I found this in aero reports forums http://www.alistek.com/forum/viewtopic.php?f=11&t=473&p=1590#p1590 it was really useful

first you have to define parser in the parser tab of your report:

class Parser(report_sxw.rml_parse):
    def __init__(self, cr, uid, name, context):
        super(Parser, self).__init__(cr, uid, name, context)
        self.localcontext.update({
            'myset':self.myset,
            'myget':self.myget,
            'storage':{}
        })


def myset(self, pair):
        if isinstance(pair, dict):
            self.localcontext['storage'].update(pair)
        return False

def myget(self, key):
    if key in self.localcontext['storage'] and self.localcontext['storage'][key]:
        return self.localcontext['storage'][key]
    return False

then in the reports you can set a variable in this way:

<if test="myset({'test':50})"></if>

then you can call it with:

<myget('test')>

example to modify an already declared variable:

<if test="myset({'test':myget('test')+1})"></if>