关于python：在为全局变量赋值之前引用的局部变量’delete’

local variable 'delete' referenced before assignment for global variable

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

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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

它在为全局变量赋值之前引用了错误的局部变量"delete"我哪里出错了？