《Python Cookbook 3rd》笔记(3.7):无穷大与 NaN

巨輪 2020-11-17 00:24:44
Python


无穷大与 NaN

问题

你想创建或测试正无穷、负无穷或 NaN(非数字) 的浮点数。

解法

Python 并没有特殊的语法来表示这些特殊的浮点值,但是可以使用 float() 来创建它们。比如:

>>> a = float('inf')
>>> b = float('-inf')
>>> c = float('nan')
>>> a
inf
>>> b
-inf
>>> c
nan
>>>

为了测试这些值的存在,使用 math.isinf() 和 math.isnan() 函数。比如:

>>> math.isinf(a)
True
>>> math.isnan(c)
True
>>>

讨论

想了解更多这些特殊浮点值的信息,可以参考 IEEE 754 规范。然而,也有一些地方需要你特别注意,特别是跟比较和操作符相关的时候。

无穷大数在执行数学计算的时候会传播,比如:

>>> a = float('inf')
>>> a + 45
inf
>>> a * 10
inf
>>> 10 / a
0.0
>>>

但是有些操作时未定义的并会返回一个 NaN 结果。比如:

>>> a = float('inf')
>>> a/a
nan
>>> b = float('-inf')
>>> a + b
nan
>>>

NaN 值会在所有操作中传播,而不会产生异常。比如:

>>> c = float('nan')
>>> c + 23
nan
>>> c / 2
nan
>>> c * 2
nan
>>> math.sqrt(c)
nan
>>>

NaN 值的一个特别的地方时它们之间的比较操作总是返回 False。比如:

>>> c = float('nan')
>>> d = float('nan')
>>> c == d
False
>>> c is d
False
>>>

由于这个原因,测试一个 NaN 值得唯一安全的方法就是使用 math.isnan() ,也就是上面演示的那样。

有时候程序员想改变 Python 默认行为,在返回无穷大或 NaN 结果的操作中抛出异常。 fpectl 模块可以用来改变这种行为,但是它在标准的 Python 构建中并没有被 启用,它是平台相关的,并且针对的是专家级程序员。

版权声明
本文为[巨輪]所创,转载请带上原文链接,感谢
https://my.oschina.net/jallenkwong/blog/4720511

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