9.4. Unicode

Unicode is a system to represent characters from all the world's different languages. When Python parses an XML document, all data is stored in memory as unicode.

You'll get to all that in a minute, but first, some background.

Historical note. 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.

To solve these problems, unicode represents each character as a 2-byte number, from 0 to 65535.[5] 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.

Of course, there is still the matter of all these legacy encoding systems. 7-bit ASCII, for instance, which stores English characters as numbers ranging from 0 to 127. (65 is capital “A”, 97 is lowercase “a”, and so forth.) English has a very simple alphabet, so it can be completely expressed in 7-bit ASCII. Western European languages like French, Spanish, and German all use an encoding system called ISO-8859-1 (also called “latin-1”), which uses the 7-bit ASCII 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 ASCII 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.

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 XML document which explicitly specifies the encoding scheme.

And on that note, let's get back to Python.

Python has had unicode support throughout the language since version 2.0. The XML package uses unicode to store all parsed XML data, but you can use unicode anywhere.

Example 9.13. Introducing unicode

>>> s = u'Dive in'            1
>>> s
u'Dive in'
>>> print s                   2
Dive in
1 To create a unicode string instead of a regular ASCII string, add the letter “u” before the string. Note that this particular string doesn't have any non-ASCII characters. That's fine; unicode is a superset of ASCII (a very large superset at that), so any regular ASCII string can also be stored as unicode.
2 When printing a string, Python will attempt to convert it to your default encoding, which is usually ASCII. (More on this in a minute.) Since this unicode string is made up of characters that are also ASCII characters, printing it has the same result as printing a normal ASCII string; the conversion is seamless, and if you didn't know that s was a unicode string, you'd never notice the difference.

Example 9.14. Storing non-ASCII characters

>>> s = u'La Pe\xf1a'         1
>>> print s                   2
Traceback (innermost last):
  File "<interactive input>", line 1, in ?
UnicodeError: ASCII encoding error: ordinal not in range(128)
>>> print s.encode('latin-1') 3
La Peña
1 The real advantage of unicode, of course, is its ability to store non-ASCII characters, like the Spanish “ñ” (n with a tilde over it). The unicode character code for the tilde-n is 0xf1 in hexadecimal (241 in decimal), which you can type like this: \xf1.
2 Remember I said that the print function attempts to convert a unicode string to ASCII so it can print it? Well, that's not going to work here, because your unicode string contains non-ASCII characters, so Python raises a UnicodeError error.
3 Here's where the conversion-from-unicode-to-other-encoding-schemes comes in. s is a unicode string, but print can only print a regular string. To solve this problem, you call the encode 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 latin-1 (also known as iso-8859-1), which includes the tilde-n (whereas the default ASCII encoding scheme did not, since it only includes characters numbered 0 through 127).

Remember I said Python usually converted unicode to ASCII 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.

Example 9.15. sitecustomize.py

# sitecustomize.py                   1
# this file can be anywhere in your Python path,
# but it usually goes in ${pythondir}/lib/site-packages/
import sys
sys.setdefaultencoding('iso-8859-1') 2
1 sitecustomize.py is a special script; Python 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 import can find it), but it usually goes in the site-packages directory within your Python lib directory.
2 setdefaultencoding function sets, well, the default encoding. This is the encoding scheme that Python will try to use whenever it needs to auto-coerce a unicode string into a regular string.

Example 9.16. Effects of setting the default encoding

>>> import sys
>>> sys.getdefaultencoding() 1
>>> s = u'La Pe\xf1a'
>>> print s                  2
La Peña
1 This example assumes that you have made the changes listed in the previous example to your sitecustomize.py file, and restarted Python. If your default encoding still says 'ascii', you didn't set up your sitecustomize.py properly, or you didn't restart Python. The default encoding can only be changed during Python 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 sys.setdefaultencoding after Python has started up. Dig into site.py and search for “setdefaultencoding” to find out how.)
2 Now that the default encoding scheme includes all the characters you use in your string, Python has no problem auto-coercing the string and printing it.

Example 9.17. Specifying encoding in .py files

If you are going to be storing non-ASCII strings within your Python code, you'll need to specify the encoding of each individual .py file by putting an encoding declaration at the top of each file. This declaration defines the .py file to be UTF-8:

#!/usr/bin/env python
# -*- coding: UTF-8 -*-

Now, what about XML? Well, every XML 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 XML document.

Example 9.18. russiansample.xml

<?xml version="1.0" encoding="koi8-r"?>       1
<title>Предисловие</title>                    2
1 This is a sample extract from a real Russian XML document; it's part of a Russian translation of this very book. Note the encoding, koi8-r, specified in the header.
2 These are Cyrillic characters which, as far as I know, spell the Russian word for “Preface”. If you open this file in a regular text editor, the characters will most likely like gibberish, because they're encoded using the koi8-r encoding scheme, but they're being displayed in iso-8859-1.

Example 9.19. Parsing russiansample.xml

>>> from xml.dom import minidom
>>> xmldoc = minidom.parse('russiansample.xml') 1
>>> title = xmldoc.getElementsByTagName('title')[0].firstChild.data
>>> title                                       2
>>> print title                                 3
Traceback (innermost last):
  File "<interactive input>", line 1, in ?
UnicodeError: ASCII encoding error: ordinal not in range(128)
>>> convertedtitle = title.encode('koi8-r')     4
>>> convertedtitle
>>> print convertedtitle                        5
1 I'm assuming here that you saved the previous example as russiansample.xml in the current directory. I am also, for the sake of completeness, assuming that you've changed your default encoding back to 'ascii' by removing your sitecustomize.py file, or at least commenting out the setdefaultencoding line.
2 Note that the text data of the title tag (now in the title variable, thanks to that long concatenation of Python functions which I hastily skipped over and, annoyingly, won't explain until the next section) -- the text data inside the XML document's title element is stored in unicode.
3 Printing the title is not possible, because this unicode string contains non-ASCII characters, so Python can't convert it to ASCII because that doesn't make sense.
4 You can, however, explicitly convert it to koi8-r, in which case you get a (regular, not unicode) string of single-byte characters (f0, d2, c5, and so forth) that are the koi8-r-encoded versions of the characters in the original unicode string.
5 Printing the koi8-r-encoded string will probably show gibberish on your screen, because your Python IDE is interpreting those characters as iso-8859-1, not koi8-r. But at least they do print. (And, if you look carefully, it's the same gibberish that you saw when you opened the original XML document in a non-unicode-aware text editor. Python converted it from koi8-r into unicode when it parsed the XML document, and you've just converted it back.)

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 Python. If your XML documents are all 7-bit ASCII (like the examples in this chapter), you will literally never think about unicode. Python will convert the ASCII data in the XML documents into unicode while parsing, and auto-coerce it back to ASCII whenever necessary, and you'll never even notice. But if you need to deal with that in other languages, Python is ready.

Further reading


[5] This, sadly, is still 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 Python 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.