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

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]

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

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).
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)