关于python：接收：“UnboundLocalError：赋值前引用的局部变量”

Receiving : “UnboundLocalError: local variable referenced before assignment”

我看到过很多人都有同样的问题，但由于问题的独特性，似乎已经根据具体情况提供了解决方案。

以下是我的代码：

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loga = [(912,"Message A1") , (1000,"Message A2") , (988,"Message A3") , (1012,"Message A4") , (1002,"Message A5")]
logb = [(926,"Message B1") , (1008,"Message B2") , (996,"Message B3") , (1019,"Message B4") , (1100,"Message B5")]
logc = [(1056,"Message C1") , (1033,"Message C2") , (999,"Message C3") , (1054,"Message C4") , (1086,"Message C5")]
logs = [loga, logb, logc]
out = []

def find_lowest_i(lst):
for i in range(len(lst)):
log = lst[i]
if log:
t = log[0][0]
if i==0 or t < lowest_t:
lowest_i = i
lowest_t = t
return lowest_i

while True:
i = find_lowest_i(logs)
print"i=", i
tpl = logs[i].pop(0)
print tpl
out.append(tpl)
print out

我的确切错误是：

"消息文件名行位置回溯19查找最低点13UnboundLocalError:分配前引用的本地变量'lowest_t'