The list into a string

1.join()

It can only be used when each element of the string can be used, or else it will error

li = ['1','2','3',"wewe","eee"]
a = '_'.join(li) # 语法：str.join(sequence)
print(a)
<<<<1_2_3_wewe_eee

2.for cycle

li = [1,2,3,"wewe","eee"]
a = ''
for i in li:
    a = a+str(i)+'_'
print(a.strip('_'))
<<<<1_2_3_wewe_eee