On Sun, 03 Jan 2010 22:36:44 -0500, Roy Smith wrote:
> In article <pan.2010.01.04.03.30...@remove.this.cybersource.com.au>,
> Steven D'Aprano <ste...@remove.this.cybersource.com.au> wrote:
>
>> This last one can be *very* problematic. In the early 1990s, I was
>> programming using a callback API that could only return an integer. The
>> standard way of indicating an error was to return -1. But what happens
>> if -1 is a legitimate return value, e.g. for a maths function?
>
> One of the truly nice features of Python is the universally
> distinguished value, None.
What happens if you need to return None as a legitimate value?
Here's a good example: iterating over a list. Python generates an
exception when you hit the end of the list. If instead, Python returned
None when the index is out of bounds, you couldn't store None in a list
without breaking code.
So we produce a special sentinel object EndOfSequence. Now we can't do
this:
for obj in ["", 12, None, EndOfSequence, [], {}]:
print dir(obj) # or some other useful operation
The fundamental flaw of using magic values is that, there will always be
some application where you want to use the magic value as a non-special
value, and then you're screwed.
This is why, for instance, it's difficult for C strings to contain a null
byte, and there are problems with text files on DOS and CP/M (and Windows
under some circumstances) that contain a ^Z byte.
--
Steven
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