Comparison with False - something I don't understand

[Steven D'Aprano]

On Sun, 05 Dec 2010 04:13:02 +0000, Tim Harig wrote:

> Anything it is an obvious
> error *should* throw an exception.

Well, maybe... there are good use-cases for returning a sentinel. E.g. 
str.find, or the use of quiet NANs in IEEE floating point and decimal 
maths.

NANs and INFs in floating point maths are a good example of the right way 
to do it. If you forget to check for a NAN, it will propagate through 
your calculation. INF will, under some circumstances where it is 
mathematically valid to do so, will disappear leaving a normal result. 
This means you only need to check your result at the very end of the 
calculation, not after every step.

str.find is more troublesome, because the sentinel -1 doesn't propagate 
and is a common source of errors:

result = string[string.find(delim):]

will return a plausible-looking but incorrect result if delim is missing 
from string. But the convenience and familiarity of str.find means it 
will probably be around forever.


-- 
Steven