https://docs.python.org/3.7/tutorial/index.html
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eval函数就是实现list、dict、tuple与str之间的转化。
str函数把list，dict，tuple转为为字符串。
# 字符串转换成列表
a = "[[1,2], [3,4], [5,6], [7,8], [9,0]]"
print(type(a))
b = eval(a)
print(b)# 字符串转换成字典
a = "{1: 'a', 2: 'b'}"
print(type(a))
b = eval(a)
print(type(b))
print(b)# 字符串转换成元组
a = "([1,2], [3,4], [5,6], [7,8], (9,0))"
print(type(a))
b=eval(a)
print(type(b))
print(b)====================================