python math domain errors in math.log function

Question

I have to use Python math.log(x) function with values of x from (0, ..., 1). Sometimes x may be too close to zero, and Python gives me an error:

ValueError: math domain error

How can I know, what is the domain of definition of math.log function?

Answer 1

As long as your input is within the half-open interval (0, 1] (not including 0), you are fine. You can't be too close to zero:

>>> math.log(sys.float_info.min) -708.3964185322641 

So simply checking for exactly zero (maybe as the result of an underflow) should be enough, or alternatively catch the exception and handle it.

EDIT: This also holds for the denormal minimum floating point number:

>>> math.log(sys.float_info.min * sys.float_info.epsilon) -744.4400719213812