set_smooth.py

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# made for https://blender.stackexchange.com/q/85049/935

bl_info = {
    "name": "Set Smooth",
    "author": "sambler",
    "version": (1, 2),
    "blender": (2, 80, 0),
    "location": "Render panel",
    "description": "Easily set smooth/flat shading to all objects.",
    "wiki_url": "https://github.com/sambler/addonsByMe/blob/master/set_smooth.py",
    "tracker_url": "https://github.com/sambler/addonsByMe/issues",
    "category": "Render",
    }

import bpy

class SetShading(bpy.types.Operator):
    bl_idname = 'object.set_all_shading'
    bl_label = 'Set shading of all objects'

    smooth : bpy.props.BoolProperty()

    def execute(self, context):
        for o in context.scene.objects:
            if o.type == 'MESH':
                for f in o.data.polygons:
                    f.use_smooth = self.smooth
        return {'FINISHED'}

def add_change_shading(self, context):
    self.layout.operator(SetShading.bl_idname, text='All smooth').smooth = True
    self.layout.operator(SetShading.bl_idname, text='All flat').smooth = False

def register():
    bpy.utils.register_class(SetShading)
    bpy.types.TOPBAR_MT_render.prepend(add_change_shading)

    wm = bpy.context.window_manager
    kc = wm.keyconfigs.addon
    if kc:
        km = kc.keymaps.new('Window', space_type='EMPTY')
        kmi = km.keymap_items.new(SetShading.bl_idname, 'F12', 'PRESS', alt=True)
        kmi.properties.smooth = True

        kmi = km.keymap_items.new(SetShading.bl_idname, 'F12', 'PRESS', alt=True, shift=True)
        kmi.properties.smooth = False

def unregister():
    wm = bpy.context.window_manager
    kc = wm.keyconfigs.addon
    if kc:
        km = kc.keymaps['Window']
        for kmi in km.keymap_items:
            if kmi.idname == SetShading.bl_idname:
                km.keymap_items.remove(kmi)

    bpy.types.TOPBAR_MT_render.remove(add_change_shading)
    bpy.utils.unregister_class(SetShading)

if __name__ == "__main__":
    register()