PME Treasure

Why a PME menu changes or breaks with a different active object

3 min read

Symptom

The same PME menu looks correct for a mesh, but a control disappears, changes, or raises an error when an Empty, Curve, another object type, or no object is active.

Cause

First confirm that the same PME menu is being called. A context router, duplicate name, or different hotkey scope can intentionally choose another menu.

If the menu identity is the same, a common cause is a Custom item drawing a property that does not exist on every possible data owner. For example, C.object.data is a Mesh for a mesh object, different data for a curve, and None for an Empty. A valid mesh property path is therefore not automatically a valid menu-wide path.

The forum example followed Blender’s active context and therefore reached a different kind of data when the selected object changed.

Check the object before drawing its property

Check the owner and property before drawing it. Written readably, the pattern is:

obj = C.active_object
data = getattr(obj, "data", None)
 
if data is not None and hasattr(data, "your_property"):
    L.prop(data, "your_property")
else:
    L.label(text="Not available for this object")

PME’s standard Custom field is a compact code field, so the equivalent stored expression is:

obj = C.active_object; data = getattr(obj, "data", None); L.prop(data, "your_property") if data is not None and hasattr(data, "your_property") else L.label(text="Not available for this object")

Replace your_property after confirming its Blender RNA identifier in the Python Console or API documentation.

Choose between two designs

  • One menu, optional controls: guard each object-specific row and show a short unavailable label or separator.
  • Different workflows by object or mode: route to separate menus with a Poll or context-sensitive menu. This is clearer when most controls differ, not just one row.

Also check

  • An Empty can be the active object while having no object data block, so check both the object and its data.
  • The historical example used auto_smooth_angle, but Blender’s mesh-normal API has since changed. Reuse the guarded check, not that property name.
  • A Poll on the whole menu can hide valid common controls along with one invalid row. A row-level guard works better when most of the menu remains useful.
  • If a property exists but its operator still fails, investigate the editor, region, and mode separately.

Sources

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