试了N中方法，最终发现大神解决了。
https://blog.csdn.net/u011085172/article/details/79044490
list = [‘a’, ‘b’, ‘c’]
方法1，可行：cursor.execute("select *from students where pNo in (‘a’, ‘b’, ‘c’)”)
方法2，可行：sql= 'select * from students where pNo in (%s)' %','.join(['%s']* len(list))
cursor.execute(sql,list)
print(sql)# select* from students where pNo in (%s,%s,%s)
方法3，不可行，原因我也没明白：
b= "','"
list2= b.join(list)
list3= "'"+list2+"'"
sql2="select * from students where pNo in (%s) "
cursor.execute(sql2,list3)
