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<title>
Additional Exercises – Programming with Python
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<h1 class="maintitle">Defensive Programming - Exercises</h1>
<blockquote class="challenge">
<blockquote class="challenge">
<h2 id="pre--and-post-conditions">Pre- and Post-Conditions</h2>
<p>Suppose you are writing a function called <code class="language-plaintext highlighter-rouge">average</code> that calculates
the average of the numbers in a list.
What pre-conditions and post-conditions would you write for it?
Compare your answer to your neighbor’s:
can you think of a function that will pass your tests but not his/hers or vice versa?</p>
<blockquote class="solution">
<h2 id="solution">Solution</h2>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="c1"># a possible pre-condition:
</span><span class="k">assert</span> <span class="nb">len</span><span class="p">(</span><span class="n">input_list</span><span class="p">)</span> <span class="o">></span> <span class="mi">0</span><span class="p">,</span> <span class="s">'List length must be non-zero'</span>
<span class="c1"># a possible post-condition:
</span><span class="k">assert</span> <span class="n">numpy</span><span class="p">.</span><span class="nb">min</span><span class="p">(</span><span class="n">input_list</span><span class="p">)</span> <span class="o"><=</span> <span class="n">average</span> <span class="o"><=</span> <span class="n">numpy</span><span class="p">.</span><span class="nb">max</span><span class="p">(</span><span class="n">input_list</span><span class="p">),</span>
<span class="s">'Average should be between min and max of input values (inclusive)'</span>
</code></pre></div> </div>
</blockquote>
</blockquote>
<blockquote class="challenge">
<h2 id="testing-assertions">Testing Assertions</h2>
<p>Given a sequence of a number of cars, the function <code class="language-plaintext highlighter-rouge">get_total_cars</code> returns
the total number of cars.</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">get_total_cars</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>
</code></pre></div> </div>
<div class="language-plaintext output highlighter-rouge"><div class="highlight"><pre class="highlight"><code>10
</code></pre></div> </div>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">get_total_cars</span><span class="p">([</span><span class="s">'a'</span><span class="p">,</span> <span class="s">'b'</span><span class="p">,</span> <span class="s">'c'</span><span class="p">])</span>
</code></pre></div> </div>
<div class="language-plaintext output highlighter-rouge"><div class="highlight"><pre class="highlight"><code>ValueError: invalid literal for int() with base 10: 'a'
</code></pre></div> </div>
<p>Explain in words what the assertions in this function check,
and for each one,
give an example of input that will make that assertion fail.</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">def</span> <span class="nf">get_total</span><span class="p">(</span><span class="n">values</span><span class="p">):</span>
<span class="k">assert</span> <span class="nb">len</span><span class="p">(</span><span class="n">values</span><span class="p">)</span> <span class="o">></span> <span class="mi">0</span>
<span class="k">for</span> <span class="n">element</span> <span class="ow">in</span> <span class="n">values</span><span class="p">:</span>
<span class="k">assert</span> <span class="nb">int</span><span class="p">(</span><span class="n">element</span><span class="p">)</span>
<span class="n">values</span> <span class="o">=</span> <span class="p">[</span><span class="nb">int</span><span class="p">(</span><span class="n">element</span><span class="p">)</span> <span class="k">for</span> <span class="n">element</span> <span class="ow">in</span> <span class="n">values</span><span class="p">]</span>
<span class="n">total</span> <span class="o">=</span> <span class="nb">sum</span><span class="p">(</span><span class="n">values</span><span class="p">)</span>
<span class="k">assert</span> <span class="n">total</span> <span class="o">></span> <span class="mi">0</span>
<span class="k">return</span> <span class="n">total</span>
</code></pre></div> </div>
<blockquote class="solution">
<h2 id="solution-1">Solution</h2>
<ul>
<li>The first assertion checks that the input sequence <code class="language-plaintext highlighter-rouge">values</code> is not empty.
An empty sequence such as <code class="language-plaintext highlighter-rouge">[]</code> will make it fail.</li>
<li>The second assertion checks that each value in the list can be turned into an integer.
Input such as <code class="language-plaintext highlighter-rouge">[1, 2,'c', 3]</code> will make it fail.</li>
<li>The third assertion checks that the total of the list is greater than 0.
Input such as <code class="language-plaintext highlighter-rouge">[-10, 2, 3]</code> will make it fail.</li>
</ul>
</blockquote>
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