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?

Asked By: Felix

||

Source

Answer 1

You are going over the supported precision, use Decimal class instead.

>>> from math import log
>>> from decimal import Decimal

>>> d = Decimal('1E-1024')
>>> log(d)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: math domain error
>>> d.ln()
Decimal('-2357.847135225902780434423250')

Answered By: ismail

Answer 2

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

Answered By: Sven Marnach

python math domain errors in math.log function

Question:

Answers: