Python eval() 函数看这一篇就够了

Python eval() 函数看这一篇就够了

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。

  • expression:表达式,上面提到evel函数用于执行一个字符串表达式,表达式的内容就放在此处。当表达式涉及到
  • globals:该部分必须是字典!必须是字典!必须是字典!否则程序会出错。当定义了globals 参数之后eval函数的作用域会被限定在globals中。
  • locals:该参数掌控局部的命名空间,功能和globals类型,不过当参数冲突时,会执行locals处的参数。

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中都有参数b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.

附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

>>> eval("pow(2,2)")
4
>>> eval("2 + 2")
4
>>> eval("n + 4")
85

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]

总结

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