Idea: Utility for building complex XPath expressions from simple ones

[janssenhenning]

This issue sketches a design to enable easier construction of complex xpath expressions from simple ones without needing to fall back to knowing the full xpath.

The current workflow for this is

#Get the simple xpath from the schema dictionary
simple = schema_dict.tag_xpath(tag_name)

#Build your complex xpath manually with string manipulation
...

#Evaluating the complex xpath

This is used in the xml_setters set_species for example to select specific species

This building of the complex xpath could be designed in a more concise way. This is just a sketch how the usage of this functionality could look. What I have in mind is only adding so called predicates and not wildcard expressions or entering relative expressions https://www.w3schools.com/xml/xpath_syntax.asp

lo_path = schema_dict.tag_xpath('lo', contains='species')

builder = XPathBuilder(lo_path, filters={
                           'species': {'index': {'<=': 3 }}, #Get the first three species nodes
                           'lo': {'type': {'==': 'SCLO'}}
                })
xpath = builder.build() #/fleurInput/atomSpecies/species[position()<=3]/lo[@type='SCLO']

The syntax of the filters dictionary is borrowed from the AiiDA Querybuilder. I don't know if this is the best approach but it seems like a robust starting point to me.

Ultimately this allows to add an argument filters or where to specify these filters for xml_setters/xml_getters

More things that can be added on to this are:

• Subclassing the basic XPathBuilder with SchemaXPathBuilder with additional validation and functionality from teh schema dictionaries
• Instead of the example /fleurInput/atomSpecies/species[position()<=3]/lo[@type='SCLO'] the recommended safe way to do these variable expressions is /fleurInput/atomSpecies/species[position()<=$index]/lo[@type=$type] and passing the actual values as variables to the Xpath engine. This can also be supported by this builder mechanism

[janssenhenning]

Closed via #96