Those reports functions are defined in report parsers classes, that's the name that those kind of classes receive on Odoo. How could we define a report parser and tell Odoo that need to use it in a report?

A report parser is a class that need to inherit from the class openerp.report.report_sxw.rml_parse and it's used to let values available to the report. Those values could be anything: objects, methods, functions, classes, packages, etc. You define those values available to the report by just overriding the __init__ method of the class and after call to the super __init__ method you need to update self.localcontext dict to add your new values that mostly are methods of the class.

A report parser is a class not attached to a model so you could write a parser, and use it for multiple reports just need to be careful about what you use it for every report.

A report parser definition, with an utility method that get the display value of a selection field and another utility to find the next value of a ir.sequence by it's code, to serve as examples:

from openerp.report import report_sxw
from openerp import api, models

class example_report_parser(report_sxw.rml_parse):
    
    def __init__(self, cr, uid, name, context):
        super(example_report_parser, self).__init__(cr, uid, name, context=context)
        self.localcontext.update({
            'next_seq': self._get_next_seq,
            'select_value': self._selection_value,
        })
    
    def _get_next_seq(self, seq_code):
        code = self.pool.get('ir.sequence').next_by_code(self.cr, self.uid, seq_code)
        return code
    
    def _selection_value(self, model, name, value):
        if value:
            field = self.pool.get(model)._fields[name]
            env = api.Environment(self.cr, self.uid, self.localcontext)
            val = dict(field.get_description(env)['selection'])[value]
            return self._translate(val)
        return ''

To use this report parser in a Qweb Report you need to declare a new class that inherit from openerp.models.AbstractModel just to define a few class properties like:

class report_test1_parser(models.AbstractModel):
    _name = 'report.parser_test_module.report_test1'
    _inherit = 'report.abstract_report'
    _template = 'parser_test_module.report_test1'
    _wrapped_report_class = example_report_parser

Where:

_name need to be conformed by 'report.' followed by the name of the report template defined, in this case the report template name is 'parser_test_module.report_test1'

_inherit need to be 'report.abstract_report' or a model that already it's inheriting from 'report.abstract_report'

_template need to be the name of the report template defined, in this case 'parser_test_module.report_test1' and the same used to conform the value of _name

_wrapped_report_class need to be the report parser class object defined

As said before, you could reuse report parser definitions to share functionality with multiple reports like:

class report_test2_parser(models.AbstractModel):
    _name = 'report.parser_test_module.report_test2'
    _inherit = 'report.abstract_report'
    _template = 'parser_test_module.report_test2'
    _wrapped_report_class = example_report_parser

With this setup you need to have defined 2 report templates with the names 'parser_test_module.report_test1' and 'parser_test_module.report_test2'. Like:

<report id="report_parser_sale_test" model="sale.order" string="Report Parser Sale Test" report_type="qweb-pdf" name="parser_test_module.report_test1" menu="True"/>

<report id="report_parser_purchase_test" model="purchase.order" string="Report Parser Purchase Test" report_type="qweb-pdf" name="parser_test_module.report_test2" menu="True"/>

In both report templates you could use the functions _get_next_seq and _selection_value but not under those names, you need to use those functions using the names used to register those functions on the self.localcontext dict.

The report templates could use those functions like this:

#For the report of sale.order

<span t-esc="next_seq('sale.order')"/>
<span t-esc="select_value('sale.order','state', o.state)"/>

#For the report of purchase.order

<span t-esc="next_seq('purchase.order')"/>
<span t-esc="select_value('purchase.order','state', o.state)"/>

For now I showed you how could you define a report parser to be used on one or more report templates even when the models are differents for those templates, and as I said before you just need to be careful about what you use at every template to don't break anything in the report template. The utility functions shows how to search data in others models using what you have available at the report parser to do it

Hope this clarify how to develop report parsers to use on the QWeb report templates