Readings/Automate-Boring-Stuff-with-Python.html

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    <h1 id="automate-the-boring-stuff-with-python">
      Automate the Boring Stuff with Python
    </h1>
    <blockquote>
      <p>
        Variables are a fine way to store data while your program is running,
        but if you want your data to persist even after your program has
        finished, you need to save it to a file. You can think of a file’s
        contents as a single string value, potentially gigabytes in size. In
        this chapter, you will learn how to use Python to create, read, and save
        files on the hard drive.
      </p>
    </blockquote>
    <p>
      Variables are a fine way to store data while your program is running, but
      if you want your data to persist even after your program has finished, you
      need to save it to a file. You can think of a file’s contents as a single
      string value, potentially gigabytes in size. In this chapter, you will
      learn how to use Python to create, read, and save files on the hard drive.
    </p>
    <p>
      A file has two key properties: a <em>filename</em> (usually written as one
      word) and a <em>path</em>. The path specifies the location of a file on
      the computer. For example, there is a file on my Windows 7 laptop with the
      filename <em>project.docx</em> in the path
      <em>C:\Users\asweigart\Documents</em>. The part of the filename after the
      last period is called the file’s <em>extension</em> and tells you a file’s
      type. <em>project.docx</em> is a Word document, and <em>Users</em>,
      <em>asweigart</em>, and <em>Documents</em> all refer to
      <em>folders</em> (also called <em>directories</em>). Folders can contain
      files and other folders. For example, <em>project.docx</em> is in the
      <em>Documents</em> folder, which is inside the <em>asweigart</em> folder,
      which is inside the <em>Users</em> folder.
      <a
        href="#calibre_link-82"
        title="Figure 8-1. A file in a hierarchy of folders"
        >Figure 8-1</a
      >
      shows this folder organization.
    </p>
    <figure>
      <img
        src="chrome-extension://cjedbglnccaioiolemnfhjncicchinao/images/000027.jpg"
        alt="A file in a hierarchy of folders"
      />
      <figcaption>A file in a hierarchy of folders</figcaption>
    </figure>
    <p>Figure 8-1. A file in a hierarchy of folders</p>
    <p>
      The <em>C:\</em> part of the path is the <em>root folder</em>, which
      contains all other folders. On Windows, the root folder is named
      <em>C:\</em> and is also called the <em>C: drive</em>. On OS X and Linux,
      the root folder is <em>/</em>. In this book, I’ll be using the
      Windows-style root folder, <em>C:\</em>. If you are entering the
      interactive shell examples on OS X or Linux, enter <code>/</code> instead.
    </p>
    <p>
      Additional <em>volumes</em>, such as a DVD drive or USB thumb drive, will
      appear differently on different operating systems. On Windows, they appear
      as new, lettered root drives, such as <em>D:\</em> or <em>E:\</em>. On OS
      X, they appear as new folders under the <em>/Volumes</em> folder. On
      Linux, they appear as new folders under the <em>/mnt</em> (“mount”)
      folder. Also note that while folder names and filenames are not case
      sensitive on Windows and OS X, they are case sensitive on Linux.
    </p>
    <h2 id="backslash-on-windows-and-forward-slash-on-os-x-and-linux">
      Backslash on Windows and Forward Slash on OS X and Linux
    </h2>
    <p>
      On Windows, paths are written using backslashes (<em>\</em>) as the
      separator between folder names. OS X and Linux, however, use the forward
      slash (<em>/</em>) as their path separator. If you want your programs to
      work on all operating systems, you will have to write your Python scripts
      to handle both cases.
    </p>
    <p>
      Fortunately, this is simple to do with the
      <code>os.path.join()</code> function. If you pass it the string values of
      individual file and folder names in your path,
      <code>os.path.join()</code> will return a string with a file path using
      the correct path separators. Enter the following into the interactive
      shell:
    </p>
    <p>&gt;&gt;&gt; <strong>import os</strong></p>
    <blockquote>
      <blockquote>
        <blockquote>
          <p>
            <strong>os.path.join(‘usr’, ‘bin’, ‘spam’)</strong> ‘usr\\bin\\spam’
          </p>
        </blockquote>
      </blockquote>
    </blockquote>
    <p>
      I’m running these interactive shell examples on Windows, so
      <code>os.path.join('usr', 'bin', 'spam')</code> returned
      <code>'usr\\bin\\spam'</code>. (Notice that the backslashes are doubled
      because each backslash needs to be escaped by another backslash
      character.) If I had called this function on OS X or Linux, the string
      would have been <code>'usr/bin/spam'</code>.
    </p>
    <p>
      The os.path.join() function is helpful if you need to create strings for
      filenames. These strings will be passed to several of the file-related
      functions introduced in this chapter. For example, the following example
      joins names from a list of filenames to the end of a folder’s name:
    </p>
    <p>
      &gt;&gt;&gt;
      <strong>myFiles = [‘accounts.txt’, ‘details.csv’, ‘invite.docx’]</strong>
    </p>
    <blockquote>
      <blockquote>
        <blockquote>
          <p><strong>for filename in myFiles:</strong></p>
        </blockquote>
      </blockquote>
    </blockquote>
    <pre><code>    print(os.path.join(&#39;C:\\\\Users\\\\asweigart&#39;, filename))</code></pre>
    <p>
      C:\Users\asweigart\accounts.txt C:\Users\asweigart\details.csv
      C:\Users\asweigart\invite.docx
    </p>
    <h2 id="the-current-working-directory">The Current Working Directory</h2>
    <p>
      Every program that runs on your computer has a
      <em>current working directory</em>, or <em>cwd</em>. Any filenames or
      paths that do not begin with the root folder are assumed to be under the
      current working directory. You can get the current working directory as a
      string value with the <code>os.getcwd()</code> function and change it with
      <code>os.chdir()</code>. Enter the following into the interactive shell:
    </p>
    <p>&gt;&gt;&gt; <strong>import os</strong></p>
    <blockquote>
      <blockquote>
        <blockquote>
          <p>
            <strong>os.getcwd()</strong> ‘C:\\Python34’
            <strong>os.chdir(‘C:\\Windows\\System32’)</strong> &gt;&gt;&gt;
            <strong>os.getcwd()</strong> ‘C:\\Windows\\System32’
          </p>
        </blockquote>
      </blockquote>
    </blockquote>
    <p>
      Here, the current working directory is set to <em>C:\Python34</em>, so the
      filename <em>project.docx</em> refers to
      <em>C:\Python34\project.docx</em>. When we change the current working
      directory to <em>C:\Windows</em>, <em>project.docx</em> is interpreted as
      <em>C:\ Windows\project.docx</em>.
    </p>
    <p>
      Python will display an error if you try to change to a directory that does
      not exist.
    </p>
    <p>
      &gt;&gt;&gt;
      <strong>os.chdir(‘C:\\ThisFolderDoesNotExist’)</strong> Traceback (most
      recent call last): File “&lt;pyshell#18&gt;”, line 1, in
      os.chdir(‘C:\\ThisFolderDoesNotExist’) FileNotFoundError: [WinError 2] The
      system cannot find the file specified: ‘C:\\ThisFolderDoesNotExist’
    </p>
    <h3 id="note">Note</h3>
    <p>
      <em>While folder is the more modern name for directory, note that</em>
      current working directory <em>(or just</em> working directory<em
        >) is the standard term, not current working folder.</em
      >
    </p>
    <h2 id="absolute-vs.-relative-paths">Absolute vs. Relative Paths</h2>
    <p>There are two ways to specify a file path.</p>
    <ul>
      <li>
        An <em>absolute path</em>, which always begins with the root folder
      </li>
      <li>
        A <em>relative path</em>, which is relative to the program’s current
        working directory
      </li>
    </ul>
    <p>
      There are also the <em>dot</em> (<code>.</code>) and
      <em>dot-dot</em> (<code>..</code>) folders. These are not real folders but
      special names that can be used in a path. A single period (“dot”) for a
      folder name is shorthand for “this directory.” Two periods (“dot-dot”)
      means “the parent folder.”
    </p>
    <p>
      <a
        href="#calibre_link-83"
        title="Figure 8-2. The relative paths for folders and files in the working directory C:\bacon"
        >Figure 8-2</a
      >
      is an example of some folders and files. When the current working
      directory is set to <em>C:\bacon</em>, the relative paths for the other
      folders and files are set as they are in the figure.
    </p>
    <figure>
      <img
        src="chrome-extension://cjedbglnccaioiolemnfhjncicchinao/images/000032.jpg"
        alt="The relative paths for folders and files in the working directory C:"
      />
      <figcaption>
        The relative paths for folders and files in the working directory C:
      </figcaption>
    </figure>
    <p>
      Figure 8-2. The relative paths for folders and files in the working
      directory <em>C:\bacon</em>
    </p>
    <p>
      The <em>.\</em> at the start of a relative path is optional. For example,
      <em>.\spam.txt</em> and <em>spam.txt</em> refer to the same file.
    </p>
    <h2 id="creating-new-folders-with-os.makedirs">
      Creating New Folders with os.makedirs()
    </h2>
    <p>
      Your programs can create new folders (directories) with the
      <code>os.makedirs()</code> function. Enter the following into the
      interactive shell:
    </p>
    <p>&gt;&gt;&gt; <strong>import os</strong></p>
    <blockquote>
      <blockquote>
        <blockquote>
          <p><strong>os.makedirs(‘C:\\delicious\\walnut\\waffles’)</strong></p>
        </blockquote>
      </blockquote>
    </blockquote>
    <p>
      This will create not just the <em>C:\delicious</em> folder but also a
      <em>walnut</em> folder inside <em>C:\delicious</em> and a
      <em>waffles</em> folder inside <em>C:\delicious\walnut</em>. That is,
      <code>os.makedirs()</code> will create any necessary intermediate folders
      in order to ensure that the full path exists.
      <a
        href="#calibre_link-84"
        title="Figure 8-3. The result of os.makedirs(&#39;C:\delicious \walnut\waffles&#39;)"
        >Figure 8-3</a
      >
      shows this hierarchy of folders.
    </p>
    <figure>
      <img
        src="chrome-extension://cjedbglnccaioiolemnfhjncicchinao/images/000036.jpg"
        alt="The result of os.makedirs(‘C:’)"
      />
      <figcaption>The result of os.makedirs(‘C:’)</figcaption>
    </figure>
    <p>
      Figure 8-3. The result of
      <code>os.makedirs('C:\\delicious \\walnut\\waffles')</code>
    </p>
    <p>
      The <code>os.path</code> module contains many helpful functions related to
      filenames and file paths. For instance, you’ve already used
      <code>os.path.join()</code> to build paths in a way that will work on any
      operating system. Since <code>os.path</code> is a module inside the
      <code>os</code> module, you can import it by simply running
      <code>import os</code>. Whenever your programs need to work with files,
      folders, or file paths, you can refer to the short examples in this
      section. The full documentation for the <code>os.path</code> module is on
      the Python website at
      <em
        ><a href="http://docs.python.org/3/library/os.path.html" class="uri"
          >http://docs.python.org/3/library/os.path.html</a
        ></em
      >.
    </p>
    <h3 id="note-1">Note</h3>
    <p>
      <em>Most of the examples that follow in this section will require the</em>
      <code>os</code>
      <em
        >module, so remember to import it at the beginning of any script you
        write and any time you restart IDLE. Otherwise, you’ll get a</em
      >
      <code>NameError: name 'os' is not defined</code> <em>error message.</em>
    </p>
    <h2 id="handling-absolute-and-relative-paths">
      Handling Absolute and Relative Paths
    </h2>
    <p>
      The <code>os.path</code> module provides functions for returning the
      absolute path of a relative path and for checking whether a given path is
      an absolute path.
    </p>
    <ul>
      <li>
        Calling <code>os.path.abspath(</code><em><code>path</code></em
        ><code>)</code> will return a string of the absolute path of the
        argument. This is an easy way to convert a relative path into an
        absolute one.
      </li>
      <li>
        Calling <code>os.path.isabs(</code><em><code>path</code></em
        ><code>)</code> will return <code>True</code> if the argument is an
        absolute path and <code>False</code> if it is a relative path.
      </li>
      <li>
        Calling <code>os.path.relpath(</code><em><code>path, start</code></em
        ><code>)</code> will return a string of a relative path from the
        <em><code>start</code></em> path to <em><code>path</code></em
        >. If <em><code>start</code></em> is not provided, the current working
        directory is used as the start path.
      </li>
    </ul>
    <p>Try these functions in the interactive shell:</p>
    <p>&gt;&gt;&gt; <strong>os.path.abspath(‘.’)</strong> ‘C:\\Python34’</p>
    <blockquote>
      <blockquote>
        <blockquote>
          <p>
            <strong>os.path.abspath(‘.\\Scripts’)</strong>
            ‘C:\\Python34\\Scripts’ <strong>os.path.isabs(‘.’)</strong> False
            <strong>os.path.isabs(os.path.abspath(‘.’))</strong> True
          </p>
        </blockquote>
      </blockquote>
    </blockquote>
    <p>
      Since <em>C:\Python34</em> was the working directory when
      <code>os.path.abspath()</code> was called, the “single-dot” folder
      represents the absolute path <code>'C:\\Python34'</code>.
    </p>
    <h3 id="note-2">Note</h3>
    <p>
      <em
        >Since your system probably has different files and folders on it than
        mine, you won’t be able to follow every example in this chapter exactly.
        Still, try to follow along using folders that exist on your
        computer.</em
      >
    </p>
    <p>
      Enter the following calls to <code>os.path.relpath()</code> into the
      interactive shell:
    </p>
    <p>
      &gt;&gt;&gt;
      <strong>os.path.relpath(‘C:\\Windows’, ‘C:\\’)</strong> ‘Windows’
    </p>
    <blockquote>
      <blockquote>
        <blockquote>
          <p>
            <strong>os.path.relpath(‘C:\\Windows’, ‘C:\\spam\\eggs’)</strong>
            ‘..\\..\\Windows’ <strong>os.getcwd()</strong> ‘C:\\Python34’
          </p>
        </blockquote>
      </blockquote>
    </blockquote>
    <p>
      Calling <code>os.path.dirname(</code><em><code>path</code></em
      ><code>)</code> will return a string of everything that comes before the
      last slash in the <code>path</code> argument. Calling
      <code>os.path.basename(</code><em><code>path</code></em
      ><code>)</code> will return a string of everything that comes after the
      last slash in the <code>path</code> argument. The dir name and base name
      of a path are outlined in
      <a
        href="#calibre_link-85"
        title="Figure 8-4. The base name follows the last slash in a path and is the same as the filename. The dir name is everything before the last slash."
        >Figure 8-4</a
      >.
    </p>
    <figure>
      <img
        src="chrome-extension://cjedbglnccaioiolemnfhjncicchinao/images/000041.png"
        alt="The base name follows the last slash in a path and is the same as the filename. The dir name is everything before the last slash."
      />
      <figcaption>
        The base name follows the last slash in a path and is the same as the
        filename. The dir name is everything before the last slash.
      </figcaption>
    </figure>
    <p>
      Figure 8-4. The base name follows the last slash in a path and is the same
      as the filename. The dir name is everything before the last slash.
    </p>
    <p>For example, enter the following into the interactive shell:</p>
    <p>
      &gt;&gt;&gt; <strong>path = ‘C:\\Windows\\System32\\calc.exe’</strong>
    </p>
    <blockquote>
      <blockquote>
        <blockquote>
          <p>
            <strong>os.path.basename(path)</strong> ‘calc.exe’
            <strong>os.path.dirname(path)</strong> ‘C:\\Windows\\System32’
          </p>
        </blockquote>
      </blockquote>
    </blockquote>
    <p>
      If you need a path’s dir name and base name together, you can just call
      <code>os.path.split()</code> to get a tuple value with these two strings,
      like so:
    </p>
    <p>
      &gt;&gt;&gt;
      <strong>calcFilePath = ‘C:\\Windows\\System32\\calc.exe’</strong>
    </p>
    <blockquote>
      <blockquote>
        <blockquote>
          <p>
            <strong>os.path.split(calcFilePath)</strong>
            (‘C:\\Windows\\System32’, ‘calc.exe’)
          </p>
        </blockquote>
      </blockquote>
    </blockquote>
    <p>
      Notice that you could create the same tuple by calling
      <code>os.path.dirname()</code> and <code>os.path.basename()</code> and
      placing their return values in a tuple.
    </p>
    <p>
      &gt;&gt;&gt;
      <strong
        >(os.path.dirname(calcFilePath), os.path.basename(calcFilePath))</strong
      >
      (‘C:\\Windows\\System32’, ‘calc.exe’)
    </p>
    <p>
      But <code>os.path.split()</code> is a nice shortcut if you need both
      values.
    </p>
    <p>
      Also, note that <code>os.path.split()</code> does <em>not</em> take a file
      path and return a list of strings of each folder. For that, use the
      <code>split()</code> string method and split on the string in
      <code>os.sep</code>. Recall from earlier that the
      <code>os.sep</code> variable is set to the correct folder-separating slash
      for the computer running the program.
    </p>
    <p>For example, enter the following into the interactive shell:</p>
    <p>
      &gt;&gt;&gt; <strong>calcFilePath.split(os.path.sep)</strong> [‘C:’,
      ‘Windows’, ‘System32’, ‘calc.exe’]
    </p>
    <p>
      On OS X and Linux systems, there will be a blank string at the start of
      the returned list:
    </p>
    <p>
      &gt;&gt;&gt; <strong>‘/usr/bin’.split(os.path.sep)</strong> [’‘, ’usr’,
      ‘bin’]
    </p>
    <p>
      The <code>split()</code> string method will work to return a list of each
      part of the path. It will work on any operating system if you pass it
      <code>os.path.sep</code>.
    </p>
    <h2 id="finding-file-sizes-and-folder-contents">
      Finding File Sizes and Folder Contents
    </h2>
    <p>
      Once you have ways of handling file paths, you can then start gathering
      information about specific files and folders. The
      <code>os.path</code> module provides functions for finding the size of a
      file in bytes and the files and folders inside a given folder.
    </p>
    <ul>
      <li>
        Calling <code>os.path.getsize(</code><em><code>path</code></em
        ><code>)</code> will return the size in bytes of the file in the
        <em><code>path</code></em> argument.
      </li>
      <li>
        Calling <code>os.listdir(</code><em><code>path</code></em
        ><code>)</code> will return a list of filename strings for each file in
        the <em><code>path</code></em> argument. (Note that this function is in
        the <code>os</code> module, not <code>os.path</code>.)
      </li>
    </ul>
    <p>
      Here’s what I get when I try these functions in the interactive shell:
    </p>
    <p>
      &gt;&gt;&gt;
      <strong>os.path.getsize(‘C:\\Windows\\System32\\calc.exe’)</strong> 776192
    </p>
    <blockquote>
      <blockquote>
        <blockquote>
          <p>
            <strong>os.listdir(‘C:\\Windows\\System32’)</strong> &gt; &gt; &gt;
            [‘0409’, ‘12520437.cpx’, ‘12520850.cpx’, ‘5U877.ax’, ‘aaclient.dll’,
            –<em>snip</em>– ‘xwtpdui.dll’, ‘xwtpw32.dll’, ‘zh-CN’, ‘zh-HK’,
            ‘zh-TW’, ‘zipfldr.dll’]
          </p>
        </blockquote>
      </blockquote>
    </blockquote>
    <p>
      As you can see, the <em>calc.exe</em> program on my computer is 776,192
      bytes in size, and I have a lot of files in <em>C:\Windows\system32</em>.
      If I want to find the total size of all the files in this directory, I can
      use <code>os.path.getsize()</code> and <code>os.listdir()</code> together.
    </p>
    <p>&gt;&gt;&gt; <strong>totalSize = 0</strong></p>
    <blockquote>
      <blockquote>
        <blockquote>
          <p>
            <strong
              >for filename in os.listdir(‘C:\\Windows\\System32’):</strong
            >
          </p>
        </blockquote>
      </blockquote>
    </blockquote>
    <pre><code>  **totalSize = totalSize + os.path.getsize(os.path.join(&#39;C:\\\\Windows\\\\System32&#39;, filename))**</code></pre>
    <blockquote>
      <blockquote>
        <blockquote>
          <p><strong>print(totalSize)</strong> 1117846456</p>
        </blockquote>
      </blockquote>
    </blockquote>
    <p>
      As I loop over each filename in the <em>C:\Windows\System32</em> folder,
      the <code>totalSize</code> variable is incremented by the size of each
      file. Notice how when I call <code>os.path.getsize()</code>, I use
      <code>os.path.join()</code> to join the folder name with the current
      filename. The integer that <code>os.path.getsize()</code> returns is added
      to the value of <code>totalSize</code>. After looping through all the
      files, I print <code>totalSize</code> to see the total size of the
      <em>C:\Windows\System32</em> folder.
    </p>
    <p>
      Many Python functions will crash with an error if you supply them with a
      path that does not exist. The <code>os.path</code> module provides
      functions to check whether a given path exists and whether it is a file or
      folder.
    </p>
    <ul>
      <li>
        Calling <code>os.path.exists(</code><em><code>path</code></em
        ><code>)</code> will return <code>True</code> if the file or folder
        referred to in the argument exists and will return <code>False</code> if
        it does not exist.
      </li>
      <li>
        Calling <code>os.path.isfile(</code><em><code>path</code></em
        ><code>)</code> will return <code>True</code> if the path argument
        exists and is a file and will return <code>False</code> otherwise.
      </li>
      <li>
        Calling <code>os.path.isdir(</code><em><code>path</code></em
        ><code>)</code> will return <code>True</code> if the path argument
        exists and is a folder and will return <code>False</code> otherwise.
      </li>
    </ul>
    <p>
      Here’s what I get when I try these functions in the interactive shell:
    </p>
    <p>&gt;&gt;&gt; <strong>os.path.exists(‘C:\\Windows’)</strong> True</p>
    <blockquote>
      <blockquote>
        <blockquote>
          <p>
            <strong>os.path.exists(‘C:\\some_made_up_folder’)</strong> False
            <strong>os.path.isdir(‘C:\\Windows\\System32’)</strong> True
            <strong>os.path.isfile(‘C:\\Windows\\System32’)</strong> False
            <strong>os.path.isdir(‘C:\\Windows\\System32\\calc.exe’)</strong>
            False
            <strong>os.path.isfile(‘C:\\Windows\\System32\\calc.exe’)</strong>
            True
          </p>
        </blockquote>
      </blockquote>
    </blockquote>
    <p>
      You can determine whether there is a DVD or flash drive currently attached
      to the computer by checking for it with the
      <code>os.path.exists()</code> function. For instance, if I wanted to check
      for a flash drive with the volume named <em>D:\</em> on my Windows
      computer, I could do that with the following:
    </p>
    <p>&gt;&gt;&gt; <strong>os.path.exists(‘D:\\’)</strong> False</p>
    <p>Oops! It looks like I forgot to plug in my flash drive.</p>
    <p>
      Once you are comfortable working with folders and relative paths, you’ll
      be able to specify the location of files to read and write. The functions
      covered in the next few sections will apply to plaintext files.
      <em>Plaintext files</em> contain only basic text characters and do not
      include font, size, or color information. Text files with the
      <em>.txt</em> extension or Python script files with the
      <em>.py</em> extension are examples of plaintext files. These can be
      opened with Windows’s Notepad or OS X’s TextEdit application. Your
      programs can easily read the contents of plaintext files and treat them as
      an ordinary string value.
    </p>
    <p>
      <em>Binary files</em> are all other file types, such as word processing
      documents, PDFs, images, spreadsheets, and executable programs. If you
      open a binary file in Notepad or TextEdit, it will look like scrambled
      nonsense, like in
      <a
        href="#calibre_link-86"
        title="Figure 8-5. The Windows calc.exe program opened in Notepad"
        >Figure 8-5</a
      >.
    </p>
    <figure>
      <img
        src="chrome-extension://cjedbglnccaioiolemnfhjncicchinao/images/000046.jpg"
        alt="The Windows calc.exe program opened in Notepad"
      />
      <figcaption>The Windows calc.exe program opened in Notepad</figcaption>
    </figure>
    <p>
      Figure 8-5. The Windows <code>calc.exe</code> program opened in Notepad
    </p>
    <p>
      Since every different type of binary file must be handled in its own way,
      this book will not go into reading and writing raw binary files directly.
      Fortunately, many modules make working with binary files easier—you will
      explore one of them, the <code>shelve</code> module, later in this
      chapter.
    </p>
    <p>There are three steps to reading or writing files in Python.</p>
    <ol type="1">
      <li>
        Call the <code>open()</code> function to return a
        <code>File</code> object.
      </li>
      <li>
        Call the <code>read()</code> or <code>write()</code> method on the
        <code>File</code> object.
      </li>
      <li>
        Close the file by calling the <code>close()</code> method on the
        <code>File</code> object.
      </li>
    </ol>
    <h2 id="opening-files-with-the-open-function">
      Opening Files with the open() Function
    </h2>
    <p>
      To open a file with the <code>open()</code> function, you pass it a string
      path indicating the file you want to open; it can be either an absolute or
      relative path. The <code>open()</code> function returns a
      <code>File</code> object.
    </p>
    <p>
      Try it by creating a text file named <em>hello.txt</em> using Notepad or
      TextEdit. Type <strong><code>Hello world!</code></strong> as the content
      of this text file and save it in your user home folder. Then, if you’re
      using Windows, enter the following into the interactive shell:
    </p>
    <p>
      &gt;&gt;&gt;
      <strong>helloFile = open(‘C:\\Users\\***</strong
      >your_home_folder<strong>*</strong>\\hello.txt’)**
    </p>
    <p>
      If you’re using OS X, enter the following into the interactive shell
      instead:
    </p>
    <p>
      &gt;&gt;&gt;
      <strong>helloFile = open(‘/Users/***</strong
      >your_home_folder<strong>*</strong>/hello.txt’)**
    </p>
    <p>
      Make sure to replace <em><code>your_home_folder</code></em> with your
      computer username. For example, my username is <em>asweigart</em>, so I’d
      enter <code>'C:\\Users\\asweigart\\ hello.txt'</code> on Windows.
    </p>
    <p>
      Both these commands will open the file in “reading plaintext” mode, or
      <em>read mode</em> for short. When a file is opened in read mode, Python
      lets you only read data from the file; you can’t write or modify it in any
      way. Read mode is the default mode for files you open in Python. But if
      you don’t want to rely on Python’s defaults, you can explicitly specify
      the mode by passing the string value <code>'r'</code> as a second argument
      to <code>open()</code>. So
      <code>open('/Users/asweigart/ hello.txt', 'r')</code> and
      <code>open('/Users/asweigart/hello.txt')</code> do the same thing.
    </p>
    <p>
      The call to <code>open()</code> returns a <code>File</code> object. A
      <code>File</code> object represents a file on your computer; it is simply
      another type of value in Python, much like the lists and dictionaries
      you’re already familiar with. In the previous example, you stored the
      <code>File</code> object in the variable <code>helloFile</code>. Now,
      whenever you want to read from or write to the file, you can do so by
      calling methods on the <code>File</code> object in <code>helloFile</code>.
    </p>
    <h2 id="reading-the-contents-of-files">Reading the Contents of Files</h2>
    <p>
      Now that you have a <code>File</code> object, you can start reading from
      it. If you want to read the entire contents of a file as a string value,
      use the <code>File</code> object’s <code>read()</code> method. Let’s
      continue with the <em>hello.txt</em> <code>File</code> object you stored
      in <code>helloFile</code>. Enter the following into the interactive shell:
    </p>
    <p>&gt;&gt;&gt; <strong>helloContent = helloFile.read()</strong></p>
    <blockquote>
      <blockquote>
        <blockquote>
          <p><strong>helloContent</strong> ‘Hello world!’</p>
        </blockquote>
      </blockquote>
    </blockquote>
    <p>
      If you think of the contents of a file as a single large string value, the
      <code>read()</code> method returns the string that is stored in the file.
    </p>
    <p>
      Alternatively, you can use the <code>readlines()</code> method to get a
      <em>list</em> of string values from the file, one string for each line of
      text. For example, create a file named <em>sonnet29.txt</em> in the same
      directory as <em>hello.txt</em> and write the following text in it:
    </p>
    <p>
      When, in disgrace with fortune and men’s eyes, I all alone beweep my
      outcast state, And trouble deaf heaven with my bootless cries, And look
      upon myself and curse my fate,
    </p>
    <p>
      Make sure to separate the four lines with line breaks. Then enter the
      following into the interactive shell:
    </p>
    <p>&gt;&gt;&gt; <strong>sonnetFile = open(‘sonnet29.txt’)</strong></p>
    <blockquote>
      <blockquote>
        <blockquote>
          <p>
            <strong>sonnetFile.readlines()</strong> &gt; &gt; &gt; [When, in
            disgrace with fortune and men’s eyes,\n’, ’ I all alone beweep my
            outcast state,\n’, And trouble deaf heaven with my bootless
            cries,\n’, And look upon myself and curse my fate,’]
          </p>
        </blockquote>
      </blockquote>
    </blockquote>
    <p>
      Note that each of the string values ends with a newline character,
      <code>\n</code>, except for the last line of the file. A list of strings
      is often easier to work with than a single large string value.
    </p>
    <p>
      Python allows you to write content to a file in a way similar to how the
      <code>print()</code> function “writes” strings to the screen. You can’t
      write to a file you’ve opened in read mode, though. Instead, you need to
      open it in “write plaintext” mode or “append plaintext” mode, or
      <em>write mode</em> and <em>append mode</em> for short.
    </p>
    <p>
      Write mode will overwrite the existing file and start from scratch, just
      like when you overwrite a variable’s value with a new value. Pass
      <code>'w'</code> as the second argument to <code>open()</code> to open the
      file in write mode. Append mode, on the other hand, will append text to
      the end of the existing file. You can think of this as appending to a list
      in a variable, rather than overwriting the variable altogether. Pass
      <code>'a'</code> as the second argument to <code>open()</code> to open the
      file in append mode.
    </p>
    <p>
      If the filename passed to <code>open()</code> does not exist, both write
      and append mode will create a new, blank file. After reading or writing a
      file, call the <code>close()</code> method before opening the file again.
    </p>
    <p>
      Let’s put these concepts together. Enter the following into the
      interactive shell:
    </p>
    <p>&gt;&gt;&gt; <strong>baconFile = open(‘bacon.txt’, ‘w’)</strong></p>
    <blockquote>
      <blockquote>
        <blockquote>
          <p>
            <strong>baconFile.write(‘Hello world!\n’)</strong> 13
            <strong>baconFile.close()</strong> &gt;&gt;&gt;
            <strong>baconFile = open(‘bacon.txt’, ‘a’)</strong> &gt;&gt;&gt;
            <strong>baconFile.write(‘Bacon is not a vegetable.’)</strong> 25
            <strong>baconFile.close()</strong> &gt;&gt;&gt;
            <strong>baconFile = open(‘bacon.txt’)</strong> &gt;&gt;&gt;
            <strong>content = baconFile.read()</strong> &gt;&gt;&gt;
            <strong>baconFile.close()</strong> &gt;&gt;&gt;
            <strong>print(content)</strong> Hello world! Bacon is not a
            vegetable.
          </p>
        </blockquote>
      </blockquote>
    </blockquote>
    <p>
      First, we open <em>bacon.txt</em> in write mode. Since there isn’t a
      <em>bacon.txt</em> yet, Python creates one. Calling
      <code>write()</code> on the opened file and passing
      <code>write()</code> the string argument
      <code>'Hello world! /n'</code> writes the string to the file and returns
      the number of characters written, including the newline. Then we close the
      file.
    </p>
    <p>
      To add text to the existing contents of the file instead of replacing the
      string we just wrote, we open the file in append mode. We write
      <code>'Bacon is not a vegetable.'</code> to the file and close it.
      Finally, to print the file contents to the screen, we open the file in its
      default read mode, call <code>read()</code>, store the resulting
      <code>File</code> object in <code>content</code>, close the file, and
      print <code>content</code>.
    </p>
    <p>
      Note that the <code>write()</code> method does not automatically add a
      newline character to the end of the string like the
      <code>print()</code> function does. You will have to add this character
      yourself.
    </p>
    <p>
      You can save variables in your Python programs to binary shelf files using
      the <code>shelve</code> module. This way, your program can restore data to
      variables from the hard drive. The <code>shelve</code> module will let you
      add Save and Open features to your program. For example, if you ran a
      program and entered some configuration settings, you could save those
      settings to a shelf file and then have the program load them the next time
      it is run.
    </p>
    <p>Enter the following into the interactive shell:</p>
    <p>&gt;&gt;&gt; <strong>import shelve</strong></p>
    <blockquote>
      <blockquote>
        <blockquote>
          <p>
            <strong>shelfFile = shelve.open(‘mydata’)</strong> &gt;&gt;&gt;
            <strong>cats = [‘Zophie’, ‘Pooka’, ‘Simon’]</strong> &gt;&gt;&gt;
            <strong>shelfFile[‘cats’] = cats</strong> &gt;&gt;&gt;
            <strong>shelfFile.close()</strong>
          </p>
        </blockquote>
      </blockquote>
    </blockquote>
    <p>
      To read and write data using the <code>shelve</code> module, you first
      import <code>shelve</code>. Call <code>shelve.open()</code> and pass it a
      filename, and then store the returned shelf value in a variable. You can
      make changes to the shelf value as if it were a dictionary. When you’re
      done, call <code>close()</code> on the shelf value. Here, our shelf value
      is stored in <code>shelfFile</code>. We create a list
      <code>cats</code> and write <code>shelfFile['cats'] = cats</code> to store
      the list in <code>shelfFile</code> as a value associated with the key
      <code>'cats'</code> (like in a dictionary). Then we call
      <code>close()</code> on <code>shelfFile</code>.
    </p>
    <p>
      After running the previous code on Windows, you will see three new files
      in the current working directory: <em>mydata.bak</em>,
      <em>mydata.dat</em>, and <em>mydata.dir</em>. On OS X, only a single
      <em>mydata.db</em> file will be created.
    </p>
    <p>
      These binary files contain the data you stored in your shelf. The format
      of these binary files is not important; you only need to know what the
      <code>shelve</code> module does, not how it does it. The module frees you
      from worrying about how to store your program’s data to a file.
    </p>
    <p>
      Your programs can use the <code>shelve</code> module to later reopen and
      retrieve the data from these shelf files. Shelf values don’t have to be
      opened in read or write mode—they can do both once opened. Enter the
      following into the interactive shell:
    </p>
    <p>&gt;&gt;&gt; <strong>shelfFile = shelve.open(‘mydata’)</strong></p>
    <blockquote>
      <blockquote>
        <blockquote>
          <p>
            <strong>type(shelfFile)</strong> &lt;class
            ‘shelve.DbfilenameShelf’&gt; <strong>shelfFile[‘cats’]</strong> &gt;
            &gt; &gt; [‘Zophie’, ‘Pooka’, ‘Simon’] &gt;&gt;&gt;
            <strong>shelfFile.close()</strong>
          </p>
        </blockquote>
      </blockquote>
    </blockquote>
    <p>
      Here, we open the shelf files to check that our data was stored correctly.
      Entering <code>shelfFile['cats']</code> returns the same list that we
      stored earlier, so we know that the list is correctly stored, and we call
      <code>close()</code>.
    </p>
    <p>
      Just like dictionaries, shelf values have <code>keys()</code> and
      <code>values()</code> methods that will return list-like values of the
      keys and values in the shelf. Since these methods return list-like values
      instead of true lists, you should pass them to the
      <code>list()</code> function to get them in list form. Enter the following
      into the interactive shell:
    </p>
    <p>&gt;&gt;&gt; <strong>shelfFile = shelve.open(‘mydata’)</strong></p>
    <blockquote>
      <blockquote>
        <blockquote>
          <p>
            <strong>list(shelfFile.keys())</strong> &gt; &gt; &gt; [‘cats’]
            &gt;&gt;&gt; <strong>list(shelfFile.values())</strong> &gt; &gt;
            &gt; [[‘Zophie’, ‘Pooka’, ‘Simon’]] &gt;&gt;&gt;
            <strong>shelfFile.close()</strong>
          </p>
        </blockquote>
      </blockquote>
    </blockquote>
    <p>
      Plaintext is useful for creating files that you’ll read in a text editor
      such as Notepad or TextEdit, but if you want to save data from your Python
      programs, use the <code>shelve</code> module.
    </p>
    <p>
      Recall from
      <a href="#calibre_link-87" title="Pretty Printing">Pretty Printing</a>
      that the <code>pprint.pprint()</code> function will “pretty print” the
      contents of a list or dictionary to the screen, while the
      <code>pprint.pformat()</code> function will return this same text as a
      string instead of printing it. Not only is this string formatted to be
      easy to read, but it is also syntactically correct Python code. Say you
      have a dictionary stored in a variable and you want to save this variable
      and its contents for future use. Using <code>pprint.pformat()</code> will
      give you a string that you can write to <em>.py</em> file. This file will
      be your very own module that you can import whenever you want to use the
      variable stored in it.
    </p>
    <p>For example, enter the following into the interactive shell:</p>
    <p>&gt;&gt;&gt; <strong>import pprint</strong></p>
    <blockquote>
      <blockquote>
        <blockquote>
          <p>
            <strong
              >cats = [{‘name’: ‘Zophie’, ‘desc’: ‘chubby’}, {‘name’: ‘Pooka’,
              ‘desc’: ‘fluffy’}]</strong
            >
            &gt;&gt;&gt; <strong>pprint.pformat(cats)</strong> “[{‘desc’:
            ‘chubby’, ‘name’: ‘Zophie’}, {‘desc’: ‘fluffy’, ‘name’: ‘Pooka’}]”
            <strong>fileObj = open(‘myCats.py’, ‘w’)</strong> &gt;&gt;&gt;
            <strong
              >fileObj.write(‘cats =’ + pprint.pformat(cats) + ‘\n’)</strong
            >
            83 <strong>fileObj.close()</strong>
          </p>
        </blockquote>
      </blockquote>
    </blockquote>
    <p>
      Here, we import <code>pprint</code> to let us use
      <code>pprint.pformat()</code>. We have a list of dictionaries, stored in a
      variable <code>cats</code>. To keep the list in
      <code>cats</code> available even after we close the shell, we use
      <code>pprint.pformat()</code> to return it as a string. Once we have the
      data in <code>cats</code> as a string, it’s easy to write the string to a
      file, which we’ll call <em>myCats.py</em>.
    </p>
    <p>
      The modules that an <code>import</code> statement imports are themselves
      just Python scripts. When the string from <code>pprint.pformat()</code> is
      saved to a <em>.py</em> file, the file is a module that can be imported
      just like any other.
    </p>
    <p>
      And since Python scripts are themselves just text files with the
      <em>.py</em> file extension, your Python programs can even generate other
      Python programs. You can then import these files into scripts.
    </p>
    <p>&gt;&gt;&gt; <strong>import myCats</strong></p>
    <blockquote>
      <blockquote>
        <blockquote>
          <p>
            <strong>myCats.cats</strong> &gt; &gt; &gt; [{‘name’: ‘Zophie’,
            ‘desc’: ‘chubby’}, {‘name’: ‘Pooka’, ‘desc’: ‘fluffy’}] &gt;&gt;&gt;
            <strong>myCats.cats[0]</strong> {‘name’: ‘Zophie’, ‘desc’: ‘chubby’}
            <strong>myCats.cats[0][‘name’]</strong> ‘Zophie’
          </p>
        </blockquote>
      </blockquote>
    </blockquote>
    <p>
      The benefit of creating a <em>.py</em> file (as opposed to saving
      variables with the <code>shelve</code> module) is that because it is a
      text file, the contents of the file can be read and modified by anyone
      with a simple text editor. For most applications, however, saving data
      using the <code>shelve</code> module is the preferred way to save
      variables to a file. Only basic data types such as integers, floats,
      strings, lists, and dictionaries can be written to a file as simple text.
      <code>File</code> objects, for example, cannot be encoded as text.
    </p>
    <p>
      Say you’re a geography teacher with 35 students in your class and you want
      to give a pop quiz on US state capitals. Alas, your class has a few bad
      eggs in it, and you can’t trust the students not to cheat. You’d like to
      randomize the order of questions so that each quiz is unique, making it
      impossible for anyone to crib answers from anyone else. Of course, doing
      this by hand would be a lengthy and boring affair. Fortunately, you know
      some Python.
    </p>
    <p>Here is what the program does:</p>
    <ul>
      <li>Creates 35 different quizzes.</li>
      <li>
        Creates 50 multiple-choice questions for each quiz, in random order.
      </li>
      <li>
        Provides the correct answer and three random wrong answers for each
        question, in random order.
      </li>
      <li>Writes the quizzes to 35 text files.</li>
      <li>Writes the answer keys to 35 text files.</li>
    </ul>
    <p>This means the code will need to do the following:</p>
    <ul>
      <li>Store the states and their capitals in a dictionary.</li>
      <li>
        Call <code>open()</code>, <code>write()</code>, and
        <code>close()</code> for the quiz and answer key text files.
      </li>
      <li>
        Use <code>random.shuffle()</code> to randomize the order of the
        questions and multiple-choice options.
      </li>
    </ul>
    <h2 id="step-1-store-the-quiz-data-in-a-dictionary">
      Step 1: Store the Quiz Data in a Dictionary
    </h2>
    <p>
      The first step is to create a skeleton script and fill it with your quiz
      data. Create a file named <em>randomQuizGenerator.py</em>, and make it
      look like the following:
    </p>
    <p>#! python3</p>
    <h1
      id="randomquizgenerator.py---creates-quizzes-with-questions-and-answers-in"
    >
      randomQuizGenerator.py - Creates quizzes with questions and answers in
    </h1>
    <h1 id="random-order-along-with-the-answer-key.">
      random order, along with the answer key.
    </h1>
    <p>❶ import random</p>
    <h1 id="the-quiz-data.-keys-are-states-and-values-are-their-capitals.">
      The quiz data. Keys are states and values are their capitals.
    </h1>
    <p>
      ❷ capitals = {‘Alabama’: ‘Montgomery’, ‘Alaska’: ‘Juneau’, ‘Arizona’:
      ‘Phoenix’, ‘Arkansas’: ‘Little Rock’, ‘California’: ‘Sacramento’,
      ‘Colorado’: ‘Denver’, ‘Connecticut’: ‘Hartford’, ‘Delaware’: ‘Dover’,
      ‘Florida’: ‘Tallahassee’, ‘Georgia’: ‘Atlanta’, ‘Hawaii’: ‘Honolulu’,
      ‘Idaho’: ‘Boise’, ‘Illinois’: ‘Springfield’, ‘Indiana’: ‘Indianapolis’,
      ‘Iowa’: ‘Des Moines’, ‘Kansas’: ‘Topeka’, ‘Kentucky’: ‘Frankfort’,
      ‘Louisiana’: ‘Baton Rouge’, ‘Maine’: ‘Augusta’, ‘Maryland’: ‘Annapolis’,
      ‘Massachusetts’: ‘Boston’, ‘Michigan’: ‘Lansing’, ‘Minnesota’: ‘Saint
      Paul’, ‘Mississippi’: ‘Jackson’, ‘Missouri’: ‘Jefferson City’, ‘Montana’:
      ‘Helena’, ‘Nebraska’: ‘Lincoln’, ‘Nevada’: ‘Carson City’, ‘New Hampshire’:
      ‘Concord’, ‘New Jersey’: ‘Trenton’, ‘New Mexico’: ‘Santa Fe’, ‘New York’:
      ‘Albany’, ‘North Carolina’: ‘Raleigh’, ‘North Dakota’: ‘Bismarck’, ‘Ohio’:
      ‘Columbus’, ‘Oklahoma’: ‘Oklahoma City’, ‘Oregon’: ‘Salem’,
      ‘Pennsylvania’: ‘Harrisburg’, ‘Rhode Island’: ‘Providence’, ‘South
      Carolina’: ‘Columbia’, ‘South Dakota’: ‘Pierre’, ‘Tennessee’: ‘Nashville’,
      ‘Texas’: ‘Austin’, ‘Utah’: ‘Salt Lake City’, ‘Vermont’: ‘Montpelier’,
      ‘Virginia’: ‘Richmond’, ‘Washington’: ‘Olympia’, ‘West Virginia’:
      ‘Charleston’, ‘Wisconsin’: ‘Madison’, ‘Wyoming’: ‘Cheyenne’}
    </p>
    <h1 id="generate-35-quiz-files.">Generate 35 quiz files.</h1>
    <p>
      ❸ for quizNum in range(35): # TODO: Create the quiz and answer key files.
    </p>
    <pre><code>   # TODO: Write out the header for the quiz.

   # TODO: Shuffle the order of the states.

   # TODO: Loop through all 50 states, making a question for each.</code></pre>
    <p>
      Since this program will be randomly ordering the questions and answers,
      you’ll need to import the <code>random</code> module ❶ to make use of its
      functions. The <code>capitals</code> variable ❷ contains a dictionary with
      US states as keys and their capitals as values. And since you want to
      create 35 quizzes, the code that actually generates the quiz and answer
      key files (marked with <code>TODO</code> comments for now) will go inside
      a <code>for</code> loop that loops 35 times ❸. (This number can be changed
      to generate any number of quiz files.)
    </p>
    <h2 id="step-2-create-the-quiz-file-and-shuffle-the-question-order">
      Step 2: Create the Quiz File and Shuffle the Question Order
    </h2>
    <p>Now it’s time to start filling in those <code>TODO</code>s.</p>
    <p>
      The code in the loop will be repeated 35 times—once for each quiz—so you
      have to worry about only one quiz at a time within the loop. First you’ll
      create the actual quiz file. It needs to have a unique filename and should
      also have some kind of standard header in it, with places for the student
      to fill in a name, date, and class period. Then you’ll need to get a list
      of states in randomized order, which can be used later to create the
      questions and answers for the quiz.
    </p>
    <p>Add the following lines of code to <em>randomQuizGenerator.py</em>:</p>
    <p>#! python3</p>
    <h1
      id="randomquizgenerator.py---creates-quizzes-with-questions-and-answers-in-1"
    >
      randomQuizGenerator.py - Creates quizzes with questions and answers in
    </h1>
    <h1 id="random-order-along-with-the-answer-key.-1">
      random order, along with the answer key.
    </h1>
    <p>–<em>snip</em>–</p>
    <h1 id="generate-35-quiz-files.-1">Generate 35 quiz files.</h1>
    <p>
      for quizNum in range(35):
      <strong># Create the quiz and answer key files.</strong> ❶
      <strong
        >quizFile = open(‘capitalsquiz%s.txt’ % (quizNum + 1), ‘w’)</strong
      >
      ❷
      <strong
        >answerKeyFile = open(‘capitalsquiz_answers%s.txt’ % (quizNum + 1),
        ‘w’)</strong
      >
    </p>
    <pre><code>   **\# Write out the header for the quiz.**</code></pre>
    <p>
      ❸ <strong>quizFile.write(‘Name:\n\nDate:\n\nPeriod:\n\n’)</strong>
      <strong
        >quizFile.write((’ ’ * 20) + ‘State Capitals Quiz (Form %s)’ % (quizNum
        + 1))</strong
      >
      <strong>quizFile.write(‘\n\n’)</strong>
    </p>
    <pre><code>   **\# Shuffle the order of the states.**
   **states = list(capitals.keys())**</code></pre>
    <p>❹ <strong>random.shuffle(states)</strong></p>
    <pre><code>   # TODO: Loop through all 50 states, making a question for each.</code></pre>
    <p>
      The filenames for the quizzes will be <em>capitalsquiz.txt</em>, where is
      a unique number for the quiz that comes from <code>quizNum</code>, the
      <code>for</code> loop’s counter. The answer key for
      <em>capitalsquiz.txt</em> will be stored in a text file named
      <em>capitalsquiz_answers.txt</em>. Each time through the loop, the
      <code>%s</code> placeholder in <code>'capitalsquiz%s.txt'</code> and
      <code>'capitalsquiz_answers%s.txt'</code> will be replaced by
      <code>(quizNum + 1)</code>, so the first quiz and answer key created will
      be <em>capitalsquiz1.txt</em> and <em>capitalsquiz_answers1.txt</em>.
      These files will be created with calls to the <code>open()</code> function
      at ❶ and ❷, with <code>'w'</code> as the second argument to open them in
      write mode.
    </p>
    <p>
      The <code>write()</code> statements at ❸ create a quiz header for the
      student to fill out. Finally, a randomized list of US states is created
      with the help of the <code>random.shuffle()</code> function ❹, which
      randomly reorders the values in any list that is passed to it.
    </p>
    <h2 id="step-3-create-the-answer-options">
      Step 3: Create the Answer Options
    </h2>
    <p>
      Now you need to generate the answer options for each question, which will
      be multiple choice from A to D. You’ll need to create another
      <code>for</code> loop—this one to generate the content for each of the 50
      questions on the quiz. Then there will be a third <code>for</code> loop
      nested inside to generate the multiple-choice options for each question.
      Make your code look like the following:
    </p>
    <p>#! python3</p>
    <h1
      id="randomquizgenerator.py---creates-quizzes-with-questions-and-answers-in-2"
    >
      randomQuizGenerator.py - Creates quizzes with questions and answers in
    </h1>
    <h1 id="random-order-along-with-the-answer-key.-2">
      random order, along with the answer key.
    </h1>
    <p>–<em>snip</em>–</p>
    <pre><code>   **\# Loop through all 50 states, making a question for each.**
   **for questionNum in range(50):**

       **\# Get right and wrong answers.**</code></pre>
    <p>
      ❶ <strong>correctAnswer = capitals[states[questionNum]]</strong> ❷
      <strong>wrongAnswers = list(capitals.values())</strong> ❸
      <strong>del wrongAnswers[wrongAnswers.index(correctAnswer)]</strong> ❹
      <strong>wrongAnswers = random.sample(wrongAnswers, 3)</strong> ❺
      <strong>answerOptions = wrongAnswers + [correctAnswer]</strong> ❻
      <strong>random.shuffle(answerOptions)</strong>
    </p>
    <pre><code>       **\# TODO: Write the question and answer options to the quiz file.**

       **\# TODO: Write the answer key to a file.**</code></pre>
    <p>
      The correct answer is easy to get—it’s stored as a value in the
      <code>capitals</code> dictionary ❶. This loop will loop through the states
      in the shuffled <code>states</code> list, from <code>states[0]</code> to
      <code>states[49]</code>, find each state in <code>capitals</code>, and
      store that state’s corresponding capital in <code>correctAnswer</code>.
    </p>
    <p>
      The list of possible wrong answers is trickier. You can get it by
      duplicating <em>all</em> the values in the
      <code>capitals</code> dictionary ❷, deleting the correct answer ❸, and
      selecting three random values from this list ❹. The
      <code>random.sample()</code> function makes it easy to do this selection.
      Its first argument is the list you want to select from; the second
      argument is the number of values you want to select. The full list of
      answer options is the combination of these three wrong answers with the
      correct answers ❺. Finally, the answers need to be randomized ❻ so that
      the correct response isn’t always choice D.
    </p>
    <h2 id="step-4-write-content-to-the-quiz-and-answer-key-files">
      Step 4: Write Content to the Quiz and Answer Key Files
    </h2>
    <p>
      All that is left is to write the question to the quiz file and the answer
      to the answer key file. Make your code look like the following:
    </p>
    <p>#! python3</p>
    <h1
      id="randomquizgenerator.py---creates-quizzes-with-questions-and-answers-in-3"
    >
      randomQuizGenerator.py - Creates quizzes with questions and answers in
    </h1>
    <h1 id="random-order-along-with-the-answer-key.-3">
      random order, along with the answer key.
    </h1>
    <p>
      –<em>snip</em>– # Loop through all 50 states, making a question for each.
      for questionNum in range(50): –<em>snip</em>–
    </p>
    <pre><code>       **\# Write the question and the answer options to the quiz file.**
       **quizFile.write(&#39;%s. What is the capital of %s?\\n&#39; % (questionNum + 1,**
           **states\[questionNum\]))**</code></pre>
    <p>
      ❶ <strong>for i in range(4):</strong> ❷
      <strong
        >quizFile.write(’ %s. %s\n’ % (‘ABCD’[i], answerOptions[i]))</strong
      >
      <strong>quizFile.write(‘\n’)</strong>
    </p>
    <pre><code>       **\# Write the answer key to a file.**</code></pre>
    <p>
      ❸
      <strong
        >answerKeyFile.write(‘%s. %s\n’ % (questionNum + 1, ‘ABCD’[</strong
      >
      <strong>answerOptions.index(correctAnswer)]))</strong>
      <strong>quizFile.close()</strong> <strong>answerKeyFile.close()</strong>
    </p>
    <p>
      A <code>for</code> loop that goes through integers <code>0</code> to
      <code>3</code> will write the answer options in the
      <code>answerOptions</code> list ❶. The expression
      <code>'ABCD'[i]</code> at ❷ treats the string <code>'ABCD'</code> as an
      array and will evaluate to <code>'A'</code>,<code>'B'</code>,
      <code>'C'</code>, and then <code>'D'</code> on each respective iteration
      through the loop.
    </p>
    <p>
      In the final line ❸, the expression
      <code>answerOptions.index(correctAnswer)</code> will find the integer
      index of the correct answer in the randomly ordered answer options, and
      <code>'ABCD'[answerOptions.index(correctAnswer)]</code> will evaluate to
      the correct answer’s letter to be written to the answer key file.
    </p>
    <p>
      After you run the program, this is how your
      <em>capitalsquiz1.txt</em> file will look, though of course your questions
      and answer options may be different from those shown here, depending on
      the outcome of your <code>random.shuffle()</code> calls:
    </p>
    <p>Name:</p>
    <p>Date:</p>
    <p>Period:</p>
    <pre><code>                State Capitals Quiz (Form 1)</code></pre>
    <ol type="1">
      <li>
        <p>
          What is the capital of West Virginia? A. Hartford B. Santa Fe C.
          Harrisburg D. Charleston
        </p>
      </li>
      <li>
        <p>
          What is the capital of Colorado? A. Raleigh B. Harrisburg C. Denver D.
          Lincoln
        </p>
      </li>
    </ol>
    <p>–<em>snip</em>–</p>
    <p>
      The corresponding <em>capitalsquiz_answers1.txt</em> text file will look
      like this:
    </p>
    <ol type="1">
      <li>D 2. C 3. A 4. C –<em>snip</em>–</li>
    </ol>
    <p>
      Say you have the boring task of filling out many forms in a web page or
      software with several text fields. The clipboard saves you from typing the
      same text over and over again. But only one thing can be on the clipboard
      at a time. If you have several different pieces of text that you need to
      copy and paste, you have to keep highlighting and copying the same few
      things over and over again.
    </p>
    <p>
      You can write a Python program to keep track of multiple pieces of text.
      This “multiclipboard” will be named <em>mcb.pyw</em> (since “mcb” is
      shorter to type than “multiclipboard”). The <em>.pyw</em> extension means
      that Python won’t show a Terminal window when it runs this program. (See
      Appendix B for more details.)
    </p>
    <p>
      The program will save each piece of clipboard text under a keyword. For
      example, when you run <code>py mcb.pyw save spam</code>, the current
      contents of the clipboard will be saved with the keyword <em>spam</em>.
      This text can later be loaded to the clipboard again by running
      <code>py mcb.pyw spam</code>. And if the user forgets what keywords they
      have, they can run <code>py mcb.pyw list</code> to copy a list of all
      keywords to the clipboard.
    </p>
    <p>Here’s what the program does:</p>
    <ul>
      <li>The command line argument for the keyword is checked.</li>
      <li>
        If the argument is <code>save</code>, then the clipboard contents are
        saved to the keyword.
      </li>
      <li>
        If the argument is <code>list</code>, then all the keywords are copied
        to the clipboard.
      </li>
      <li>Otherwise, the text for the keyword is copied to the clipboard.</li>
    </ul>
    <p>This means the code will need to do the following:</p>
    <ul>
      <li>Read the command line arguments from <code>sys.argv</code>.</li>
      <li>Read and write to the clipboard.</li>
      <li>Save and load to a shelf file.</li>
    </ul>
    <p>
      If you use Windows, you can easily run this script from the Run… window by
      creating a batch file named <em>mcb.bat</em> with the following content:
    </p>
    <p>
      <span class="citation" data-cites="pyw.exe"
        ><span class="citation" data-cites="pyw.exe"
          ><span class="citation" data-cites="pyw.exe">@pyw.exe</span></span
        ></span
      >
      C:\Python34\mcb.pyw %*
    </p>
    <h2 id="step-1-comments-and-shelf-setup">
      Step 1: Comments and Shelf Setup
    </h2>
    <p>
      Let’s start by making a skeleton script with some comments and basic
      setup. Make your code look like the following:
    </p>
    <p>#! python3</p>
    <h1 id="mcb.pyw---saves-and-loads-pieces-of-text-to-the-clipboard.">
      mcb.pyw - Saves and loads pieces of text to the clipboard.
    </h1>
    <p>❶ # Usage: py.exe mcb.pyw save - Saves clipboard to keyword.</p>
    <h1 id="py.exe-mcb.pyw---loads-keyword-to-clipboard.">
      py.exe mcb.pyw - Loads keyword to clipboard.
    </h1>
    <h1 id="py.exe-mcb.pyw-list---loads-all-keywords-to-clipboard.">
      py.exe mcb.pyw list - Loads all keywords to clipboard.
    </h1>
    <p>❷ import shelve, pyperclip, sys</p>
    <p>❸ mcbShelf = shelve.open(‘mcb’)</p>
    <h1 id="todo-save-clipboard-content.">TODO: Save clipboard content.</h1>
    <h1 id="todo-list-keywords-and-load-content.">
      TODO: List keywords and load content.
    </h1>
    <p>mcbShelf.close()</p>
    <p>
      It’s common practice to put general usage information in comments at the
      top of the file ❶. If you ever forget how to run your script, you can
      always look at these comments for a reminder. Then you import your modules
      ❷. Copying and pasting will require the <code>pyperclip</code> module, and
      reading the command line arguments will require the
      <code>sys</code> module. The <code>shelve</code> module will also come in
      handy: Whenever the user wants to save a new piece of clipboard text,
      you’ll save it to a shelf file. Then, when the user wants to paste the
      text back to their clipboard, you’ll open the shelf file and load it back
      into your program. The shelf file will be named with the prefix
      <em>mcb</em> ❸.
    </p>
    <h2 id="step-2-save-clipboard-content-with-a-keyword">
      Step 2: Save Clipboard Content with a Keyword
    </h2>
    <p>
      The program does different things depending on whether the user wants to
      save text to a keyword, load text into the clipboard, or list all the
      existing keywords. Let’s deal with that first case. Make your code look
      like the following:
    </p>
    <p>#! python3</p>
    <h1 id="mcb.pyw---saves-and-loads-pieces-of-text-to-the-clipboard.-1">
      mcb.pyw - Saves and loads pieces of text to the clipboard.
    </h1>
    <p>–<em>snip</em>–</p>
    <p>
      <strong># Save clipboard content.</strong> ❶
      <strong>if len(sys.argv) == 3 and sys.argv[1].lower() == ‘save’:</strong>
      ❷ <strong>mcbShelf[sys.argv[2]] = pyperclip.paste()</strong>
      <strong>elif len(sys.argv) == 2:</strong> ❸ # TODO: List keywords and load
      content.
    </p>
    <p>mcbShelf.close()</p>
    <p>
      If the first command line argument (which will always be at index
      <code>1</code> of the <code>sys.argv</code> list) is
      <code>'save'</code> ❶, the second command line argument is the keyword for
      the current content of the clipboard. The keyword will be used as the key
      for <code>mcbShelf</code>, and the value will be the text currently on the
      clipboard ❷.
    </p>
    <p>
      If there is only one command line argument, you will assume it is either
      <code>'list'</code> or a keyword to load content onto the clipboard. You
      will implement that code later. For now, just put a
      <code>TODO</code> comment there ❸.
    </p>
    <h2 id="step-3-list-keywords-and-load-a-keywords-content">
      Step 3: List Keywords and Load a Keyword’s Content
    </h2>
    <p>
      Finally, let’s implement the two remaining cases: The user wants to load
      clipboard text in from a keyword, or they want a list of all available
      keywords. Make your code look like the following:
    </p>
    <p>#! python3</p>
    <h1 id="mcb.pyw---saves-and-loads-pieces-of-text-to-the-clipboard.-2">
      mcb.pyw - Saves and loads pieces of text to the clipboard.
    </h1>
    <p>–<em>snip</em>–</p>
    <h1 id="save-clipboard-content.">Save clipboard content.</h1>
    <p>
      if len(sys.argv) == 3 and sys.argv[1].lower() == ‘save’:
      mcbShelf[sys.argv[2]] = pyperclip.paste() elif len(sys.argv) == 2:
      <strong># List keywords and load content.</strong> ❶
      <strong>if sys.argv[1].lower() == ‘list’:</strong> ❷
      <strong>pyperclip.copy(str(list(mcbShelf.keys())))</strong>
      <strong>elif sys.argv[1] in mcbShelf:</strong> ❸
      <strong>pyperclip.copy(mcbShelf[sys.argv[1]])</strong>
    </p>
    <p>mcbShelf.close()</p>
    <p>
      If there is only one command line argument, first let’s check whether it’s
      <code>'list'</code> ❶. If so, a string representation of the list of shelf
      keys will be copied to the clipboard ❷. The user can paste this list into
      an open text editor to read it.
    </p>
    <p>
      Otherwise, you can assume the command line argument is a keyword. If this
      keyword exists in the <code>mcbShelf</code> shelf as a key, you can load
      the value onto the clipboard ❸.
    </p>
    <p>
      And that’s it! Launching this program has different steps depending on
      what operating system your computer uses. See Appendix B for details for
      your operating system.
    </p>
    <p>
      Recall the password locker program you created in
      <a
        href="chrome-extension://cjedbglnccaioiolemnfhjncicchinao/chapter6"
        title="Chapter 6. Manipulating Strings"
        >Chapter 6</a
      >
      that stored the passwords in a dictionary. Updating the passwords required
      changing the source code of the program. This isn’t ideal because average
      users don’t feel comfortable changing source code to update their
      software. Also, every time you modify the source code to a program, you
      run the risk of accidentally introducing new bugs. By storing the data for
      a program in a different place than the code, you can make your programs
      easier for others to use and more resistant to bugs.
    </p>
    <p>
      Files are organized into folders (also called directories), and a path
      describes the location of a file. Every program running on your computer
      has a current working directory, which allows you to specify file paths
      relative to the current location instead of always typing the full (or
      absolute) path. The <code>os.path</code> module has many functions for
      manipulating file paths.
    </p>
    <p>
      Your programs can also directly interact with the contents of text files.
      The <code>open()</code> function can open these files to read in their
      contents as one large string (with the <code>read()</code> method) or as a
      list of strings (with the <code>readlines()</code> method). The
      <code>open()</code> function can open files in write or append mode to
      create new text files or add to existing text files, respectively.
    </p>
    <p>
      In previous chapters, you used the clipboard as a way of getting large
      amounts of text into a program, rather than typing it all in. Now you can
      have your programs read files directly from the hard drive, which is a big
      improvement, since files are much less volatile than the clipboard.
    </p>
    <p>
      In the next chapter, you will learn how to handle the files themselves, by
      copying them, deleting them, renaming them, moving them, and more.
    </p>
    <p>
      <span id="calibre_link-2751"></span><span id="calibre_link-2752"></span>
    </p>
    <p>Q:</p>
    <p>What is a relative path relative to?</p>
    <p>
      <span id="calibre_link-2753"></span><span id="calibre_link-2754"></span>
    </p>
    <p>Q:</p>
    <p>What does an absolute path start with?</p>
    <p>
      <span id="calibre_link-2755"></span><span id="calibre_link-2756"></span>
    </p>
    <p>Q:</p>
    <p>
      What do the <code>os.getcwd()</code> and <code>os.chdir()</code> functions
      do?
    </p>
    <p>
      <span id="calibre_link-2757"></span><span id="calibre_link-2758"></span>
    </p>
    <p>Q:</p>
    <p>What are the <code>.</code> and <code>..</code> folders?</p>
    <p>
      <span id="calibre_link-2759"></span><span id="calibre_link-2760"></span>
    </p>
    <p>Q:</p>
    <p>
      In <em>C:.txt</em>, which part is the dir name, and which part is the base
      name?
    </p>
    <p>
      <span id="calibre_link-2761"></span><span id="calibre_link-2762"></span>
    </p>
    <p>Q:</p>
    <p>
      What are the three “mode” arguments that can be passed to the
      <code>open()</code> function?
    </p>
    <p>
      <span id="calibre_link-2763"></span><span id="calibre_link-2764"></span>
    </p>
    <p>Q:</p>
    <p>What happens if an existing file is opened in write mode?</p>
    <p>
      <span id="calibre_link-2765"></span><span id="calibre_link-2766"></span>
    </p>
    <p>Q:</p>
    <p>
      What is the difference between the <code>read()</code> and
      <code>readlines()</code> methods?
    </p>
    <p>
      <span id="calibre_link-2767"></span><span id="calibre_link-2768"></span>
    </p>
    <p>Q:</p>
    <ol type="1">
      <li>What data structure does a shelf value resemble?</li>
    </ol>
    <p>For practice, design and write the following programs.</p>
    <h2 id="extending-the-multiclipboard">Extending the Multiclipboard</h2>
    <p>
      Extend the multiclipboard program in this chapter so that it has a
      <code>delete &lt;keyword&gt;</code> command line argument that will delete
      a keyword from the shelf. Then add a <code>delete</code> command line
      argument that will delete <em>all</em> keywords.
    </p>
    <p>
      Create a Mad Libs program that reads in text files and lets the user add
      their own text anywhere the word <em>ADJECTIVE</em>, <em>NOUN</em>,
      <em>ADVERB</em>, or <em>VERB</em> appears in the text file. For example, a
      text file may look like this:
    </p>
    <p>
      The ADJECTIVE panda walked to the NOUN and then VERB. A nearby NOUN was
      unaffected by these events.
    </p>
    <p>
      The program would find these occurrences and prompt the user to replace
      them.
    </p>
    <p>
      Enter an adjective: <strong>silly</strong> Enter a noun:
      <strong>chandelier</strong> Enter a verb: <strong>screamed</strong> Enter
      a noun: <strong>pickup truck</strong>
    </p>
    <p>The following text file would then be created:</p>
    <p>
      The silly panda walked to the chandelier and then screamed. A nearby
      pickup truck was unaffected by these events.
    </p>
    <p>
      The results should be printed to the screen and saved to a new text file.
    </p>
    <p>
      Write a program that opens all ._txt_ files in a folder and searches for
      any line that matches a user-supplied regular expression. The results
      should be printed to the screen.
    </p>
    <p><a href="https://automatetheboringstuff.com/chapter8/">Source</a></p>
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