将任何字符串作为python表达式求值：
eval()方法：
eval(source[, globals[, locals]]) -> value
Evaluate the source in the context of globals and locals.
The source may be a string rePResenting a Python expression
or a code object as returned by compile().
The globals must be a dictionary and locals can be any mapping,
defaulting to the current globals and locals.
If only globals is given, locals defaults to it.
用法：
>>> eval('1+1==2')
True
>>> eval('1+1==3')
False
>>> eval('9567+1085 == 10652')
True
>>> eval('"A"+"B"')
'AB'
>>> eval('"MARK".translate({65:79})')
'MORK'
>>> eval('"AAAAA".count("A")')
>>> eval("x*5",{}, {})
Traceback (most recent call last):
File "<pyshell#120>", line 1, in <module>
eval("x*5",{}, {})
File "<string>", line 1, in <module>
NameError: name 'x' is not defined
>>> eval("x*5",{"x":x},{})
25
>>> import math
>>> eval("math.sqrt(x)",{"x":x},{})
Traceback (most recent call last):
File "<pyshell#123>", line 1, in <module>
eval("math.sqrt(x)",{"x":x},{})
File "<string>", line 1, in <module>
NameError: name 'math' is not defined
注：
给eval()函数传递的第二、第三个参数担当了求值表达式是全局和局部名字空间的角色
eval()是不安全的，为了安全的求值不受信任的表达式，需要定义一个将"__builtins__"映射为none的全局名字空间字典。在内部，“内建”函数包含在一个叫“__builtins__"的伪模块内。
re.findall() -- 返回字符串中所有字母
set() -- 返回字符串中所有不同的字母