Value mapping hardcoding mechanism

aeat.py

@@ -623,10 +623,7 @@ class IssuedTrytonInvoiceMapper(mapping.IssuedInvoiceMapper):
     serial_number = attrgetter('number')
     issue_date = attrgetter('invoice_date')
     invoice_kind = attrgetter('sii_operation_key')
-
-    @classmethod
-    def rectified_invoice_kind(cls, invoice):
-        return 'I'
+    rectified_invoice_kind = mapping.hardcode('I')
     rectified_base = attrgetter('untaxed_amount')
     rectified_amount = attrgetter('tax_amount')
     total_amount = attrgetter('total_amount')
@@ -650,10 +647,7 @@ class RecievedTrytonInvoiceMapper(mapping.RecievedInvoiceMapper):
     serial_number = attrgetter('reference')
     issue_date = attrgetter('invoice_date')
     invoice_kind = attrgetter('sii_operation_key')
-
-    @classmethod
-    def rectified_invoice_kind(cls, invoice):
-        return 'I'
+    rectified_invoice_kind = mapping.hardcode('I')
     specialkey_or_trascendence = attrgetter('sii_received_key')
     description = attrgetter('description')
     not_exempt_kind = attrgetter('sii_subjected')

pyAEATsii/mapping.py

@@ -4,6 +4,7 @@ __all__ = [
     'build_query_filter',
     'IssuedInvoiceMapper',
     'RecievedInvoiceMapper',
+    'hardcode',
 ]
 
 _DATE_FMT = '%d-%m-%Y'
@@ -37,6 +38,19 @@ def get_headers(name=None, vat=None, comm_kind=None, version='0.7'):
     }
 
 
+class _HardcodedValue(object):
+
+    def __init__(self, value):
+        self.value = value
+
+    def __call__(self, *args, **kwargs):
+        return self.value
+
+
+def hardcode(value):
+    return _HardcodedValue(value)
+
+
 class IssuedInvoiceMapper(object):
 
     @classmethod