eval函数在Python中具有非常重要的地位，熟练的使用eval函数能够为我们的Python编程提供很多的便利之处。在本文中我将详细记录eval函数在Python中的使用方法及它带来便利时带来的一些其他危害，希望您阅读完本文后能够有所收获。欢迎在文章下方留言共同交流学习。

Python eval 函数

一、语法和参数

  在Python中evel()函数的语法格式为eval(expression, globals=None, locals=None)，注意后面还有globals参数和locals参数。eval()函数用于执行一个字符串表达式，并且返回该表达式的值。与eval相近的有exec函数，该函数将会在另一篇文章详细讲解。

二、expression参数示例

a=10;
print(eval("a+1"))

运行结果为11

【解析】：因为此处没有指定globals和locals，所以直接执行expression部分的内容。该程序的效果等价于a=10 print(a+1)

三、globals参数示例

a=10;
g={'a':4}
print(eval("a+1",g))

运行结果为5

【解析】：因为现在指定了globals，所以在expression部分的作用域就是globals指定的字典范围内。所以此时外面的a=10被屏蔽，取用字典中的值。

四、locals参数示例

a=10
b=20
c=30
g={'a':6,'b':8}
t={'b':100,'c':10}
print(eval('a+b+c',g,t))

运行结果为116
【解析】：根据上面题目的练习我们知道了当有globals和locals时作用的范围域是在globals和locals中，所以a=10,b=20,c=30不会被应用。a和c的值分别去字典g和字典t中的值，当globals和locals中都有相同参数时取locals中的值。所以a=6,b=100,c=10

五、eval函数的危险之处

  eval函数非常的方便，我们可以使用一行代码就实现计算器的功能print(eval(input('请输入')))。但是因为它具有可以将字符串转成表达式执行的特性，所以它也就可以去执行系统命令。这样很容易被别有用心的人用来执行系统命令，删除关键系统文件。

六、eval()函数官方文档

   The arguments are a string and optional globals and locals. If provided, globals must be a 
dictionary. If provided, locals can be any mapping object.
   The expression argument is parsed and evaluated as a Python expression (technically speaking, 
a condition list) using the globals and locals dictionaries as global and local namespace. If the
globals dictionary is present and lacks ‘__builtins__’, the current globals are copied into 
globals before expression is parsed. This means that expression normally has full access to the 
standard builtins module and restricted environments are propagated. If the locals dictionary is 
omitted it defaults to the globals dictionary. If both dictionaries are omitted, the expression 
is executed in the environment where eval() is called. The return value is the result of the 
evaluated expression. Syntax errors are reported as exceptions. Example:

>>> x = 1
>>> eval('x+1')
2

   This function can also be used to execute arbitrary code objects (such as those created by 
compile()). In this case pass a code object instead of a string. If the code object has been
compiled with 'exec' as the mode argument, eval()‘s return value will be None.
   Hints: dynamic execution of statements is supported by the exec() function. The globals() and 
locals() functions returns the current global and local dictionary, respectively, which may be 
useful to pass around for use by eval() or exec().
   See ast.literal_eval() for a function that can safely evaluate strings with expressions 
containing only literals.