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Lecture 2 - CS50's Web Programming with Python and JavaScript.htm
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Lecture 2 - CS50's Web Programming with Python and JavaScript.htm
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<!DOCTYPE html>
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<meta name="viewport" content="initial-scale=1, width=device-width"><meta property="og:description" content="This course picks up where Harvard University's CS50 leaves off, diving more deeply into the design and implementation of web apps with Python, JavaScript, and SQL using frameworks like Django, React, and Bootstrap. Topics include database design, scalability, security, and user experience. Through hands-on projects, students learn to write and use APIs, create interactive UIs, and leverage cloud services like GitHub and Heroku. By semester’s end, students emerge with knowledge and experience in principles, languages, and tools that empower them to design and deploy applications on the Internet.">
<meta property="og:image" content="https://img.youtube.com/vi/24Kf3v7kZyE/maxresdefault.jpg"><meta property="og:title" content="Lecture 2 - CS50's Web Programming with Python and JavaScript">
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<a data-id="" id="lecture-2" style="top: -81.3333px;"></a><h1 class="no_toc"><a data-id="" href="#lecture-2">Lecture 2</a></h1>
<ul id="markdown-toc">
<li><a href="#introduction" id="markdown-toc-introduction">Introduction</a></li>
<li><a href="#python" id="markdown-toc-python">Python</a></li>
<li><a href="#variables" id="markdown-toc-variables">Variables</a></li>
<li><a href="#formatting-strings" id="markdown-toc-formatting-strings">Formatting Strings</a></li>
<li><a href="#conditions" id="markdown-toc-conditions">Conditions</a></li>
<li><a href="#sequences" id="markdown-toc-sequences">Sequences</a> <ul>
<li><a href="#strings" id="markdown-toc-strings">Strings</a></li>
<li><a href="#lists" id="markdown-toc-lists">Lists</a></li>
<li><a href="#tuples" id="markdown-toc-tuples">Tuples</a></li>
<li><a href="#sets" id="markdown-toc-sets">Sets</a></li>
<li><a href="#dictionaries" id="markdown-toc-dictionaries">Dictionaries</a></li>
<li><a href="#loops" id="markdown-toc-loops">Loops</a></li>
</ul>
</li>
<li><a href="#functions" id="markdown-toc-functions">Functions</a></li>
<li><a href="#modules" id="markdown-toc-modules">Modules</a></li>
<li><a href="#object-oriented-programming" id="markdown-toc-object-oriented-programming">Object-Oriented Programming</a></li>
<li><a href="#functional-programming" id="markdown-toc-functional-programming">Functional Programming</a> <ul>
<li><a href="#decorators" id="markdown-toc-decorators">Decorators</a></li>
<li><a href="#lambda-functions" id="markdown-toc-lambda-functions">Lambda Functions</a></li>
</ul>
</li>
<li><a href="#exceptions" id="markdown-toc-exceptions">Exceptions</a></li>
</ul>
<a data-id="" id="introduction" style="top: -81.3333px;"></a><h2><a data-id="" href="#introduction">Introduction</a></h2>
<ul class="fa-ul">
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span>So
far, we’ve discussed how to build simple web pages using HTML and CSS,
and how to use Git and GitHub in order to keep track of changes to our
code and collaborate with others.</li>
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span>Today, we’ll dive into Python, one of the two main programming languages we’ll use throughout this course.</li>
</ul>
<a data-id="" id="python" style="top: -81.3333px;"></a><h2><a data-id="" href="#python">Python</a></h2>
<p><img src="Lecture%202%20-%20CS50's%20Web%20Programming%20with%20Python%20and%20JavaScript_files/python.png" alt="Python Logo"></p>
<ul class="fa-ul">
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span>
<p>Python is a very powerful and widely-used language that will
allow us to quickly build fairly complicated web applications. In this
course, we’ll be using Python 3, although Python 2 is still in use in
some places. When looking at outside resources, be careful to make sure
they’re using the same version.</p>
</li>
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span>
<p>Let’s start where we start with many programming languages: Hello, world. This program, written in Python, would look like this:</p>
</li>
</ul>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">print</span><span class="p">(</span><span class="s">"Hello, world!"</span><span class="p">)</span>
</code></pre></div></div>
<ul class="fa-ul">
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span>To break down what’s going on in that line, there is a <code class="language-plaintext highlighter-rouge">print</code> <strong>function</strong> built in to the python language, that takes an <strong>argument</strong> in parentheses, and displays that argument on the command line.</li>
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span>To
actually write and run this program on your computers, you’ll first
type this line into your text editor of choice, and then save the file
as <code class="language-plaintext highlighter-rouge">something.py</code>. Next, you’ll head over to your terminal, navigate to the directory containing your file, and type <code class="language-plaintext highlighter-rouge">python something.py</code>. In the case of the above program, the words “Hello, world!” will then be displayed in the terminal.</li>
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span>Depending on how your computer is set up, you may have to type <code class="language-plaintext highlighter-rouge">python3</code> instead of <code class="language-plaintext highlighter-rouge">python</code> before the file name, and you may even have to <a href="https://www.python.org/downloads/">download Python</a> if you haven’t already. After installing Python, we recommend that you also <a href="https://pip.pypa.io/en/stable/installing/">download Pip</a>, as you’ll need that later in the course.</li>
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span>When you type <code class="language-plaintext highlighter-rouge">python file.py</code> in your terminal, a program called an <strong>interpreter</strong>,
which you downloaded together with Python, reads through your file line
by line, and executes each line of the code. This is different than
languages like <strong>C</strong> or <strong>Java</strong>, which need to be <strong>compiled</strong> into machine code before they can be run.</li>
</ul>
<a data-id="" id="variables" style="top: -81.3333px;"></a><h2><a data-id="" href="#variables">Variables</a></h2>
<p>A key part of any programming language is the ability to create and
manipulate variables. In order to assign a value to a variable in
Python, the syntax looks like this:</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">a</span> <span class="o">=</span> <span class="mi">28</span>
<span class="n">b</span> <span class="o">=</span> <span class="mf">1.5</span>
<span class="n">c</span> <span class="o">=</span> <span class="s">"Hello!"</span>
<span class="n">d</span> <span class="o">=</span> <span class="bp">True</span>
<span class="n">e</span> <span class="o">=</span> <span class="bp">None</span>
</code></pre></div></div>
<p>Each of these lines is taking the value to the right of the <code class="language-plaintext highlighter-rouge">=</code>, and storing it in the variable name to the left.</p>
<p>Unlike in some other programming languages, Python variable types are
inferred, meaning that while each variable does have a type, we do not
have to explicitly state which type it is when we create the variable.
Some of the most common variable types are:</p>
<ul class="fa-ul">
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><strong>int</strong>: An integer</li>
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><strong>float</strong>: A decimal number</li>
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><strong>chr</strong>: A single character</li>
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><strong>str</strong>: A string, or sequence of characters</li>
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><strong>bool</strong>: A value that is either <code class="language-plaintext highlighter-rouge">True</code> or <code class="language-plaintext highlighter-rouge">False</code></li>
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span><strong>NoneType</strong>: A special value (<code class="language-plaintext highlighter-rouge">None</code>) indicating the absence of a value.</li>
</ul>
<p>Now, we’ll work on writing a more interesting program that can take
input from the user and say hello to that user. To do this, we’ll use
another built in function called <code class="language-plaintext highlighter-rouge">input</code>
which displays a prompt to the user, and returns whatever the user
provides as input. For example, we can write the following in a file
called <code class="language-plaintext highlighter-rouge">name.py</code>:</p>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">name</span> <span class="o">=</span> <span class="nb">input</span><span class="p">(</span><span class="s">"Name: "</span><span class="p">)</span>
<span class="k">print</span><span class="p">(</span><span class="s">"Hello, "</span> <span class="o">+</span> <span class="n">name</span><span class="p">)</span>
</code></pre></div></div>
<p>When run on the terminal, this is what the program looks like:</p>
<p><img src="Lecture%202%20-%20CS50's%20Web%20Programming%20with%20Python%20and%20JavaScript_files/hello.png" alt="Hello"></p>
<p>A couple of things to point out here:</p>
<ul class="fa-ul">
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span>In the first line, instead of assigning the variable name to an explicit value, we’re assigning it to whatever the <code class="language-plaintext highlighter-rouge">input</code> function returns.</li>
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span>In the second line, we’re using the <code class="language-plaintext highlighter-rouge">+</code> operator to combine, or <strong>concatenate</strong>, two strings. In python, the <code class="language-plaintext highlighter-rouge">+</code> operator can be used to add numbers or concatenate strings and lists.</li>
</ul>
<a data-id="" id="formatting-strings" style="top: -81.3333px;"></a><h2><a data-id="" href="#formatting-strings">Formatting Strings</a></h2>
<ul class="fa-ul">
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span>While we can use the <code class="language-plaintext highlighter-rouge">+</code>
operator to combine strings as we did above, in the latest versions of
python, there are even easier ways to work with strings, known as <a href="https://realpython.com/python-f-strings/">formatted strings</a>, or <strong>f-strings</strong> for short.</li>
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span>To indicate that we’re using formatted strings, we simply add an <code class="language-plaintext highlighter-rouge">f</code> before the quotation marks. For example, instead of using <code class="language-plaintext highlighter-rouge">"Hello, " + name</code> as we did above, we could write <code class="language-plaintext highlighter-rouge">f"Hello, {name}"</code> for the same result. We can even plug a function into this string if we want, and turn our program above into the single line:</li>
</ul>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="k">print</span><span class="p">(</span><span class="sa">f</span><span class="s">"Hello, </span><span class="si">{</span><span class="nb">input</span><span class="p">(</span><span class="s">"Name: "</span><span class="p">)</span><span class="si">}</span><span class="s">"</span><span class="p">)</span>
</code></pre></div></div>
<a data-id="" id="conditions" style="top: -81.3333px;"></a><h2><a data-id="" href="#conditions">Conditions</a></h2>
<ul class="fa-ul">
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span>Just like in other programming languages, Python gives us the ability to run different segments of code based on different <a href="https://realpython.com/python-conditional-statements/">conditions</a>. For example, in the program below, we’ll change our output depending on the number a user types in:</li>
</ul>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">num</span> <span class="o">=</span> <span class="nb">input</span><span class="p">(</span><span class="s">"Number: "</span><span class="p">)</span>
<span class="k">if</span> <span class="n">num</span> <span class="o">></span> <span class="mi">0</span><span class="p">:</span>
<span class="k">print</span><span class="p">(</span><span class="s">"Number is positive"</span><span class="p">)</span>
<span class="k">elif</span> <span class="n">num</span> <span class="o"><</span> <span class="mi">0</span><span class="p">:</span>
<span class="k">print</span><span class="p">(</span><span class="s">"Number is negative"</span><span class="p">)</span>
<span class="k">else</span><span class="p">:</span>
<span class="k">print</span><span class="p">(</span><span class="s">"Number is 0"</span><span class="p">)</span>
</code></pre></div></div>
<ul class="fa-ul">
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span>Getting into how the above program works, conditionals in python contain a keyword (<code class="language-plaintext highlighter-rouge">if</code>, <code class="language-plaintext highlighter-rouge">elif</code>, or <code class="language-plaintext highlighter-rouge">else</code>) and then (except in the <code class="language-plaintext highlighter-rouge">else</code> case) a boolean expression, or an expression that evaluates to either <code class="language-plaintext highlighter-rouge">True</code> or <code class="language-plaintext highlighter-rouge">False</code>. Then, all of the code we want to run if a certain expression is true is <strong>indented</strong> directly below the statement. Indentation is required as part of the Python syntax.</li>
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span>However, when we run this program, we run into an <a href="https://docs.python.org/3/tutorial/errors.html">exception</a> that looks like this:</li>
</ul>
<p><img src="Lecture%202%20-%20CS50's%20Web%20Programming%20with%20Python%20and%20JavaScript_files/cond.png" alt="condition"></p>
<ul class="fa-ul">
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span>An
exception is what happens when an error occurs while we’re running our
python code, and over time you’ll get better and better at interpreting
these errors, which is a very valuable skill to have.</li>
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span>Let’s look a bit more closely at this specific exception: If we look at the bottom, we’ll see that we ran into a <code class="language-plaintext highlighter-rouge">TypeError</code>,
which generally means Python expected a certain variable to be of one
type, but found it to be of another type. In this case, the exception
tells us that we cannot use the <code class="language-plaintext highlighter-rouge">></code> symbol to compare a <code class="language-plaintext highlighter-rouge">str</code> and <code class="language-plaintext highlighter-rouge">int</code>, and then above we can see that this comparison occurs in line 2.</li>
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span>In this case, it’s obvious that <code class="language-plaintext highlighter-rouge">0</code> is an integer, so it must be the case that our <code class="language-plaintext highlighter-rouge">num</code> variable is a string. This is happening because it turns out that the <code class="language-plaintext highlighter-rouge">input</code> function always returns a string, and we have to specify that it should be turned into (or <strong>cast</strong> into) an integer using the <code class="language-plaintext highlighter-rouge">int</code> function. This means our first line would now look like:</li>
</ul>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">num</span> <span class="o">=</span> <span class="nb">int</span><span class="p">(</span><span class="nb">input</span><span class="p">(</span><span class="s">"Number: "</span><span class="p">))</span>
</code></pre></div></div>
<ul class="fa-ul">
<li data-marker="*"><span class="fa-li"><i class="fas fa-square"></i></span>Now, the program will work just as we intended!</li>
</ul>
<a data-id="" id="sequences" style="top: -81.3333px;"></a><h2><a data-id="" href="#sequences">Sequences</a></h2>
<p>One of the most powerful parts of the Python language is its ability to work with <strong>sequences</strong> of data in addition to individual variables.</p>
<p>There are several types of sequences that are similar in some ways,
but different in others. When explaining those differences, we’ll use
the terms <strong>mutable/immutable</strong> and <strong>ordered/unordered</strong>. <strong>Mutable</strong> means that once a sequence has been defined, we can change individual elements of that sequence, and <strong>ordered</strong> means that the order of the objects matters.</p>
<a data-id="" id="strings" style="top: -81.3333px;"></a><h3><a data-id="" href="#strings">Strings</a></h3>
<dl>
<dt>: <strong>Ordered</strong>: Yes</dt>
<dd>
<p><strong>Mutable</strong>: No</p>
</dd>
<dd>
<p>We’ve already looked at strings a little bit, but instead of just
variables, we can think of a string as a sequence of characters. This
means we can access individual elements within the string! For example:</p>
</dd>
</dl>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="n">name</span> <span class="o">=</span> <span class="s">"Harry"</span>
<span class="k">print</span><span class="p">(</span><span class="n">name</span><span class="p">[</span><span class="mi">0</span><span class="p">])</span>
<span class="k">print</span><span class="p">(</span><span class="n">name</span><span class="p">[</span><span class="mi">1</span><span class="p">])</span>
</code></pre></div></div>
<p>prints out the first (or index-0) character in the string, which in this case happens to be <code class="language-plaintext highlighter-rouge">H</code>, and then prints out the second (or index-1) character, which is <code class="language-plaintext highlighter-rouge">a</code>.</p>
<a data-id="" id="lists" style="top: -81.3333px;"></a><h3><a data-id="" href="#lists">Lists</a></h3>
<dl>
<dt>: <strong>Ordered</strong>: Yes</dt>
<dd>
<p><strong>Mutable</strong>: Yes</p>
</dd>
<dd>
<p>A <a href="https://www.w3schools.com/python/python_lists.asp">Python list</a>
allows you to store any variable types. We create a list using square
brackets and commas, as shown below. Similarly to strings, we can print
an entire list, or some individual elements. We can also add elements to
a list using <code class="language-plaintext highlighter-rouge">append</code>, and sort a list using <code class="language-plaintext highlighter-rouge">sort</code></p>
</dd>
</dl>
<div class="language-python highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="c1"># This is a Python comment
</span><span class="n">names</span> <span class="o">=</span> <span class="p">[</span><span class="s">"Harry"</span><span class="p">,</span> <span class="s">"Ron"</span><span class="p">,</span> <span class="s">"Hermione"</span><span class="p">]</span>
<span class="c1"># Print the entire list:
</span><span class="k">print</span><span class="p">(</span><span class="n">names</span><span class="p">)</span>
<span class="c1"># Print the second element of the list:
</span><span class="k">print</span><span class="p">(</span><span class="n">names</span><span class="p">[</span><span class="mi">1</span><span class="p">])</span>
<span class="c1"># Add a new name to the list:
</span><span class="n">names</span><span class="p">.</span><span class="n">append</span><span class="p">(</span><span class="s">"Draco"</span><span class="p">)</span>
<span class="c1"># Sort the list: