Juan, I had the same error, this is because with some thing related with "context" and immutable property.

Try with this code (adapt it to your needs):

@api.v7
def render_html(self, cr, uid, ids, data=None, context=None):
      report_obj = self.pool['report']
      report = report_obj._get_report_from_name(cr, uid, self._template)
      docs = self.pool[report.model].browse(cr, uid, ids, context=context)
      docargs = {
          'doc_ids': ids,
          'doc_model': report.model,
          'docs': docs,
      }
      return report_obj.render(cr, uid, [], report.report_name, docargs, context=context)

Hola Juan, te funciono la plantilla?

 puedo mostrar la plantilla pero no consigo que rellene los datos de los tags 

You create parser like this (see method currency_text):

<span t-if="o.currency_id" t-esc="currency_text(o.amount_total, o.currency_id.name, o.partner_id.lang or 'pl_PL')"/>

class report_invoice(models.AbstractModel):
    _name = 'report.account.report_invoice'
    _template = 'account__pl.report_invoice_pl'

    def currency_text(self, sum, currency, language):
        return currency_to_text(sum, currency, language)

    @api.multi
    def render_html(self, data=None):
                                                                     
        report_obj = self.env['report']
        report = report_obj._get_report_from_name(self._template)        
       
        docargs = {
            'currency_text': self.currency_text,        
            'doc_ids': self._ids,
            'doc_model': report.model,
            'docs': self,
        }

        return report_obj.render(self._template, docargs)

UPDATE:

In my test system, your code works for me without any problem. Code:

        <report
            id = "contrato_laboral"
            string = "Contrato laboral"
            model = "hr.contract"
            report_type = "qweb-pdf"
            file = "hr.contrato_laboral"
            name = "hr.contrato_laboral"/>

        <template id="hr.contrato_laboral">
            <t t-call="report.html_container">
                <t t-foreach="docs" t-as="o">
                    <t t-call="report.external_layout">
                        <div class="page">
                            <span t-esc="get_formato()"/>
                        </div>
                    </t>
                </t>
            </t>
        </template>

class hr_contract_type(orm.Model):
    _name = "hr.contract.type"
    _description = "Contract Type"
    _inherit = "hr.contract.type"
    _columns = {
        'format': fields.html('Formato contrato', help="Campo HTML para definir el formato del contrato"),
    }
hr_contract_type()

class report_contrato_laboral(models.AbstractModel):
    _name = 'report.hr.contrato_laboral'
    _template = 'hr.contrato_laboral'
    
    def get_formato(self):
        return 'Hola Mundo!!!'
    
    @api.multi
    def render_html(self, data=None):
        report_obj = self.env['report']
        report = report_obj._get_report_from_name(self._template)

        docargs = {
            'get_formato':self.get_formato,
            'doc_ids': self._ids,
            'doc_model': report.model,
            'docs': self,
        }
        
        return report_obj.render(self._template, docargs)
report_contrato_laboral()

Hi zbik, thanks for your answer, i tried this:

Class to add field and define custom render for report:

from openerp.osv import fields, osv
from openerp import api, models

class hr_contract_type(osv.osv):
    _name = "hr.contract.type"
    _description = "Contract Type"
    _inherit = "hr.contract.type"
    _columns = {
        'format': fields.html('Formato contrato', help="Campo HTML para definir el formato del contrato"),
    }
hr_contract_type()

class report_contrato_laboral(osv.AbstractModel):
    _name = 'report.hr.contrato_laboral'
    _template = 'hr.contrato_laboral'
    _wrapped_report_class = None
    
    def get_formato(self):
        return 'Hola Mundo!!!'
    
    def render_html(self, data=None):
        report_obj = self.env['report']
        report = report_obj._get_report_from_name(self._template)

        docargs = {
            'get_formato':self.get_formato,
            'doc_ids': self._ids,
            'doc_model': report.model,
            'docs': self,
        }
        
        return report_obj.render(self._template, docargs)
report_contrato_laboral()

XML to define report:

<?xml version="1.0" encoding="utf-8"?>
<openerp>
    <data>
        <report
            id = "contrato_laboral"
            string = "Contrato laboral"
            model = "hr.contract"
            report_type = "qweb-pdf"
            file = "hr.contrato_laboral"
            name = "hr.contrato_laboral"/>
    </data>
</openerp>

XML to define template

<?xml version="1.0" encoding="utf-8"?>
<openerp>
<data>
<template id="hr.contrato_laboral">
    <t t-call="report.html_container">
        <t t-foreach="docs" t-as="o">
            <t t-call="report.external_layout">
                <div class="page">
                    <span t-esc="get_formato()"/>
                </div>
            </t>
        </t>
    </t>
</template>
</data>
</openerp>

This is the error that is shown when i clic the print option "Contrato laboral": Uncaught SyntaxError: Unexpected end of input

Do you see an error in the code?

Best regards,

Juan David López