Add init method to lists

capnp/lib/capnp.pxd

@@ -115,6 +115,8 @@ cdef class _DynamicListBuilder:
     cpdef adopt(self, index, _DynamicOrphan orphan)
     cpdef disown(self, index)
 
+    cpdef init(self, index, size)
+
 cdef to_python_reader(C_DynamicValue.Reader self, object parent)
 cdef to_python_builder(C_DynamicValue.Builder self, object parent)
 cdef _to_dict(msg, bint verbose, bint ordered)

capnp/lib/capnp.pyx

@@ -534,6 +534,17 @@ cdef class _DynamicListBuilder:
         """
         return _DynamicOrphan()._init(self.thisptr.disown(index), self._parent)
 
+    cpdef init(self, index, size):
+        """A method for initializing an element in a list
+
+        :type index: int
+        :param index: The index of the element in the list
+
+        :type size: int
+        :param size: Size of the element to be initialized.
+        """
+        return to_python_builder(self.thisptr.init(index, size), self._parent)
+
     def __str__(self):
         return <char*>printListBuilder(self.thisptr).flatten().cStr()
 

test/addressbook.capnp

@@ -37,3 +37,6 @@ struct AddressBook {
   people @0 :List(Person);
 }
 
+struct NestedList {
+  list @0 :List(List(Int32));
+}

test/test_struct.py

@@ -218,3 +218,16 @@ def test_to_dict_ordered(addressbook):
     else:
         with pytest.raises(Exception):
             person.to_dict(ordered=True)
+
+def test_nested_list(addressbook):
+    struct = addressbook.NestedList.new_message()
+    struct.init('list', 2)
+
+    struct.list.init(0, 1)
+    struct.list.init(1, 2)
+
+    struct.list[0][0] = 1
+    struct.list[1][0] = 2
+    struct.list[1][1] = 3
+
+    assert struct.to_dict()["list"] == [[1], [2,3]]