XPath expressions

Please note

This page is a stub and will be updated in due course, in particular with the list of common XPath extension functions provided by PT.

PT property expressions make prolific use of XPath. The reference Python implementation of PT is based on lxml, which supports the XPath 1.0 feature set.

Prefixes

Although labels conventionally make the main PDS4 data dictionary schema the default (prefix-less) namespace, XPath does not have a notion of a default namespace. PT will therefore map the default namespace of the active source (i.e. the XPath evaluation context) to the pds prefix. This means that property XPath expressions are required to prefix elements in the default namespace with pds, e.g. //pds:Time_Coordinates/pds:start_date_time.

Extension functions

XPath extensions are functions made available within XPath expressions either by PT itself (to address common needs) or a specific mission or instrument team. Such functions are placed under their own function namespace to differentiate them from the standard array of XPath functions, and from other extension groups. The common set of functions provided by Passthrough can be found under the pt namespace.