diff --git a/addons/hr_payroll/hr_payroll.py b/addons/hr_payroll/hr_payroll.py
index 42e37f1d996..c692e9116e6 100644
--- a/addons/hr_payroll/hr_payroll.py
+++ b/addons/hr_payroll/hr_payroll.py
@@ -462,7 +462,7 @@ class hr_payslip(osv.osv):
         def _sum_salary_rule_category(localdict, category, amount):
             if category.parent_id:
                 localdict = _sum_salary_rule_category(localdict, category.parent_id, amount)
-            localdict['categories'][category.code] = category.code in localdict['categories'] and localdict['categories'][category.code] + amount or amount
+            localdict['categories'].dict[category.code] = category.code in localdict['categories'].dict and localdict['categories'].dict[category.code] + amount or amount
             return localdict
 
         class BrowsableObject(object):
@@ -474,7 +474,7 @@ class hr_payslip(osv.osv):
                 self.dict = dict
 
             def __getattr__(self, attr):
-                return self.dict.__getitem__(attr)
+                return attr in self.dict and self.dict.__getitem__(attr) or 0.0
 
         class InputLine(BrowsableObject):
             """a class that will be used into the python code, mainly for usability purposes"""
@@ -528,6 +528,7 @@ class hr_payslip(osv.osv):
         #we keep a dict with the result because a value can be overwritten by another rule with the same code
         result_dict = {}
         rules = {}
+        categories_dict = {}
         blacklist = []
         payslip_obj = self.pool.get('hr.payslip')
         inputs_obj = self.pool.get('hr.payslip.worked_days')
@@ -539,13 +540,14 @@ class hr_payslip(osv.osv):
         inputs = {}
         for input_line in payslip.input_line_ids:
             inputs[input_line.code] = input_line
-
+        
+        categories_obj = BrowsableObject(self.pool, cr, uid, payslip.employee_id.id, categories_dict)
         input_obj = InputLine(self.pool, cr, uid, payslip.employee_id.id, inputs)
         worked_days_obj = WorkedDays(self.pool, cr, uid, payslip.employee_id.id, worked_days)
         payslip_obj = Payslips(self.pool, cr, uid, payslip.employee_id.id, payslip)
         rules_obj = BrowsableObject(self.pool, cr, uid, payslip.employee_id.id, rules)
 
-        localdict = {'categories': {}, 'rules': rules_obj, 'payslip': payslip_obj, 'worked_days': worked_days_obj, 'inputs': input_obj}
+        localdict = {'categories': categories_obj, 'rules': rules_obj, 'payslip': payslip_obj, 'worked_days': worked_days_obj, 'inputs': input_obj}
         #get the ids of the structures on the contracts and their parent id as well
         structure_ids = self.pool.get('hr.contract').get_all_structures(cr, uid, contract_ids, context=context)
         #get the rules of the structure and thier children
@@ -567,10 +569,10 @@ class hr_payslip(osv.osv):
                     #check if there is already a rule computed with that code
                     previous_amount = rule.code in localdict and localdict[rule.code] or 0.0
                     #set/overwrite the amount computed for this rule in the localdict
-                    localdict[rule.code] = amount
+                    localdict[rule.code] = amount * qty
                     rules[rule.code] = rule
                     #sum the amount for its salary category
-                    localdict = _sum_salary_rule_category(localdict, rule.category_id, amount - previous_amount)
+                    localdict = _sum_salary_rule_category(localdict, rule.category_id, (amount * qty) - previous_amount)
                     #create/overwrite the rule in the temporary results
                     result_dict[key] = {
                         'salary_rule_id': rule.id,
@@ -783,7 +785,7 @@ class hr_salary_rule(osv.osv):
 # employee: hr.employee object
 # contract: hr.contract object
 # rules: object containing the rules code (previously computed)
-# categories: dictionary containing the computed salary rule categories (sum of amount of all rules belonging to that category). Keys are the category codes.
+# categories: object containing the computed salary rule categories (sum of amount of all rules belonging to that category). 
 # worked_days: object containing the computed worked days.
 # inputs: object containing the computed inputs.
 
@@ -797,14 +799,14 @@ result = contract.wage * 0.10''',
 # payslip: object containing the payslips
 # employee: hr.employee object
 # contract: hr.contract object
-# rules: object containing the rules code (as hr.salary.rule object)
-# categories: dictionary containing the computed salary rule categories (sum of amount of all rules belonging to that category). Keys are the category codes.
+# rules: object containing the rules code (previously computed)
+# categories: object containing the computed salary rule categories (sum of amount of all rules belonging to that category). 
 # worked_days: object containing the computed worked days
 # inputs: object containing the computed inputs
 
 # Note: returned value have to be set in the variable 'result'
 
-result = rules.NET > categories['NET'] * 0.10''',
+result = rules.NET > categories.NET * 0.10''',
         'condition_range': 'contract.wage',
         'sequence': 5,
         'appears_on_payslip': True,
diff --git a/addons/hr_payroll/hr_payroll_data.xml b/addons/hr_payroll/hr_payroll_data.xml
index c7ba6e62062..93212a550bc 100644
--- a/addons/hr_payroll/hr_payroll_data.xml
+++ b/addons/hr_payroll/hr_payroll_data.xml
@@ -49,7 +49,7 @@
             <field name="category_id" ref="hr_payroll.GROSS"/>
             <field name="condition_select">none</field>
             <field name="amount_select">code</field>
-            <field name="amount_python_compute">result = categories['BASIC'] + ('ALW' in categories and categories['ALW'] or 0.0)</field>
+            <field name="amount_python_compute">result = categories.BASIC + categories.ALW</field>
         </record>
 
         <record id="hr_rule_net" model="hr.salary.rule">
@@ -59,7 +59,7 @@
             <field name="category_id" ref="hr_payroll.NET"/>
             <field name="condition_select">none</field>
             <field name="amount_select">code</field>
-            <field name="amount_python_compute">result = categories['BASIC'] + ('ALW' in categories and categories['ALW'] or 0.0) + ('DED' in categories and categories['DED'] or 0.0)</field>
+            <field name="amount_python_compute">result = categories.BASIC + categories.ALW + categories.DED</field>
         </record>