python-repr()和val()函数，,1. repr()
python-repr()和val()函数，,1. repr()
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")852、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 theCoordinateclass:
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)) == cgiven 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)
python-repr()和val()函数
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