Compiling XPath Expressions
In this chapter, we use the Evaluate
function from the 4XPath API to apply
XPath expressions against node sets. For programmatic use of XPath
within Python, the 4XPath API is readily available and offers
considerable power.
Most of the XPath API is geared towards supporting XPath expressions, as XPath is a standard. But for the programmer embedding XPath processing functionality into their applications, there is some optimization found in 4XPath.
The Compile
and
Context
functions aid the developer
to create compiled XPath expressions for repeated use against multiple
documents. For example, if you are accepting large numbers of XML
documents from customers or suppliers, you may want to apply an XPath
expression to each one (as it arrives) to figure out what to do with it,
or where to route it within your organization. Having your XPath
expression readily compiled and applied against each unique document
adds speed to your application, as you’ve done away with the need to
parse the XPath expression.
The Compile
function returns an
expression object that supports an evaluate
method similar to the Evaluate
function used thus far in this
chapter. However, the method expects a Context
object, not a node. The task of
compiling an expression, and then using the compiled version, is fairly
simple:
expression = Compile("ship/@name") context = Context(dom.documentElement) nodes = expression.evaluate(context)
The first step is to generate an expression; the second ...
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