Question

让我们考虑下面的代码

代码：

#!/usr/bin/env python

class Foo():
    def __init__(self, b):
        self.a = 0.0
        self.b = b
    def count_a(self):
        self.a += 0.1

foo = Foo(1)
for i in range(0, 15):
    foo.count_a()
    print "a =", foo.a, "b =", foo.b, '"a == b" ->', foo.a == foo.b

输出：

a = 0.2 b = 1 "a == b" -> False
a = 0.4 b = 1 "a == b" -> False
a = 0.6 b = 1 "a == b" -> False
a = 0.8 b = 1 "a == b" -> False
a = 1.0 b = 1 "a == b" -> True
a = 1.2 b = 1 "a == b" -> False
a = 1.4 b = 1 "a == b" -> False
a = 1.6 b = 1 "a == b" -> False
a = 1.8 b = 1 "a == b" -> False
a = 2.0 b = 1 "a == b" -> False
a = 2.2 b = 1 "a == b" -> False
a = 2.4 b = 1 "a == b" -> False
a = 2.6 b = 1 "a == b" -> False
a = 2.8 b = 1 "a == b" -> False
a = 3.0 b = 1 "a == b" -> False

但如果我将代码11更改为foo = Foo(2)，则输出将转为：

a = 0.2 b = 2 "a == b" -> False
a = 0.4 b = 2 "a == b" -> False
a = 0.6 b = 2 "a == b" -> False
a = 0.8 b = 2 "a == b" -> False
a = 1.0 b = 2 "a == b" -> False
a = 1.2 b = 2 "a == b" -> False
a = 1.4 b = 2 "a == b" -> False
a = 1.6 b = 2 "a == b" -> False
a = 1.8 b = 2 "a == b" -> False
a = 2.0 b = 2 "a == b" -> False *
a = 2.2 b = 2 "a == b" -> False
a = 2.4 b = 2 "a == b" -> False
a = 2.6 b = 2 "a == b" -> False
a = 2.8 b = 2 "a == b" -> False
a = 3.0 b = 2 "a == b" -> False

您会看到输出a = 2.0 b = 2 "a == b" -> False非常奇怪。我想我可能会误解Python中的一些OOP概念。请向我解释为什么会发生意外输出以及如何解决这个问题。

Answer 1

这与面向对象无关 - 它与计算机在内部表示浮点数的方式以及舍入错误有关。 http://floating-point-gui.de/basic/

这里的Python特性是浮点数的默认字符串表示形式，它将在小于十字形位置的位置进行舍入，而不是用于漂亮打印的内部表示。

虽然对于需要正确比较的人来说，尊重浮点数的规模，Python已经引入了一个很好的PEP 485机制，它将math.isclose函数添加到标准库中。

Answer 2

除了jsbueno的正确解释之外，请记住Python通常允许转换＆＃34;基本类型＆＃34;对自己。

即。 str（＆＃34; a＆＃34;）==＆＃34; a＆＃34;

因此，如果除了原因之外还需要解决方法，只需将int / float mix转换为所有浮点并测试它们。

a = 2.0 
b = 2 
print "a == b", float(a) == float(b)

输出：

a == b True

为什么比较两个属性类型`float`和`int`与相同的值在Python中得到`False`？

2 个答案: