Python?eval()?函數(shù)看這一篇就夠了

更新時間：2022年01月19日 15:22:28 作者：127.0.0.1/24

eval(str)函數(shù)很強大,官方解釋為將字符串str當成有效的表達式來求值并返回計算結果,下面這篇文章主要給大家介紹了關于Python?eval()?函數(shù)的相關資料,需要的朋友可以參考下

一、語法和參數(shù)

在Python中evel()函數(shù)的語法格式為eval(expression, globals=None, locals=None)，注意后面還有globals參數(shù)和locals參數(shù)。eval()函數(shù)用于執(zhí)行一個字符串表達式，并且返回該表達式的值。與eval相近的有exec函數(shù)，該函數(shù)將會在另一篇文章詳細講解。

expression：表達式，上面提到evel函數(shù)用于執(zhí)行一個字符串表達式，表達式的內(nèi)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。當定義了globals 參數(shù)之后eval函數(shù)的作用域會被限定在globals中。
locals：該參數(shù)掌控局部的命名空間，功能和globals類型，不過當參數(shù)沖突時，會執(zhí)行l(wèi)ocals處的參數(shù)。

二、expression參數(shù)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(zhí)行expression部分的內(nèi)容。該程序的效果等價于a=10 print(a+1)

三、globals參數(shù)示例

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

運行結果為5

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

四、locals參數(shù)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(jù)上面題目的練習我們知道了當有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(shù)b時取locals中的值。所以a=6,b=100,c=10

五、eval函數(shù)的危險之處

eval函數(shù)非常的方便，我們可以使用一行代碼就實現(xiàn)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(zhí)行的特性，所以它也就可以去執(zhí)行系統(tǒng)命令。這樣很容易被別有用心的人用來執(zhí)行系統(tǒng)命令，刪除關鍵系統(tǒng)文件。

六、eval()函數(shù)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(shù)常見作用有

1、計算字符串中有效的表達式，并返回結果

>>> 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]