[issue18842] Add float.is_finite is_nan is_infinite to match Decimal methods

[Steven D'Aprano]

Steven D'Aprano added the comment:

On Sun, Jan 29, 2017 at 08:23:05AM +0000, Martin Panter wrote:
> Why do you name the methods is_finite() etc with underscores, when the 
> existing methods math.isfinite() etc do not have underscores? Seems it 
> would add unnecessary confusion.

The idea is to enable duck-typing between float and Decimal. Instead of:

if isinstance(x, float):
    math.isfinite(x)
else:
    x.is_finite()

we can just say

x.is_finite() 

on both floats and decimals.

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<http://bugs.python.org/issue18842>
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