eval(str) 函数很强大，官方解释：将字符串当成有效的表达式来求值并返回计算结果。

1.计算字符串中出现的有效表达式，并返回结果

>>> eval('1+2')
3
>>> eval('pow(2,3)')
8
>>> n=10
>>> eval('n-1')
9
>>> def hello():
...    print('hello')
...
>>> eval('hello()')
hello
>>> eval('hello')()
hello

　　

2.将字符串转换成相应的对象

>>> a = "[[1,2],[3,4],[5,6],[7,8]]"
>>> b = eval(a)
>>> b
[[1,2],[3,4],[5,6],[7,8]]
>>> a = "{1:'xy',2:'cc'}"
>>> c = eval(a)
>>> c
{1:'xy',2:'cc'}
>>> a = "(1,2,3,4)"
>>> d = eval(a)
>>> d
(1,2,3,4)