ValueError: negative number cannot be raised to a fractional power

问题:

When I tried this in terminal

>>> (-3.66/26.32)**0.2 

I got the following error

Traceback (most recent call last):   File "<stdin>", line 1, in <module> ValueError: negative number cannot be raised to a fractional power 

However, I am able to do this in two steps like,

>>> (-3.66/26.32) -0.13905775075987842 >>> -0.13905775075987842 ** 0.2 -0.6739676327771593 

Why this behaviour? What is the way to solve this in single line?

回答1:

Raising to a power takes precedence over the unary minus sign.

So you have -(0.13905775075987842 ** 0.2) and not (-0.13905775075987842) ** 0.2 as you expect:

>>> -0.13905775075987842 ** 0.2 -0.6739676327771593 >>> (-0.13905775075987842) ** 0.2 Traceback (most recent call last):   File "<stdin>", line 1, in <module> ValueError: negative number cannot be raised to a fractional power 

If you want it to work you should write (-3.66/26.32 + 0j)**0.2

>>> (-3.66/26.32 + 0j)**0.2 (0.5452512685753758+0.39614823506888347j) 

Or switch Python 3 as noted by @TimPietzcker.

回答2:

Switch to Python 3 which automatically promotes the result to a complex number:

>>> (-3.66/26.32)**0.2 (0.5452512685753758+0.39614823506888347j) 

文章来源: ValueError: negative number cannot be raised to a fractional power