== Putting a pleasant Python surprise to use

Although I've been programming in Python for a few years now, it keeps
surprising me with little bits and pieces. Here's a neat Python language
feature that I recently used for the first time (discovered originally
through Bram Cohen's LiveJournal).

A common programming pattern is 'search for a something to work on, but
stop if you don't find anything'. In Python one might write it something
like this (taken more or less from DWiki's source):

>  found = False
>  for dir in utils.walk_to_root(curdir):
>  	page = dir.child("__readme")
>  	if page.exists():
>  		found = True
>  		break
>  if not found:
>  	return ''
>  # Go on to use the __readme file we found in some directory.

Python allows you to put 'else' conditions on loops (both _for_ and
_while_ loops); the else condition is executed if the loop completed
instead of being _break_'d from. This lets us simplify this pattern down
to:

>  for dir in utils.walk_to_root(curdir):
>  	page = dir.child("__readme")
>  	if page.exists():
>  		break
>  else:
>  	return ''

If there's no ((__readme)) file to be found from the current directory
up to the root, we just return nothing; otherwise, we'll process it.
This DWiki code is the first occasion I've had to use this feature since
I discovered it, and I'm pleased to finally have been able to.

(As you can now see, not *all* the entries in this blog are going to
be long and meandering.)