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<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml" lang="" xml:lang="">
<head>
<meta charset="utf-8" />
<meta name="generator" content="pandoc" />
<meta
name="viewport"
content="width=device-width, initial-scale=1.0, user-scalable=yes"
/>
<title>Automate-Boring-Stuff-with-Python</title>
<style type="text/css">
code {
white-space: pre-wrap;
}
span.smallcaps {
font-variant: small-caps;
}
span.underline {
text-decoration: underline;
}
div.column {
display: inline-block;
vertical-align: top;
width: 50%;
}
</style>
</head>
<body>
<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>>>> <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>
>>>
<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('C:\\\\Users\\\\asweigart', 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>>>> <strong>import os</strong></p>
<blockquote>
<blockquote>
<blockquote>
<p>
<strong>os.getcwd()</strong> ‘C:\\Python34’
<strong>os.chdir(‘C:\\Windows\\System32’)</strong> >>>
<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>
>>>
<strong>os.chdir(‘C:\\ThisFolderDoesNotExist’)</strong> Traceback (most
recent call last): File “<pyshell#18>”, 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>>>> <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('C:\delicious \walnut\waffles')"
>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>>>> <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>
>>>
<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>
>>> <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>
>>>
<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>
>>>
<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>
>>> <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>
>>> <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>
>>>
<strong>os.path.getsize(‘C:\\Windows\\System32\\calc.exe’)</strong> 776192
</p>
<blockquote>
<blockquote>
<blockquote>
<p>
<strong>os.listdir(‘C:\\Windows\\System32’)</strong> > > >
[‘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>>>> <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('C:\\\\Windows\\\\System32', 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>>>> <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>>>> <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>
>>>
<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>
>>>
<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>>>> <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>>>> <strong>sonnetFile = open(‘sonnet29.txt’)</strong></p>
<blockquote>
<blockquote>
<blockquote>
<p>
<strong>sonnetFile.readlines()</strong> > > > [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>>>> <strong>baconFile = open(‘bacon.txt’, ‘w’)</strong></p>
<blockquote>
<blockquote>
<blockquote>
<p>
<strong>baconFile.write(‘Hello world!\n’)</strong> 13
<strong>baconFile.close()</strong> >>>
<strong>baconFile = open(‘bacon.txt’, ‘a’)</strong> >>>
<strong>baconFile.write(‘Bacon is not a vegetable.’)</strong> 25
<strong>baconFile.close()</strong> >>>
<strong>baconFile = open(‘bacon.txt’)</strong> >>>
<strong>content = baconFile.read()</strong> >>>
<strong>baconFile.close()</strong> >>>
<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>>>> <strong>import shelve</strong></p>
<blockquote>
<blockquote>
<blockquote>
<p>
<strong>shelfFile = shelve.open(‘mydata’)</strong> >>>
<strong>cats = [‘Zophie’, ‘Pooka’, ‘Simon’]</strong> >>>
<strong>shelfFile[‘cats’] = cats</strong> >>>
<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>>>> <strong>shelfFile = shelve.open(‘mydata’)</strong></p>
<blockquote>
<blockquote>
<blockquote>
<p>
<strong>type(shelfFile)</strong> <class
‘shelve.DbfilenameShelf’> <strong>shelfFile[‘cats’]</strong> >
> > [‘Zophie’, ‘Pooka’, ‘Simon’] >>>
<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>>>> <strong>shelfFile = shelve.open(‘mydata’)</strong></p>
<blockquote>
<blockquote>
<blockquote>
<p>
<strong>list(shelfFile.keys())</strong> > > > [‘cats’]
>>> <strong>list(shelfFile.values())</strong> > >
> [[‘Zophie’, ‘Pooka’, ‘Simon’]] >>>
<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>>>> <strong>import pprint</strong></p>
<blockquote>
<blockquote>
<blockquote>
<p>
<strong
>cats = [{‘name’: ‘Zophie’, ‘desc’: ‘chubby’}, {‘name’: ‘Pooka’,
‘desc’: ‘fluffy’}]</strong
>
>>> <strong>pprint.pformat(cats)</strong> “[{‘desc’:
‘chubby’, ‘name’: ‘Zophie’}, {‘desc’: ‘fluffy’, ‘name’: ‘Pooka’}]”
<strong>fileObj = open(‘myCats.py’, ‘w’)</strong> >>>
<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>>>> <strong>import myCats</strong></p>
<blockquote>
<blockquote>