.add</code> will only add the element if the set does not already contain it, as sets cannot contain duplicates. In addition, sets are unordered whereas <code>list.append</code> will add the element to the end of the sequence.</p>
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<h2 id="mutating-dictionaries">Mutating dictionaries</h2>
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<p>The most common ways for dictionaries to be mutated is by adding a new key-value pair, or changing the associated value for a key-value pair in the dictionary. This does not use a function call, but rather the same syntax as assigning by list index.</p>
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<div class="sourceCode" id="cb5"><pre class="sourceCode python"><code class="sourceCode python"><span id="cb5-1"><a href="#cb5-1"></a><span class="op">>>></span> items <span class="op">=</span> {<span class="st">'a'</span>: <span class="dv">1</span>, <span class="st">'b'</span>: <span class="dv">2</span>}</span>
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<span id="cb5-2"><a href="#cb5-2"></a><span class="op">>>></span> items[<span class="st">'c'</span>] <span class="op">=</span> <span class="dv">3</span></span>
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<span id="cb5-3"><a href="#cb5-3"></a><span class="op">>>></span> items</span>
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<span id="cb5-4"><a href="#cb5-4"></a>{<span class="st">'a'</span>: <span class="dv">1</span>, <span class="st">'b'</span>: <span class="dv">2</span>, <span class="st">'c'</span>: <span class="dv">3</span>}</span></code></pre></div>
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<p>The second assignment statement adds a new key-value pair to <code>items</code>, with the key being <code>'c'</code> and the items being <code>3</code>. In this case, the left-hand side of the assignment is not a variable but instead an expression representing a component of <code>items</code>, in this case the key <code>'c'</code> in the dictionary. When this assignment statement is evaluated, the right-hand side value <code>3</code> is stored in the dictionary items as the corresponding value for <code>'c'</code>.</p>
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<p>Assignment statements in this form can also be used to mutate the dictionary by taking an existing key-value pair and replacing the value with a different one. Here’s an example of that:</p>
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<div class="sourceCode" id="cb6"><pre class="sourceCode python"><code class="sourceCode python"><span id="cb6-1"><a href="#cb6-1"></a><span class="op">>>></span> items[<span class="st">'a'</span>] <span class="op">=</span> <span class="dv">100</span></span>
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<span id="cb6-2"><a href="#cb6-2"></a><span class="op">>>></span> items</span>
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<span id="cb6-3"><a href="#cb6-3"></a>{<span class="st">'a'</span>: <span class="dv">100</span>, <span class="st">'b'</span>: <span class="dv">2</span>, <span class="st">'c'</span>: <span class="dv">3</span>}</span></code></pre></div>
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<h2 id="mutating-data-classes">Mutating data classes</h2>
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<p>Python data classes are mutable by default.<label for="sn-1" class="margin-toggle sidenote-number"></label><input type="checkbox" id="sn-1" class="margin-toggle"/><span class="sidenote"> Technically there is a way to create immutable data classes, but this is beyond the scope of this course.</span> To illustrate this, we’ll return to our <code>Person</code> class:</p>
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<div class="sourceCode" id="cb7"><pre class="sourceCode python"><code class="sourceCode python"><span id="cb7-1"><a href="#cb7-1"></a><span class="at">@dataclass</span></span>
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<span id="cb7-2"><a href="#cb7-2"></a><span class="kw">class</span> Person:</span>
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<span id="cb7-3"><a href="#cb7-3"></a> <span class="co">"""A person with some basic demographic information.</span></span>
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<span id="cb7-4"><a href="#cb7-4"></a></span>
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<span id="cb7-5"><a href="#cb7-5"></a><span class="co"> Representation Invariants:</span></span>
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<span id="cb7-6"><a href="#cb7-6"></a><span class="co"> - self.age >= 0</span></span>
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<span id="cb7-7"><a href="#cb7-7"></a></span>
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<span id="cb7-8"><a href="#cb7-8"></a><span class="co"> """</span></span>
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<span id="cb7-9"><a href="#cb7-9"></a> given_name: <span class="bu">str</span></span>
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<span id="cb7-10"><a href="#cb7-10"></a> family_name: <span class="bu">str</span></span>
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<span id="cb7-11"><a href="#cb7-11"></a> age: <span class="bu">int</span></span>
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<span id="cb7-12"><a href="#cb7-12"></a> address: <span class="bu">str</span></span></code></pre></div>
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<p>We mutate instances of data classes by modifying their attributes. We do this by assigning to their attributes directly, using <em>dot notation</em> on the left side of an assignment statement.</p>
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<div class="sourceCode" id="cb8"><pre class="sourceCode python"><code class="sourceCode python"><span id="cb8-1"><a href="#cb8-1"></a><span class="op">>>></span> p <span class="op">=</span> Person(<span class="st">'David'</span>, <span class="st">'Liu'</span>, <span class="dv">100</span>, <span class="st">'40 St. George Street'</span>)</span>
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<span id="cb8-2"><a href="#cb8-2"></a><span class="op">>>></span> p.age <span class="op">=</span> <span class="dv">200</span></span>
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<span id="cb8-3"><a href="#cb8-3"></a><span class="op">>>></span> p</span>
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<span id="cb8-4"><a href="#cb8-4"></a>Person(given_name<span class="op">=</span><span class="st">'David'</span>, family_name<span class="op">=</span><span class="st">'Liu'</span>, age<span class="op">=</span><span class="dv">200</span>, address<span class="op">=</span><span class="st">'40 St. George Street'</span>)</span></code></pre></div>
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<p>One note of caution here: as you start mutating data class instances, you must always remember to respect the representation invariants associated with that data class. For example, setting <code>p.age = -1</code> would violate the <code>Person</code> representation invariant. To protect against this, <code>python_ta</code> checks representation invariants whenever you assign to attributes of data classes, as long as the <code>python_ta.contracts.check_all_contracts</code> function has been called in your file.</p>
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