feat: add Null value which stands in for a forced XML nil value

This allows things to be specifically unset, which was not previously possible.
At present the reverse path for decoding this has not been implemented.

broadworks_ocip/base.py

@@ -15,6 +15,7 @@
 
 import attr
 from lxml import etree
+from null_object import Null
 
 from broadworks_ocip.exceptions import OCIErrorAPISetup
 from broadworks_ocip.exceptions import OCIErrorAttributeMissing
@@ -73,7 +74,10 @@ def __init__(self, **kwargs):
                     raise OCIErrorAttributeMissing(
                         message=f"{cname}: Required attribute {elem.name} is missing",
                     )
-                if elem.is_array:
+                if value is Null:
+                    # always allowed
+                    pass
+                elif elem.is_array:
                     if not isinstance(value, list):
                         raise TypeError(
                             f"{cname}: Expected {elem.name} to be a list but it is {type(value)}",
@@ -212,14 +216,16 @@ def etree_sub_element_(
         Returns:
             etree: etree.Element() for this class
         """
-        if value is None:
-            if sub_element.is_required:
-                etree.SubElement(
-                    element,
-                    sub_element.xmlname,
-                    {"{http://www.w3.org/2001/XMLSchema-instance}nil": "true"},
-                    nsmap=self._default_nsmap(),
-                )
+        # if value is Null or (value is None and sub_element.is_required):
+        if value is Null or (value is None and sub_element.is_required):
+            etree.SubElement(
+                element,
+                sub_element.xmlname,
+                {"{http://www.w3.org/2001/XMLSchema-instance}nil": "true"},
+                nsmap=self._default_nsmap(),
+            )
+        elif value is None:
+            pass
         elif sub_element.is_table:
             # any table should be a list of namedtuple elements
             if type(value) is list and len(value) > 0:

pyproject.toml

@@ -37,6 +37,7 @@ exclude = [
 python = ">=3.6.2,<4.0"
 lxml = "^4.7.1"
 attrs = "^21.4.0"
+null-object = "^0.1.0"
 
 [tool.poetry.dev-dependencies]
 pytest = "^6.2.5"

tests/test_basic_command_xml.py

@@ -4,6 +4,7 @@
 
 import pytest  # noqa: F401
 from lxml import etree
+from null_object import Null
 
 from broadworks_ocip import BroadworksAPI
 
@@ -425,13 +426,14 @@ def test_nested_elements():
         phone_number="123456789",
         extension="1219",
         # sip_alias_list=api.get_type_object("ReplacementSIPAliasList",sip_alias=[]),
+        sip_alias_list=Null,
         endpoint=dict(
             trunk_addressing=api.get_type_object(
                 "TrunkAddressingMultipleContactModify",
-                # trunk_group_device_endpoint=None,
+                trunk_group_device_endpoint=Null,
                 enterprise_trunk_name="ET02",
-                # alternate_trunk_identity=None,
-                # physical_location=None,
+                alternate_trunk_identity=Null,
+                physical_location=Null,
             ),
         ),
     )
@@ -444,13 +446,13 @@ def test_nested_elements():
             b"<userId>[email protected]</userId>"
             b"<phoneNumber>123456789</phoneNumber>"
             b"<extension>1219</extension>"
-            # b'<sipAliasList xsi:nil="true"/>'
+            b'<sipAliasList xsi:nil="true"/>'
             b"<endpoint>"
             b"<trunkAddressing>"
-            # b'<trunkGroupDeviceEndpoint xsi:nil="true"/>'
+            b'<trunkGroupDeviceEndpoint xsi:nil="true"/>'
             b"<enterpriseTrunkName>ET02</enterpriseTrunkName>"
-            # b'<alternateTrunkIdentity xsi:nil="true"/>'
-            # b'<physicalLocation xsi:nil="true"/>'
+            b'<alternateTrunkIdentity xsi:nil="true"/>'
+            b'<physicalLocation xsi:nil="true"/>'
             b"</trunkAddressing>"
             b"</endpoint>"
             b"</command>"