LAX.py

a simple way to read lists of fields from flat XML records

Many XML formats are very simple: all the fields are needed, there is no tree hierarchy, all the text inside of the tags is used, and the text is short (it can easily fit inside of memory). SAX is pretty good for this but it's still somewhat complicated to use. DOM is designed to handle tree structures so is a bit too much for a simple flat data structure.

This module implements a new, simpler API, which I'll call LAX. It only works well when the elements are small and non-hierarchical. LAX has three callbacks.

start()

the first method called

element(tag, attrs, text)

called once for each element, after the

element has been fully read. (Ie, called when the endElement would be called.) The tag is the element name, the attrs is the attribute object that would be used in a startElement, and the text is all the text between the two tags. The text is the concatenation of all the characters() calls.

end()

the last method called (unless there was an error)

LAX.LAX is an content handler which converts the SAX events to LAX events. Here is an example use:

>>> from Martel import Word, Whitespace, Group, Integer, Rep1, AnyEol >>> format = Rep1(Group("line", Word("name") + Whitespace() + ... Integer("age")) + AnyEol()) >>> parser = format.make_parser() >>> >>> from Martel import LAX >>> class PrintFields(LAX.LAX): ... def element(self, tag, attrs, text): ... print tag, "has", repr(text) ... >>> parser.setContentHandler(PrintFields()) >>> text = "Maggie 3\nPorter 1\n" >>> parser.parseString(text) name has Maggie age has 3 line has Maggie 3 name has Porter age has 1 line has Porter 1 >>>

Callbacks take some getting used to. Many people prefer an iterative solution which returns all of the fields of a given record at one time. The default implementation of LAX.LAX helps this case. The start method initializes a local variable named groups, which is dictionary. When the element method is called, the information is added to groups; the key is the element name and the value is the list of text strings. It's a list because the same field name may occur multiple times.

If you need the element attributes as well as the name, use the LAX.LAXAttrs class, which stores a list of 2-ples (text, attrs) instead of just the text.

For examples:

  >>> iterator = format.make_iterator("line")
  >>> for record in iterator.iterateString(text, LAX.LAX()):
  ...     print record.groups["name"][0], "is", record.groups["age"][0]
  ...
  Maggie is 3
  Porter is 1
  >>>

If you only want a few fields, you can pass the list to constructor, as in:

>>> lax = LAX.LAX(["name", "sequence"]) >>>