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<div class="section" lang="en">
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<div>
<h2 class="title"><a name="kgp.unicode"></a>9.4. Unicode
</h2>
</div>
</div>
<div></div>
</div>
<div class="abstract">
<p>Unicode is a system to represent characters from all the world's different languages. When <span class="application">Python</span> parses an <span class="acronym">XML</span> document, all data is stored in memory as unicode.
</p>
</div>
<p>You'll get to all that in a minute, but first, some background.</p>
<p><b>Historical note. </b>Before unicode, there were separate character encoding systems for each language, each using the same numbers (0-255) to represent
that language's characters. Some languages (like Russian) have multiple conflicting standards about how to represent the
same characters; other languages (like Japanese) have so many characters that they require multiple-byte character sets.
Exchanging documents between systems was difficult because there was no way for a computer to tell for certain which character
encoding scheme the document author had used; the computer only saw numbers, and the numbers could mean different things.
Then think about trying to store these documents in the same place (like in the same database table); you would need to store
the character encoding alongside each piece of text, and make sure to pass it around whenever you passed the text around.
Then think about multilingual documents, with characters from multiple languages in the same document. (They typically used
escape codes to switch modes; poof, you're in Russian koi8-r mode, so character 241 means this; poof, now you're in Mac Greek
mode, so character 241 means something else. And so on.) These are the problems which unicode was designed to solve.
</p>
<p>To solve these problems, unicode represents each character as a 2-byte number, from 0 to 65535.<sup>[<a name="d0e23786" href="#ftn.d0e23786">5</a>]</sup> Each 2-byte number represents a unique character used in at least one of the world's languages. (Characters that are used
in multiple languages have the same numeric code.) There is exactly 1 number per character, and exactly 1 character per number.
Unicode data is never ambiguous.
</p>
<p>Of course, there is still the matter of all these legacy encoding systems. 7-bit <span class="acronym">ASCII</span>, for instance, which stores English characters as numbers ranging from 0 to 127. (65 is capital “<span class="quote"><tt class="literal">A</tt></span>”, 97 is lowercase “<span class="quote"><tt class="literal">a</tt></span>”, and so forth.) English has a very simple alphabet, so it can be completely expressed in 7-bit <span class="acronym">ASCII</span>. Western European languages like French, Spanish, and German all use an encoding system called ISO-8859-1 (also called “<span class="quote">latin-1</span>”), which uses the 7-bit <span class="acronym">ASCII</span> characters for the numbers 0 through 127, but then extends into the 128-255 range for characters like n-with-a-tilde-over-it
(241), and u-with-two-dots-over-it (252). And unicode uses the same characters as 7-bit <span class="acronym">ASCII</span> for 0 through 127, and the same characters as ISO-8859-1 for 128 through 255, and then extends from there into characters
for other languages with the remaining numbers, 256 through 65535.
</p>
<p>When dealing with unicode data, you may at some point need to convert the data back into one of these other legacy encoding
systems. For instance, to integrate with some other computer system which expects its data in a specific 1-byte encoding
scheme, or to print it to a non-unicode-aware terminal or printer. Or to store it in an <span class="acronym">XML</span> document which explicitly specifies the encoding scheme.
</p>
<p>And on that note, let's get back to <span class="application">Python</span>.
</p>
<p><span class="application">Python</span> has had unicode support throughout the language since version 2.0. The <span class="acronym">XML</span> package uses unicode to store all parsed <span class="acronym">XML</span> data, but you can use unicode anywhere.
</p>
<div class="example"><a name="d0e23841"></a><h3 class="title">Example 9.13. Introducing unicode</h3><pre class="screen">
<tt class="prompt">>>> </tt><span class="userinput">s = u<span class='pystring'>'Dive in'</span></span> <a name="kgp.unicode.1.1"></a><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12">
<tt class="prompt">>>> </tt><span class="userinput">s</span>
<span class="computeroutput">u'Dive in'</span>
<tt class="prompt">>>> </tt><span class="userinput"><span class='pykeyword'>print</span> s</span> <a name="kgp.unicode.1.2"></a><img src="../images/callouts/2.png" alt="2" border="0" width="12" height="12">
<span class="computeroutput">Dive in</span></pre><div class="calloutlist">
<table border="0" summary="Callout list">
<tr>
<td width="12" valign="top" align="left"><a href="#kgp.unicode.1.1"><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">To create a unicode string instead of a regular <span class="acronym">ASCII</span> string, add the letter “<span class="quote"><tt class="literal">u</tt></span>” before the string. Note that this particular string doesn't have any non-<span class="acronym">ASCII</span> characters. That's fine; unicode is a superset of <span class="acronym">ASCII</span> (a very large superset at that), so any regular <span class="acronym">ASCII</span> string can also be stored as unicode.
</td>
</tr>
<tr>
<td width="12" valign="top" align="left"><a href="#kgp.unicode.1.2"><img src="../images/callouts/2.png" alt="2" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">When printing a string, <span class="application">Python</span> will attempt to convert it to your default encoding, which is usually <span class="acronym">ASCII</span>. (More on this in a minute.) Since this unicode string is made up of characters that are also <span class="acronym">ASCII</span> characters, printing it has the same result as printing a normal <span class="acronym">ASCII</span> string; the conversion is seamless, and if you didn't know that <tt class="varname">s</tt> was a unicode string, you'd never notice the difference.
</td>
</tr>
</table>
</div>
</div>
<div class="example"><a name="d0e23908"></a><h3 class="title">Example 9.14. Storing non-<span class="acronym">ASCII</span> characters
</h3><pre class="screen">
<tt class="prompt">>>> </tt><span class="userinput">s = u<span class='pystring'>'La Pe\xf1a'</span></span> <a name="kgp.unicode.2.1"></a><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12">
<tt class="prompt">>>> </tt><span class="userinput"><span class='pykeyword'>print</span> s</span> <a name="kgp.unicode.2.2"></a><img src="../images/callouts/2.png" alt="2" border="0" width="12" height="12">
<span class="traceback">Traceback (innermost last):
File "<interactive input>", line 1, in ?
UnicodeError: ASCII encoding error: ordinal not in range(128)</span>
<tt class="prompt">>>> </tt><span class="userinput"><span class='pykeyword'>print</span> s.encode(<span class='pystring'>'latin-1'</span>)</span> <a name="kgp.unicode.2.3"></a><img src="../images/callouts/3.png" alt="3" border="0" width="12" height="12">
<span class="computeroutput">La Peña</span></pre><div class="calloutlist">
<table border="0" summary="Callout list">
<tr>
<td width="12" valign="top" align="left"><a href="#kgp.unicode.2.1"><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">The real advantage of unicode, of course, is its ability to store non-<span class="acronym">ASCII</span> characters, like the Spanish “<span class="quote"><tt class="literal">ñ</tt></span>” (<tt class="literal">n</tt> with a tilde over it). The unicode character code for the tilde-n is <tt class="literal">0xf1</tt> in hexadecimal (241 in decimal), which you can type like this: <tt class="literal">\xf1</tt>.
</td>
</tr>
<tr>
<td width="12" valign="top" align="left"><a href="#kgp.unicode.2.2"><img src="../images/callouts/2.png" alt="2" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">Remember I said that the <tt class="function">print</tt> function attempts to convert a unicode string to <span class="acronym">ASCII</span> so it can print it? Well, that's not going to work here, because your unicode string contains non-<span class="acronym">ASCII</span> characters, so <span class="application">Python</span> raises a <tt class="errorname">UnicodeError</tt> error.
</td>
</tr>
<tr>
<td width="12" valign="top" align="left"><a href="#kgp.unicode.2.3"><img src="../images/callouts/3.png" alt="3" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">Here's where the conversion-from-unicode-to-other-encoding-schemes comes in. <tt class="varname">s</tt> is a unicode string, but <tt class="function">print</tt> can only print a regular string. To solve this problem, you call the <tt class="function">encode</tt> method, available on every unicode string, to convert the unicode string to a regular string in the given encoding scheme,
which you pass as a parameter. In this case, you're using <tt class="literal">latin-1</tt> (also known as <tt class="literal">iso-8859-1</tt>), which includes the tilde-n (whereas the default <span class="acronym">ASCII</span> encoding scheme did not, since it only includes characters numbered 0 through 127).
</td>
</tr>
</table>
</div>
</div>
<p>Remember I said <span class="application">Python</span> usually converted unicode to <span class="acronym">ASCII</span> whenever it needed to make a regular string out of a unicode string? Well, this default encoding scheme is an option which
you can customize.
</p>
<div class="example"><a name="d0e24009"></a><h3 class="title">Example 9.15. <tt class="filename">sitecustomize.py</tt></h3><pre class="programlisting">
<span class='pycomment'># sitecustomize.py </span><a name="kgp.unicode.3.1"></a><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12">
<span class='pycomment'># this file can be anywhere in your Python path,</span>
<span class='pycomment'># but it usually goes in ${pythondir}/lib/site-packages/</span>
<span class='pykeyword'>import</span> sys
sys.setdefaultencoding(<span class='pystring'>'iso-8859-1'</span>) <a name="kgp.unicode.3.2"></a><img src="../images/callouts/2.png" alt="2" border="0" width="12" height="12">
</pre><div class="calloutlist">
<table border="0" summary="Callout list">
<tr>
<td width="12" valign="top" align="left"><a href="#kgp.unicode.3.1"><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left"><tt class="filename">sitecustomize.py</tt> is a special script; <span class="application">Python</span> will try to import it on startup, so any code in it will be run automatically. As the comment mentions, it can go anywhere
(as long as <tt class="literal">import</tt> can find it), but it usually goes in the <tt class="filename">site-packages</tt> directory within your <span class="application">Python</span> <tt class="filename">lib</tt> directory.
</td>
</tr>
<tr>
<td width="12" valign="top" align="left"><a href="#kgp.unicode.3.2"><img src="../images/callouts/2.png" alt="2" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left"><tt class="function">setdefaultencoding</tt> function sets, well, the default encoding. This is the encoding scheme that <span class="application">Python</span> will try to use whenever it needs to auto-coerce a unicode string into a regular string.
</td>
</tr>
</table>
</div>
</div>
<div class="example"><a name="d0e24048"></a><h3 class="title">Example 9.16. Effects of setting the default encoding</h3><pre class="screen">
<tt class="prompt">>>> </tt><span class="userinput"><span class='pykeyword'>import</span> sys</span>
<tt class="prompt">>>> </tt><span class="userinput">sys.getdefaultencoding()</span> <a name="kgp.unicode.4.1"></a><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12">
<span class="computeroutput">'iso-8859-1'</span>
<tt class="prompt">>>> </tt><span class="userinput">s = u<span class='pystring'>'La Pe\xf1a'</span></span>
<tt class="prompt">>>> </tt><span class="userinput"><span class='pykeyword'>print</span> s</span> <a name="kgp.unicode.4.2"></a><img src="../images/callouts/2.png" alt="2" border="0" width="12" height="12">
<span class="computeroutput">La Peña</span></pre><div class="calloutlist">
<table border="0" summary="Callout list">
<tr>
<td width="12" valign="top" align="left"><a href="#kgp.unicode.4.1"><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">This example assumes that you have made the changes listed in the previous example to your <tt class="filename">sitecustomize.py</tt> file, and restarted <span class="application">Python</span>. If your default encoding still says <tt class="literal">'ascii'</tt>, you didn't set up your <tt class="filename">sitecustomize.py</tt> properly, or you didn't restart <span class="application">Python</span>. The default encoding can only be changed during <span class="application">Python</span> startup; you can't change it later. (Due to some wacky programming tricks that I won't get into right now, you can't even
call <tt class="function">sys.setdefaultencoding</tt> after <span class="application">Python</span> has started up. Dig into <tt class="filename">site.py</tt> and search for “<span class="quote"><tt class="literal">setdefaultencoding</tt></span>” to find out how.)
</td>
</tr>
<tr>
<td width="12" valign="top" align="left"><a href="#kgp.unicode.4.2"><img src="../images/callouts/2.png" alt="2" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">Now that the default encoding scheme includes all the characters you use in your string, <span class="application">Python</span> has no problem auto-coercing the string and printing it.
</td>
</tr>
</table>
</div>
</div>
<div class="example"><a name="d0e24123"></a><h3 class="title">Example 9.17. Specifying encoding in <tt class="filename">.py</tt> files
</h3>
<p>If you are going to be storing non-ASCII strings within your <span class="application">Python</span> code, you'll need to specify the encoding of each individual <tt class="filename">.py</tt> file by putting an encoding declaration at the top of each file. This declaration defines the <tt class="filename">.py</tt> file to be UTF-8:
</p><pre class="programlisting">
<span class='pycomment'>#!/usr/bin/env python</span>
<span class='pycomment'># -*- coding: UTF-8 -*-</span>
</pre></div>
<p>Now, what about <span class="acronym">XML</span>? Well, every <span class="acronym">XML</span> document is in a specific encoding. Again, ISO-8859-1 is a popular encoding for data in Western European languages. KOI8-R
is popular for Russian texts. The encoding, if specified, is in the header of the <span class="acronym">XML</span> document.
</p>
<div class="example"><a name="d0e24153"></a><h3 class="title">Example 9.18. <tt class="filename">russiansample.xml</tt></h3><pre class="screen"><span class="computeroutput">
<?xml version="1.0" encoding="koi8-r"?> </span><a name="kgp.unicode.5.1"></a><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12"><span class="computeroutput">
<preface>
<title>Предисловие</title> </span><a name="kgp.unicode.5.2"></a><img src="../images/callouts/2.png" alt="2" border="0" width="12" height="12"><span class="computeroutput">
</preface></span></pre><div class="calloutlist">
<table border="0" summary="Callout list">
<tr>
<td width="12" valign="top" align="left"><a href="#kgp.unicode.5.1"><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">This is a sample extract from a real Russian <span class="acronym">XML</span> document; it's part of a Russian translation of this very book. Note the encoding, <tt class="literal">koi8-r</tt>, specified in the header.
</td>
</tr>
<tr>
<td width="12" valign="top" align="left"><a href="#kgp.unicode.5.2"><img src="../images/callouts/2.png" alt="2" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">These are Cyrillic characters which, as far as I know, spell the Russian word for “<span class="quote">Preface</span>”. If you open this file in a regular text editor, the characters will most likely like gibberish, because they're encoded
using the <tt class="literal">koi8-r</tt> encoding scheme, but they're being displayed in <tt class="literal">iso-8859-1</tt>.
</td>
</tr>
</table>
</div>
</div>
<div class="example"><a name="d0e24188"></a><h3 class="title">Example 9.19. Parsing <tt class="filename">russiansample.xml</tt></h3><pre class="screen">
<tt class="prompt">>>> </tt><span class="userinput"><span class='pykeyword'>from</span> xml.dom <span class='pykeyword'>import</span> minidom</span>
<tt class="prompt">>>> </tt><span class="userinput">xmldoc = minidom.parse(<span class='pystring'>'russiansample.xml'</span>)</span> <a name="kgp.unicode.6.1"></a><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12">
<tt class="prompt">>>> </tt><span class="userinput">title = xmldoc.getElementsByTagName(<span class='pystring'>'title'</span>)[0].firstChild.data</span>
<tt class="prompt">>>> </tt><span class="userinput">title</span> <a name="kgp.unicode.6.2"></a><img src="../images/callouts/2.png" alt="2" border="0" width="12" height="12">
<span class="computeroutput">u'\u041f\u0440\u0435\u0434\u0438\u0441\u043b\u043e\u0432\u0438\u0435'</span>
<tt class="prompt">>>> </tt><span class="userinput"><span class='pykeyword'>print</span> title</span> <a name="kgp.unicode.6.3"></a><img src="../images/callouts/3.png" alt="3" border="0" width="12" height="12">
<span class="traceback">Traceback (innermost last):
File "<interactive input>", line 1, in ?
UnicodeError: ASCII encoding error: ordinal not in range(128)</span>
<tt class="prompt">>>> </tt><span class="userinput">convertedtitle = title.encode(<span class='pystring'>'koi8-r'</span>)</span> <a name="kgp.unicode.6.4"></a><img src="../images/callouts/4.png" alt="4" border="0" width="12" height="12">
<tt class="prompt">>>> </tt><span class="userinput">convertedtitle</span>
<span class="computeroutput">'\xf0\xd2\xc5\xc4\xc9\xd3\xcc\xcf\xd7\xc9\xc5'</span>
<tt class="prompt">>>> </tt><span class="userinput"><span class='pykeyword'>print</span> convertedtitle</span> <a name="kgp.unicode.6.5"></a><img src="../images/callouts/5.png" alt="5" border="0" width="12" height="12">
<span class="computeroutput">Предисловие</span></pre><div class="calloutlist">
<table border="0" summary="Callout list">
<tr>
<td width="12" valign="top" align="left"><a href="#kgp.unicode.6.1"><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">I'm assuming here that you saved the previous example as <tt class="filename">russiansample.xml</tt> in the current directory. I am also, for the sake of completeness, assuming that you've changed your default encoding back
to <tt class="literal">'ascii'</tt> by removing your <tt class="filename">sitecustomize.py</tt> file, or at least commenting out the <tt class="function">setdefaultencoding</tt> line.
</td>
</tr>
<tr>
<td width="12" valign="top" align="left"><a href="#kgp.unicode.6.2"><img src="../images/callouts/2.png" alt="2" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">Note that the text data of the <tt class="sgmltag-element">title</tt> tag (now in the <tt class="varname">title</tt> variable, thanks to that long concatenation of <span class="application">Python</span> functions which I hastily skipped over and, annoyingly, won't explain until the next section) -- the text data inside the
<span class="acronym">XML</span> document's <tt class="sgmltag-element">title</tt> element is stored in unicode.
</td>
</tr>
<tr>
<td width="12" valign="top" align="left"><a href="#kgp.unicode.6.3"><img src="../images/callouts/3.png" alt="3" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">Printing the title is not possible, because this unicode string contains non-<span class="acronym">ASCII</span> characters, so <span class="application">Python</span> can't convert it to <span class="acronym">ASCII</span> because that doesn't make sense.
</td>
</tr>
<tr>
<td width="12" valign="top" align="left"><a href="#kgp.unicode.6.4"><img src="../images/callouts/4.png" alt="4" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">You can, however, explicitly convert it to <tt class="literal">koi8-r</tt>, in which case you get a (regular, not unicode) string of single-byte characters (<tt class="literal">f0</tt>, <tt class="literal">d2</tt>, <tt class="literal">c5</tt>, and so forth) that are the <tt class="literal">koi8-r</tt>-encoded versions of the characters in the original unicode string.
</td>
</tr>
<tr>
<td width="12" valign="top" align="left"><a href="#kgp.unicode.6.5"><img src="../images/callouts/5.png" alt="5" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">Printing the <tt class="literal">koi8-r</tt>-encoded string will probably show gibberish on your screen, because your <span class="application">Python</span> <span class="acronym">IDE</span> is interpreting those characters as <tt class="literal">iso-8859-1</tt>, not <tt class="literal">koi8-r</tt>. But at least they do print. (And, if you look carefully, it's the same gibberish that you saw when you opened the original
<span class="acronym">XML</span> document in a non-unicode-aware text editor. <span class="application">Python</span> converted it from <tt class="literal">koi8-r</tt> into unicode when it parsed the <span class="acronym">XML</span> document, and you've just converted it back.)
</td>
</tr>
</table>
</div>
</div>
<p>To sum up, unicode itself is a bit intimidating if you've never seen it before, but unicode data is really very easy to handle
in <span class="application">Python</span>. If your <span class="acronym">XML</span> documents are all 7-bit <span class="acronym">ASCII</span> (like the examples in this chapter), you will literally never think about unicode. <span class="application">Python</span> will convert the <span class="acronym">ASCII</span> data in the <span class="acronym">XML</span> documents into unicode while parsing, and auto-coerce it back to <span class="acronym">ASCII</span> whenever necessary, and you'll never even notice. But if you need to deal with that in other languages, <span class="application">Python</span> is ready.
</p>
<div class="furtherreading">
<h3>Further reading</h3>
<ul>
<li><a href="http://www.unicode.org/">Unicode.org</a> is the home page of the unicode standard, including a brief <a href="http://www.unicode.org/standard/principles.html">technical introduction</a>.
</li>
<li><a href="http://www.reportlab.com/i18n/python_unicode_tutorial.html">Unicode Tutorial</a> has some more examples of how to use <span class="application">Python</span>'s unicode functions, including how to force <span class="application">Python</span> to coerce unicode into <span class="acronym">ASCII</span> even when it doesn't really want to.
</li>
<li><a href="http://www.python.org/peps/pep-0263.html">PEP 263</a> goes into more detail about how and when to define a character encoding in your <tt class="filename">.py</tt> files.
</li>
</ul>
</div>
<div class="footnotes">
<h3 class="footnotetitle">Footnotes</h3>
<div class="footnote">
<p><sup>[<a name="ftn.d0e23786" href="#d0e23786">5</a>] </sup>This, sadly, is <span class="emphasis"><em>still</em></span> an oversimplification. Unicode now has been extended to handle ancient Chinese, Korean, and Japanese texts, which had so
many different characters that the 2-byte unicode system could not represent them all. But <span class="application">Python</span> doesn't currently support that out of the box, and I don't know if there is a project afoot to add it. You've reached the
limits of my expertise, sorry.
</p>
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