Python 中 eval 函数的神奇用法
Python 中 Eval 函数的用法
eval(str)函数很强大，官方解释为：将字符串str当成有效的表达式来求值并返回计算结果。所以，结合math当成一个计算器很好用。
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
Python 3.6.8 |Anaconda, Inc.| (default, Dec 29 2018, 19:04:46) Type 'copyright', 'credits' or 'license' for more information IPython 7.4.0 -- An enhanced Interactive Python. Type '?' for help. In [1]: eval('1+1') Out[1]: 2 In [2]: eval('pow(2,2)') Out[2]: 4 In [3]: n = 10 In [4]: eval('n+1') Out[4]: 11 In [11]: def hello(): ...: print('hello') ...: In [12]: eval('hello()') hello In [13]: eval('hello')() hello
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Python 3.6.8 | Anaconda , Inc . | ( default , Dec 29 2018 , 19 : 04 : 46 )
Type 'copyright' , 'credits' or 'license' for more information
IPython 7.4.0 -- An enhanced Interactive Python . Type '?' for help .
In [ 1 ] : eval ( '1+1' )
Out [ 1 ] : 2
In [ 2 ] : eval ( 'pow(2,2)' )
Out [ 2 ] : 4
In [ 3 ] : n = 10
In [ 4 ] : eval ( 'n+1' )
Out [ 4 ] : 11
In [ 11 ] : def hello ( ) :
. . . : print ( 'hello' )
. . . :
In [ 12 ] : eval ( 'hello()' )
hello
In [ 13 ] : eval ( 'hello' ) ( )
hello
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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)
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>>> 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 )
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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(eval(`list1`)) >>> a = eval(`list1`) >>> a [1, 2, 3, 4, 5]
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>>> 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 ]
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