python-repr()和val()函数

1. repr() 函数将对象转化为供解释器读取的形式。

语法

以下是 repr() 方法的语法:

repr(object)

参数

• object -- 对象。

返回值

返回一个对象的 string 格式。

str和repr都是用来将数字，列表等类型转化为字符串的形式，但不同之处在于str更加类似于C语言中使用printf输出的内容，而repr输出的内容会直接将变量的类型连带着表现出来，从下图可以看出，对明显带有类型标志的变量而言，str和repr的转换具有明显的差别，如long型数字和字符串的‘’符号，而对于并没有非常大区别的记录数据如整型数字，二者并没有太大的差别。

 

2. 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

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

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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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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]

e.g. Your task is to define the following two methods for the Coordinate class:

1. Add an __eq__ method that returns True if coordinates refer to same point in the plane (i.e., have the same x and y coordinate).

2. Define __repr__, a special method that returns a string that looks like a valid Python expression that could be used to recreate an object with the same value. In other words, eval(repr(c)) == c given the definition of __eq__ from part 1.

class Coordinate(object):
    def __init__(self,x,y):
        self.x = x
        self.y = y

    def getX(self):
        # Getter method for a Coordinate object's x coordinate.
        # Getter methods are better practice than just accessing an attribute directly
        return self.x

    def getY(self):
        # Getter method for a Coordinate object's y coordinate
        return self.y

    def __str__(self):
        return '<' + str(self.getX()) + ',' + str(self.getY()) + '>'

    def __eq__(self, other):
        # First make sure `other` is of the same type 
        assert type(other) == type(self)
        # Since `other` is the same type, test if coordinates are equal
        return self.getX() == other.getX() and self.getY() == other.getY()

    def __repr__(self):
        return 'Coordinate(' + str(self.getX()) + ',' + str(self.getY()) + ')'

Test: equal 1

Output:

> print(c1)
<1,-8>
> print(c2)
<1,-8>
> print(c1 == c2)
True

Test: equal 2

Output:

> print(c1)
<20,20>
> print(c2)
<20,20>
> print(c1 == c2)
True

Test: not equal 1

Output:

> print(c1)
<-16,-4>
> print(c2)
<14,20>
> print(c1 == c2)
False

Test: not equal 2

Output:

> print(c1)
<7,13>
> print(c2)
<-2,-1>
> print(c1 == c2)
False

Test: repr

Output:

> print(c1)
<17,38>
> print(repr(c1))
Coordinate(17,38)

Test: repr randomized

Output:

> print(c1)
<-12,-20>
> print(repr(c1))
Coordinate(-12,-20)