Fix WheelSlot writing invalid empty MediaFileName="" on every slot

CHANGELOG.md

@@ -164,7 +164,6 @@ having to use the .utils package. See more details below.
 
 ```python
 modes_info = gdtf_fixture.dmx_modes.as_dict()
-
 ```
 
 ##### .utils.get\_geometry\_by\_name
@@ -176,7 +175,7 @@ gdtf_fixture.geometries.get_geometry_by_name("geometry name")
 ##### .utils.get\_geometry\_by\_type
 
 ```python
-#this is a static method and requires a root_geometry
+# this is a static method and requires a root_geometry
 gdtf_fixture.geometries.get_geometry_by_type(geometry_root, geometry_type)
 ```
 

pygdtf/__init__.py

@@ -791,8 +791,9 @@ def _read_xml(self, xml_node: "Element", xml_parent: Optional["Element"] = None)
         color_str = xml_node.attrib.get("Color")
         self.color = ColorCIE(str_repr=color_str) if color_str else ColorCIE()
         self.filter = NodeLink("FilterCollect", xml_node.attrib.get("Filter"))
-        self.media_file_name = Resource(
-            name=xml_node.attrib.get("MediaFileName", ""), extension="png"
+        media_file_name = xml_node.attrib.get("MediaFileName")
+        self.media_file_name = (
+            Resource(name=media_file_name, extension="png") if media_file_name else None
         )
         self.facets = [PrismFacet(xml_node=i) for i in xml_node.findall("Facet")]
         animation_system_node = xml_node.find("AnimationSystem")

tests/test_wheel_slot_media_file_name.py

@@ -0,0 +1,108 @@
+# MIT License
+#
+# Copyright (C) 2026 Marvin
+#
+# This file is part of pygdtf.
+#
+# Permission is hereby granted, free of charge, to any person obtaining a copy
+# of this software and associated documentation files (the "Software"), to deal
+# in the Software without restriction, including without limitation the rights
+# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+# copies of the Software, and to permit persons to whom the Software is
+# furnished to do so, subject to the following conditions:
+#
+# The above copyright notice and this permission notice shall be included in all
+# copies or substantial portions of the Software.
+#
+# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+# SOFTWARE.
+
+from pathlib import Path
+from xml.etree import ElementTree
+
+import pygdtf
+
+# WheelSlot._read_xml() read a missing MediaFileName as "" instead of None
+# (default on xml_node.attrib.get), so to_xml() - which only checks "is this
+# not None" - always wrote MediaFileName="" back out, even for slots that
+# never had that attribute in the source. Harmless to the XSD (empty string
+# is still a valid value) but pollutes every wheel slot on round-trip.
+_DSC_XML = """<?xml version="1.0" encoding="UTF-8"?>
+<GDTF DataVersion="1.2">
+  <FixtureType Name="Test" ShortName="Test" LongName="Test" Manufacturer="Test"
+    Description="Test" FixtureTypeID="00000000-0000-0000-0000-000000000000"
+    Thumbnail="" ThumbnailOffsetX="0" ThumbnailOffsetY="0" RefFT="" CanHaveChildren="No">
+    <AttributeDefinitions>
+      <ActivationGroups/>
+      <FeatureGroups>
+        <FeatureGroup Name="Gobo" Pretty="Gobo">
+          <Feature Name="Gobo"/>
+        </FeatureGroup>
+      </FeatureGroups>
+      <Attributes>
+        <Attribute Name="Gobo1" Pretty="G1" Feature="Gobo.Gobo" PhysicalUnit="None"/>
+      </Attributes>
+    </AttributeDefinitions>
+    <Wheels>
+      <Wheel Name="Gobo Wheel">
+        <Slot Name="Open"/>
+        <Slot Name="Gobo 1" MediaFileName="gobo01"/>
+      </Wheel>
+    </Wheels>
+    <PhysicalDescriptions>
+      <ColorSpace Mode="sRGB"/>
+      <AdditionalColorSpaces/>
+      <Gamuts/>
+      <Filters/>
+      <Emitters/>
+      <DMXProfiles/>
+      <CRIs/>
+      <Connectors/>
+      <Properties/>
+    </PhysicalDescriptions>
+    <Models>
+      <Model Name="Base" PrimitiveType="Cube"/>
+    </Models>
+    <Geometries>
+      <Geometry Name="Base" Model="Base"/>
+    </Geometries>
+    <DMXModes>
+      <DMXMode Name="Default" Geometry="Base">
+        <DMXChannels>
+          <DMXChannel Offset="1" Geometry="Base">
+            <LogicalChannel Attribute="Gobo1">
+              <ChannelFunction Name="Open" Attribute="Gobo1" Default="0/1"/>
+            </LogicalChannel>
+          </DMXChannel>
+        </DMXChannels>
+      </DMXMode>
+    </DMXModes>
+  </FixtureType>
+</GDTF>
+"""
+
+
+def test_wheel_slot_roundtrip_omits_empty_media_file_name(tmp_path: Path):
+    dsc_file = tmp_path / "description.xml"
+    dsc_file.write_text(_DSC_XML, encoding="utf-8")
+
+    with pygdtf.FixtureType(dsc_file=str(dsc_file)) as fixture:
+        writer = pygdtf.FixtureTypeWriter(fixture)
+        output = tmp_path / "roundtrip.xml"
+        writer.write_gdtf(output)
+
+    root = ElementTree.fromstring(output.read_bytes())
+    slots = root.findall("./FixtureType/Wheels/Wheel/Slot")
+    assert len(slots) == 2
+
+    open_slot, gobo_slot = slots
+    assert open_slot.get("Name") == "Open"
+    assert "MediaFileName" not in open_slot.attrib
+
+    assert gobo_slot.get("Name") == "Gobo 1"
+    assert gobo_slot.get("MediaFileName") == "gobo01"