蜗牛之路

eval(str)函数很强大，官方解释为：将字符串str当成有效的表达式来求值并返回计算结果。所以，结合math当成一个计算器很好用。

eval()函数常见作用有：
1、计算字符串中有效的表达式，并返回结果

Python 3.6.8 |Anaconda, Inc.| (default, Dec 29 2018, 19:04:46)
Type 'copyright', 'credits' or 'license' for more information
IPython 7.4.0 -- An enhanced Interactive Python. Type '?' for help.

In [1]: eval('1+1')
Out[1]: 2

In [2]: eval('pow(2,2)')
Out[2]: 4

In [3]: n = 10

In [4]: eval('n+1')
Out[4]: 11

In [11]: def hello():
    ...:     print('hello')
    ...:

In [12]: eval('hello()')
hello

In [13]: eval('hello')()
hello

2、将字符串转成相应的对象（如list、tuple、dict和string之间的转换）

>>> a = "[[1,2], [3,4], [5,6], [7,8], [9,0]]"
>>> b = eval(a)
>>> b
[[1, 2], [3, 4], [5, 6], [7, 8], [9, 0]]
>>> a = "{1:'xx',2:'yy'}"
>>> c = eval(a)
>>> c
{1: 'xx', 2: 'yy'}
>>> a = "(1,2,3,4)"
>>> d = eval(a)
>>> d
(1, 2, 3, 4)

3、将利用反引号转换的字符串再反转回对象

>>> list1 = [1,2,3,4,5]
>>> `list1`
'[1, 2, 3, 4, 5]'
>>> type(`list1`)
<type 'str'>
>>> type(eval(`list1`))
<type 'list'>
>>> a = eval(`list1`)
>>> a
[1, 2, 3, 4, 5]