indexed fields serialization fixed.

paxb/mappers.py

@@ -182,11 +182,18 @@ def __init__(self, name, ns=None, ns_map=None, idx=None, required=True):
         self.idx = idx
         self.required = required
 
-    def xml(self, obj, root, name=None, ns=None, ns_map=None, _=None, encoder=default_encoder):
-        # TODO idx
+    def xml(self, obj, root, name=None, ns=None, ns_map=None, idx=None, encoder=default_encoder):
         name = first(self.name, name)
         ns = first(self.ns, ns)
         ns_map = merge_dicts(self.ns_map, ns_map)
+        idx = first(idx, self.idx, 1)
+
+        existing_elements = root.findall(qname(ns=ns_map.get(ns), name=name), ns_map)
+        if idx > len(existing_elements) + 1:
+            raise exc.SerializationError(
+                "serialization can't be completed because {name}[{cur}] is going to be serialized, "
+                "but {name}[{prev}] is not serialized.".format(name=name, cur=idx, prev=idx-1)
+            )
 
         if obj is None:
             if self.required:
@@ -241,7 +248,8 @@ def xml(self, obj, root, name=None, ns=None, ns_map=None, idx=None, encoder=defa
         existing_elements = root.findall(qname(ns=ns_map.get(ns), name=self.name), ns_map)
         if idx > len(existing_elements) + 1:
             raise exc.SerializationError(
-                "element {} at index {} is going to be serialized, but the previous one is omitted".format(name, idx)
+                "serialization can't be completed because {name}[{cur}] is going to be serialized, "
+                "but {name}[{prev}] is not serialized.".format(name=name, cur=idx, prev=idx - 1)
             )
         if idx == len(existing_elements) + 1:
             element = et.Element(qname(ns=ns_map.get(ns), name=self.name))
@@ -321,11 +329,18 @@ def __init__(self, cls, name=None, ns=None, ns_map=None, idx=None, required=True
         self.idx = idx
         self.required = required
 
-    def xml(self, obj, root, name=None, ns=None, ns_map=None, _=None, encoder=default_encoder):
-        # TODO idx
+    def xml(self, obj, root, name=None, ns=None, ns_map=None, idx=None, encoder=default_encoder):
         name = first(self.name, name)
         ns = first(self.ns, ns)
         ns_map = merge_dicts(self.ns_map, ns_map)
+        idx = first(idx, self.idx, 1)
+
+        existing_elements = root.findall(qname(ns=ns_map.get(ns), name=name), ns_map)
+        if idx > len(existing_elements) + 1:
+            raise exc.SerializationError(
+                "serialization can't be completed because {name}[{cur}] is going to be serialized, "
+                "but {name}[{prev}] is not serialized.".format(name=name, cur=idx, prev=idx-1)
+            )
 
         if obj is None:
             if self.required:

tests/test_errors.py

@@ -109,3 +109,17 @@ def test_order():
         @pb.model(order=('element1', 'element2'))
         class TestModel:
             element1 = pb.attr()
+
+
+def test_indexes_deserialization():
+
+    @pb.model()
+    class TestModel:
+        field = pb.field(idx=2)
+
+    obj = TestModel(field="value1")
+    with pytest.raises(
+            exc.SerializationError,
+            match=r"serialization can't be completed because field\[2\] is going to be serialized, "
+                  r"but field\[1\] is not serialized."):
+        pb.to_xml(obj)