intermediatePython/_build/epub/open_function.html at testing · dpcoding/intermediatePython

History

137 lines (124 loc) · 10.3 KB

Raw

100

101

102

103

104

105

106

107

108

109

110

111

112

113

114

115

116

117

118

119

120

121

122

123

124

125

126

127

128

129

130

131

132

133

134

135

136

137

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN"

"http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">

<head>

<title>Open function</title>

</head>

<h3>Navigation</h3>

<ul>

<a href="targeting_python_2_3.html" title="Targeting Python 2+3"

accesskey="N">next</a></li>

<a href="for_-_else.html" title="For - Else"

accesskey="P">previous</a> |</li>

<li class="nav-item nav-item-0"><a href="index.html">Python Tips 0.1 documentation</a> »</li>

</ul>

</div>

<h1>Open function</h1>

<a class="reference external" href="http://docs.python.org/dev/library/functions.html#open">open</a> [http://docs.python.org/dev/library/functions.html#open] opens

a file. Pretty simple, eh? Most of the time, we see it being used like

this:

<div class="code python highlight-python"><div class="highlight"><pre>f = open('photo.jpg', 'r+')

jpgdata = f.read()

f.close()

</pre></div>

</div>

The reason I am writing this article is that most of the time, I see

open used like this. There are three errors in the above code. Can

you spot them all? If not, read on. By the end of this article, you’ll

know what’s wrong in the above code, and, more importantly, be able to

avoid these mistakes in your own code. Let’s start with the basics:

The return of open is a file handle, given out from the operating system

to your Python application. You will want to return this file handle

once you’re finished with the file, if only so that your application

won’t reach the limit of the number of open file handle it can have at

once.

Explicitly calling <code class="docutils literal">close</code> closes the file handle, but only if the

read was successful. If there is any error just after <code class="docutils literal">f = open(...)</code>,

<code class="docutils literal">f.close()</code> will not be called (depending on the Python interpreter,

the file handle may still be returned, but that’s another story). To

make sure that the file gets closed whether an exception occurs or not,

pack it into a

<code class="docutils literal">`with</code> <<a class="reference external" href="http://freepythontips.wordpress.com/2013/07/28/the-with-statement/">http://freepythontips.wordpress.com/2013/07/28/the-with-statement/</a>>`__

statement:

<div class="code python highlight-python"><div class="highlight"><pre>with open('photo.jpg', 'r+') as f:

jpgdata = f.read()

</pre></div>

</div>

The first argument of <code class="docutils literal">open</code> is the filename. The second one (the

mode) determines how the file gets opened.

<li>If you want to read and write the file, pass in <code class="docutils literal">r+</code></li>

<li>If you want to overwrite the file, pass in <code class="docutils literal">w</code></li>

<li>If you want to append to the file, pass in <code class="docutils literal">a</code></li>

</ul>

While there are a couple of other valid mode strings, chances are you

won’t ever use them. The mode matters not only because it changes the

behavior, but also because it may result in permission errors. For

example, if we were to open a jpg-file in a write-protected directory,

<code class="docutils literal">open(.., 'r+')</code> would fail. The mode can contain one further

character; we can open the file in binary (you’ll get a string of bytes)

or text mode (a string of characters).

In general, if the format is written by humans, it tends to be text

mode. <code class="docutils literal">jpg</code> image files are not generally written by humans (and are

indeed not readable to humans), and you should therefore open them in

binary mode by adding a <code class="docutils literal">b</code> to the text string (if you’re following

the opening example, the correct mode would be <code class="docutils literal">rb</code>). If you open

something in text mode (i.e. add a <code class="docutils literal">t</code>, or nothing apart from

<code class="docutils literal">r/r+/w/a</code>), you must also know which encoding to use - for a

computer, all files are just bytes, not characters.

Unfortunately, <code class="docutils literal">open</code> does not allow explicit encoding specification

in Python 2.x. However, the function

<code class="docutils literal">`io.open</code> <<a class="reference external" href="http://docs.python.org/2/library/io.html#io.open">http://docs.python.org/2/library/io.html#io.open</a>>`__ is

available in both Python 2.x and 3.x (where it is an alias of <code class="docutils literal">open</code>),

and does the right thing. You can pass in the encoding with the

<code class="docutils literal">encoding</code> keyword. If you don’t pass in any encoding, a system- (and

Python-) specific default will be picked. You may be tempted to rely on

these defaults, but the defaults are often wrong, or the default

encoding cannot actually express all characters (this will happen on

Python 2.x and/or Windows). So go ahead and pick an encoding. <code class="docutils literal">utf-8</code>

is a terrific one. When you write a file, you can just pick the encoding

to your liking (or the liking of the program that will eventually read

your file).

How do you find out which encoding a file you read has? Well,

unfortunately, there is no sureproof way to detect the encoding - the

same bytes can represent different, but equally valid characters in

different encodings. Therefore, you must rely on metadata (for example,

in HTTP headers) to know the encoding. Increasingly, formats just define

the encoding to be UTF-8.

Armed with this knowledge, let’s write a program that reads a file,

determines whether it’s JPG (hint: These files start with the bytes

<code class="docutils literal">FF D8</code>), and writes a text file that describe the input file.

<div class="code python highlight-python"><div class="highlight"><pre>import io

with open('photo.jpg', 'rb') as inf:

jpgdata = inf.read()

if jpgdata.startswith(b'\xff\xd8'):

text = u'This is a jpeg file (%d bytes long)\n'

else:

text = u'This is a random file (%d bytes long)\n'

with io.open('summary.txt', 'w', encoding='utf-8') as outf:

outf.write(text % len(jpgdata))

</pre></div>

</div>

I am sure that now you would use <code class="docutils literal">open</code> correctly!

</div>

</div>

</div>

</body>

</html>

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

FilesExpand file tree

open_function.html

Latest commit

History

open_function.html

File metadata and controls