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large output in XML using unicode and namespaces

Project description

loxun is a Python module to write large output in XML using Unicode and namespaces. Of course you can also use it for small XML output with plain 8 bit strings and no namespaces.

loxun’s features are:

  • small memory foot print: the document is created on the fly by writing to an output stream, no need to keep all of it in memory.

  • easy to use namespaces: simply add a namespace and refer to it using the standard namespace:tag syntax.

  • mix unicode and string: pass both unicode or plain 8 bit strings to any of the methods. Internally loxun converts them to unicode, so once a parameter got accepted by the API you can rely on it not causing any messy UnicodeError trouble.

  • automatic escaping: no need to manually handle special characters such as < or & when writing text and attribute values.

  • robustness: while you write the document, sanity checks are performed on everything you do. Many silly mistakes immediately result in an XmlError, for example missing end elements or references to undeclared namespaces.

  • open source: distributed under the GNU Lesser General Public License 3 or later.

Here is a very basic example. First you have create an output stream. In many cases this would be a file, but for the sake of simplicity we use a StringIO here:

>>> from StringIO import StringIO
>>> out = StringIO()
>>> xml = XmlWriter(out)

Now write the content:

>>> xml.addNamespace("xhtml", "http://www.w3.org/1999/xhtml")
>>> xml.startTag("xhtml:html")
>>> xml.startTag("xhtml:body")
>>> xml.text("Hello world!")
>>> xml.tag("xhtml:img", {"src": "smile.png", "alt": ":-)"})
>>> xml.endTag()
>>> xml.endTag()
>>> xml.close()

And the result is:

>>> print out.getvalue().rstrip("\r\n")
<?xml version="1.0" encoding="utf-8"?>
<xhtml:html xlmns:xhtml="http://www.w3.org/1999/xhtml">
  <xhtml:body>
    Hello world!
    <xhtml:img alt=":-)" src="smile.png" />
  </xhtml:body>
</xhtml:html>

Writing a simple document

The following example creates a very simple XHTML document.

To make it simple, the output goes to a string, but you could also use a file that has been created using codecs.open(filename, "wb", encoding).

>>> from StringIO import StringIO
>>> out = StringIO()

First create an XmlWriter to write the XML code to the specified output:

>>> xml = XmlWriter(out)

This automatically adds the XML prolog:

>>> print out.getvalue().rstrip("\r\n")
<?xml version="1.0" encoding="utf-8"?>

Next add the <html> start tag:

>>> xml.startTag("html")

Now comes the <body>. To pass attributes, specify them in a dictionary. So in order to add:

<body id="top">

use:

>>> xml.startTag("body", {"id": "top"})

Let’ add a little text so there is something to look at:

>>> xml.text("Hello world!")

Wrap it up: close all elements and the document.

>>> xml.endTag()
>>> xml.endTag()
>>> xml.close()

And this is what we get:

>>> print out.getvalue().rstrip("\r\n")
<?xml version="1.0" encoding="utf-8"?>
<html>
  <body id="top">
    Hello world!
  </body>
</html>

Now the same thing but with a namespace. First create the prolog and header like above:

>>> out = StringIO()
>>> xml = XmlWriter(out)

Next add the namespace:

>>> xml.addNamespace("xhtml", "http://www.w3.org/1999/xhtml")

Now elements can use qualified tag names using a colon (:) to separate namespace and tag name:

>>> xml.startTag("xhtml:html")
>>> xml.startTag("xhtml:body")
>>> xml.text("Hello world!")
>>> xml.endTag()
>>> xml.endTag()
>>> xml.close()

As a result, tag names are now prefixed with “xhtml:”:

>>> print out.getvalue().rstrip("\r\n")
<?xml version="1.0" encoding="utf-8"?>
<xhtml:html xlmns:xhtml="http://www.w3.org/1999/xhtml">
  <xhtml:body>
    Hello world!
  </xhtml:body>
</xhtml:html>

Working with non ASCII characters

Sometimes you want to use characters outside the ASCII range, for example German Umlauts, the Euro symbol or Japanese Kanji. The easiest and performance wise best way is to use Unicode strings. For example:

>>> from StringIO import StringIO
>>> out = StringIO()
>>> xml = XmlWriter(out, prolog=False)
>>> xml.text(u"The price is \u20ac 100") # Unicode of Euro symbol
>>> out.getvalue().rstrip("\r\n")
'The price is \xe2\x82\xac 100'

Notice the “u” before the string passed to text(), it declares the string to be a unicode string that can hold any character, even those that are beyond the 8 bit range.

Also notice that in the output the Euro symbol looks very different from the input. This is because the output encoding is UTF-8 (the default), which has the advantage of keeping all ASCII characters the same and turning any characters with a code or 128 or more into a sequence of 8 bit bytes that can easily fit into an output stream to a binary file or StringIO.

If you have to stick to classic 8 bit string parameters, loxun attempts to convert them to unicode. By default it assumes ASCII encoding, which does not work out as soon as you use a character outside the ASCII range:

>>> from StringIO import StringIO
>>> out = StringIO()
>>> xml = XmlWriter(out, prolog=False)
>>> xml.text("The price is \xa4 100") # ISO-8859-15 code of Euro symbol
Traceback (most recent call last):
    ...
UnicodeDecodeError: 'ascii' codec can't decode byte 0xa4 in position 13: ordinal not in range(128)

In this case you have to tell the writer the encoding you use by specifying the the sourceEncoding:

>>> from StringIO import StringIO
>>> out = StringIO()
>>> xml = XmlWriter(out, prolog=False, sourceEncoding="iso-8859-15")

Now everything works out again:

>>> xml.text("The price is \xa4 100") # ISO-8859-15 code of Euro symbol
>>> out.getvalue().rstrip("\r\n")
'The price is \xe2\x82\xac 100'

Of course in practice you will not mess around with hex codes to pass your texts. Instead you just specify the source encoding using the mechanisms described in PEP 263, Defining Python Source Code Encodings:

Changing the XML prolog

When you create a writer, it automatically write an XML prolog processing instruction to the output. This is what the default prolog looks like:

>>> from StringIO import StringIO
>>> out = StringIO()
>>> xml = XmlWriter(out)
>>> print out.getvalue().rstrip("\r\n")
<?xml version="1.0" encoding="utf-8"?>

You can change the version or encoding:

>>> out = StringIO()
>>> xml = XmlWriter(out, encoding=u"ascii", version=u"1.1")
>>> print out.getvalue().rstrip("\r\n")
<?xml version="1.1" encoding="ascii"?>

To completely omit the prolog, set the parameter prolog=False:

>>> out = StringIO()
>>> xml = XmlWriter(out, prolog=False)
>>> out.getvalue()
''

Version history

Version 0.4, 21-May-2010

  • Added option sourceEncoding to simplify processing of classic strings. The manual section “Working with non ASCII characters” explains how to use it.

Version 0.3, 17-May-2010

  • Added scoped namespaces which are removed automatically by endTag().

  • Changed text() to normalize newlines and white space if pretty printing is enabled.

  • Moved writing of XML prolog to the constructor and removed prolog(). To omit the prolog, specify prolog=False when creating the XmlWriter. If you later want to write the prolog yourself, use processingInstruction().

  • Renamed *Element() to *Tag because they really only write tags, not whole elements.

Version 0.2, 16-May-2010

  • Added comment(), cdata() and processingInstruction() to write these specific XML constructs.

  • Added indentation and automatic newline to text if pretty printing is enabled.

  • Removed newline from prolog in case pretty printing is disabled.

  • Fixed missing “?” in prolog.

Version 0.1, 15-May-2010

  • Initial release.

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