pitfall for your amusement

[Erik Max Francis]

"Russell E. Owen" wrote:

> The default value of alist will not stay [] (an empty list) but
> instead
> is affected by whatever you pass in for "alist". Very tricky and
> unpleasant.
> 
> Here's my usual solution:
> def okfunc(alist=None):
>    alist = alist or []

If you're using None as a sentinel value, it's probably better to test
for None-ness (via alist is None) rather than test for truth, since many
things can be false which are not None.

It's unlikely that in this particular example it would pose a problem,
but in the more general case it could.  The idiom _I_ use is:

	def f(l=None):
	    if l is None:
	        l = []
	    ...

-- 
 Erik Max Francis / max at alcyone.com / http://www.alcyone.com/max/
 __ San Jose, CA, USA / 37 20 N 121 53 W / &tSftDotIotE
/  \ If a thing is worth doing, then it is worth doing badly.
\__/ G.K. Chesterton
    ZOE / http://www.alcyone.com/pyos/zoe/
 A simple Python OpenGL rendering engine.