为什么比较具有相同值的两个属性类型“float”和“int”在 Python 中得到“False”？

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

但是如果我将 11 行的代码更改为 foo = Foo(2)，输出将变为：

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。我想我可能误解了 Python 中的一些 OOP 概念。请向我解释为什么会出现这种意外输出以及如何解决这个问题。

Answer 1

这与面向对象无关——它与计算机内部表示浮点数的方式和舍入误差有关。 http://floating-point-gui.de/basic/

此处的 Python 特殊性是浮点数的默认字符串表示形式，它将在比内部表示形式更少的小数位舍入它们以进行漂亮打印。

虽然，对于需要正确比较的人，尊重浮点数的比例，Python 引入了一个很好的机制 PEP 485，它在标准中添加了 math.isclose 函数图书馆。

Answer 2

除了 jsbueno 的正确解释外，请记住 Python 通常允许将 "basic types" 转换为自己。

即海峡（"a"）=="a"

因此，如果除了原因之外您还需要解决方法，只需将您的 int/float 混合转换为所有浮点数并测试它们。

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

输出：

a == b True

为什么比较具有相同值的两个属性类型“float”和“int”在 Python 中得到“False”？

Why comparing two attributes type `float` and `int` with the same values get `False` in Python?

python

floating-point-comparison