Short-circuit Logic

[Steven D'Aprano]

On Sun, 26 May 2013 04:11:56 -0700, Ahmed Abdulshafy wrote:

> Hi,
> I'm having a hard time wrapping my head around short-circuit logic
> that's used by Python, coming from a C/C++ background; so I don't
> understand why the following condition is written this way!
> 
>      if not allow_zero and abs(x) < sys.float_info.epsilon:
>                 print("zero is not allowed")

Follow the logic.

If allow_zero is a true value, then "not allow_zero" is False, and the 
"and" clause cannot evaluate to true. (False and X is always False.) So 
print is not called.

If allow_zero is a false value, then "not allow_zero" is True, and the 
"and" clause depends on the second argument. (True and X is always X.) So
abs(x) < sys.float_info.epsilon is tested, and if that is True, print is 
called.

By the way, I don't think much of this logic. Values smaller than epsilon 
are not necessarily zero:

py> import sys
py> epsilon = sys.float_info.epsilon
py> x = epsilon/10000
py> x == 0
False
py> x * 3 == 0
False
py> x + epsilon == 0
False
py> x + epsilon == epsilon
False

The above logic throws away many perfectly good numbers and treats them 
as zero even though they aren't.


> The purpose of this snippet is to print the given line when allow_zero
> is False and x is 0.

Then the snippet utterly fails at that, since it prints the line for many 
values of x which can be distinguished from zero. The way to test whether 
x equals zero is:

x == 0

What the above actually tests for is whether x is so small that (1.0+x) 
cannot be distinguished from 1.0, which is not the same thing. It is also 
quite arbitrary. Why 1.0? Why not (0.0001+x)? Or (0.00000001+x)? Or 
(10000.0+x)?



-- 
Steven