Python eval() 函數看這一篇就夠瞭

Posted on by

一、語法和參數

在Python中evel()函數的語法格式為eval(expression, globals=None, locals=None)，註意後面還有globals參數和locals參數。eval()函數用於執行一個字符串表達式，並且返回該表達式的值。與eval相近的有exec函數，該函數將會在另一篇文章詳細講解。

  • expression：表達式，上面提到evel函數用於執行一個字符串表達式，表達式的內容就放在此處。當表達式涉及到
  • globals：該部分必須是字典！必須是字典！必須是字典！否則程序會出錯。當定義瞭globals 參數之後eval函數的作用域會被限定在globals中。
  • locals：該參數掌控局部的命名空間，功能和globals類型，不過當參數沖突時，會執行locals處的參數。

二、expression參數示例

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

運行結果為11

【解析】：因為此處沒有指定globals和locals，所以直接執行expression部分的內容。該程序的效果等價於a=10 print(a+1)

三、globals參數示例

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

運行結果為5

【解析】：因為現在指定瞭globals，所以在expression部分的作用域就是globals指定的字典范圍內。所以此時外面的a=10被屏蔽，取用字典中的值。

四、locals參數示例

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

運行結果為116

【解析】：根據上面題目的練習我們知道瞭當有globals和locals時作用的范圍域是在globals和locals中，所以a=10,b=20,c=30不會被應用。a和c的值分別去字典g和字典t中的值，當globals和locals中都有參數b時取locals中的值。所以a=6,b=100,c=10

五、eval函數的危險之處

eval函數非常的方便，我們可以使用一行代碼就實現計算器的功能print(eval(input('請輸入')))。但是因為它具有可以將字符串轉成表達式執行的特性，所以它也就可以去執行系統命令。這樣很容易被別有用心的人用來執行系統命令，刪除關鍵系統文件。

六、eval()函數官方文檔

   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()函數常見作用有

1、計算字符串中有效的表達式，並返回結果

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

2、將字符串轉成相應的對象（如list、tuple、dict和string之間的轉換）

>>> 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、將利用反引號轉換的字符串再反轉回對象

>>> 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]

總結

到此這篇關於Python eval() 函數的文章就介紹到這瞭,更多相關Python eval() 函數內容請搜索WalkonNet以前的文章或繼續瀏覽下面的相關文章希望大傢以後多多支持WalkonNet！

推薦閱讀：