python之全局变量赋值前引用的局部变量 'delete'

我在项目开始时设置了一个全局变量 delete，就在导入的库下方和任何类之前，但是当我有这段代码时:

def motion_notify_callback(event): 
        if (ispymol == True): 
 
            if event.inaxes is ax:       
            x = event.xdata 
            y = event.ydata 
            x = round(x,0) 
            y = round(y,0) 
            x = int(x) 
            y = int(y) 
            coord = (x,y) 
 
 
            for i in range(0,len(number_list)): 
 
                if (coord == number_list[i]): 
                if (delete == True): 
                    pymol.cmd.do("delete CurrentCont") 
                    delete = False 
                pymol.cmd.do("distance CurrentCont, chain"+lc+" and resi "+resi1[i]+" and name CA, chain"+lc+" and resi "+resi2[i]+" and name CA") 
 
                    delete = True 
 
            for i in range(0,len(rres)): 
            if (coord == mappingpredcont[i]): 
               if (delete == True): 
                       pymol.cmd.do("delete CurrentCont") 
                       delete =False 
               pymol.cmd.do("distance CurrentCont, chain"+lc+" and resi "+predresi1[i]+" and name CA, chain"+lc+" and resi "+predresi2[i]+" and name CA") 
 
                   delete = True 

在全局变量赋值之前引用了错误的局部变量'delete' 我哪里错了？