== Small details can matter (or: a little nifty Python bit)

It's common for Python routines that search for the position of
something in a bigger thing (such as characters in a string or list
entries in a list) to return -1 when they don't find anything.

I've written Python code for years without really thinking about
this. Clearly the routines have to do something when they fail,
whether it's raise an exception or return a marker value, and -1 is
both simple and not a valid index into an object.

Only recently (while writing some DWiki code) did it occur to me that
if what you want is 'everything after where this thing is, or the
entire object if it's not there', a return value of -1 is perfect
because you can just write:

>  	pos = thing.searchFor(whatever)
>  	tail = thing[pos+1:]

If _whatever_ is found in the thing, this clearly works. If it's not
found, _pos_ is -1 and the whole expression becomes '_thing[0:]_',
which in Python is a synonym for all of _thing_ (since Python objects
are indexed from zero).

There are certainly other plausible 'not found' marker values, for
example _None_ or _False_. But then this little bit wouldn't work
and you'd have to write more code.

There's another clever little property -1 has as an error return: it's
considered true in Python, not false. This is useful, because 0 is
considered false; that the 'not found' value is *not* false saves you
from writing superficially simple code like:

>  	pos = someString.find(somechar)
>  	if not pos:
>  		# do something on failure

If the 'not found' marker was considered false, this code would work
until the day _somechar_ was found at position 0, which might be quite
a while. Since it isn't, this code fails immediately and thus gets
fixed immediately.

I usually think of the choice of what to return as the 'not found'
value as something pretty simple and arbitrary; as long as it's an
invalid value and easily checked, it's not something that matters
much. But as I've just come to understand, sometimes the small details
really *can* matter, because if they're done right they make your life
simpler in little quiet ways.