Writing a string.ishex function

[Steven D'Aprano]

On Thu, 14 Jan 2010 18:36:12 +0000, MRAB wrote:


>> And here are your unit tests.  Every line should print "True".
>> 
>> print ishex('123') is True
>> print ishex('abc') is True
>> print ishex('xyz') is False
>> print ishex('0123456789abcdefABCDEF') is True
>> print ishex('0123456789abcdefABCDEFG') is False
>> 
> Don't use 'is', use '=='.

I beg to differ. In this case, since ishex is intended to return True or 
False and not 1.0 or Decimal(0), the correct test for the purposes of 
unit-testing is the identity test, not equality.

However, for the purposes of *displaying* the result, rather than saying:

print ishex('123') is True

one should obviously say:

print ishex('123') is True is True

No, wait, that should be:

print ishex('123') is True is True is True

No wait, this is better:

print ishex('123') is True is True is True is True

*wink*

Seriously, just say print ishex('123'). It already returns True or False, 
you don't need to compare it to True or False to get a True or False 
answer.


> BTW, ishex('') should return False.


Only if you want to be inconsistent with other isFoo string functions:

>>> ''.isalpha()
False
>>> ''.isupper()
False
>>> ''.islower()
False
>>> ''.isspace()
False


Also, if ishex(s) returns True, then a reasonable person will expect that 
calling int(s, 16) should succeed. But if s is the empty string, it will 
fail.


-- 
Steven