Blender - Copy material to selected (whilst retaining material slots)

import bpy

# Get the active object (the source)
source = bpy.context.active_object
if not source:
    print("No active object selected.")
else:
    # Get the active material from the active slot
    active_mat = source.active_material
    if not active_mat:
        print(f"{source.name} has no active material.")
    else:
        # Apply to all selected objects except the source
        for obj in bpy.context.selected_objects:
            if obj == source:
                continue
            if not obj.material_slots:
                print(f"{obj.name} has no material slots, skipping.")
                continue

            # Assign the same material to all slots, without changing slot count
            for slot in obj.material_slots:
                slot.material = active_mat

            print(f"Applied {active_mat.name} to all slots of {obj.name}.")