import bpy

class SimpleOperator(bpy.types.Operator):
    """Tooltip"""
    bl_idname = "object.simple_operator"
    bl_label = "Simple Object Operator"
    bl_options = {'REGISTER','UNDO'}

    axis : bpy.props.EnumProperty (name="Axis", items=[('x','X',''),('y','Y',''),('z','Z','')], default="y")
    threshold : bpy.props.FloatProperty (name="Threshold", min=0, default=0)
    flip : bpy.props.BoolProperty (name="Flip", default = False)

    def draw(self, context):
        layout = self.layout
        layout.use_property_split = True
        column = layout.column()

        row = column.row()
        row.prop(self, "axis", expand=True)
        layout.prop(self, "threshold", text="Threshold")
        layout.prop(self, "flip", text="Flip")


    @classmethod
    def poll(cls, context):
        return context.active_object is not None

    def main(self, context):
        if self.axis == 'x':
            if self.flip == False:
                return 'POSITIVE_X'
            elif self.flip == True:
                return 'NEGATIVE_X'

        if self.axis == 'y':
            if self.flip == False:
                return 'POSITIVE_Y'
            elif self.flip == True:
                return 'NEGATIVE_Y'

        if self.axis == 'z':
            if self.flip == False:
                return 'POSITIVE_Z'
            elif self.flip == True:
                return 'NEGATIVE_Z'

    def execute(self, context):
        print (self.main(context))
        bpy.ops.mesh.symmetrize(threshold=self.threshold , direction=self.main(context))
        return {'FINISHED'}


def register():
    bpy.utils.register_class(SimpleOperator)


def unregister():
    bpy.utils.unregister_class(SimpleOperator)


if __name__ == "__main__":
    register()