Default if none

Steven D'Aprano steve at REMOVE-THIS-cybersource.com.au
Fri Apr 30 05:22:12 CEST 2010


On Thu, 29 Apr 2010 04:12:29 -0700, Astley Le Jasper wrote:

> ... oh ... that simple. Now I feel dumb.

It's really difficult to tell what you're talking about, but I assume 
that you're talking about Chris' solution:

x or y or z

Be careful, as Chris' solution is rather risky (read his disclaimer 
again). And the code you give does NOT solve the question you ask. Have 
you tested it with something like this?

>>> default_if_none(0) == 0
False
>>> default_if_none(0) is None
True

Your code destroys perfectly legitimate values and replaces them with 
None. That's almost certainly not what you actually want. Chris' solution 
isn't much better, but he warned you about it. Read his post carefully.

>>> x, y, z = 0, None, None
>>> x or y or z == 0
False
>>> x or y or z is None
True


What are you trying to accomplish? Return the first arg that is not None, 
otherwise return None? You need to code your test more carefully, by 
testing for None and nothing but None:

def default_if_none(*args):
    for arg in args:
        if arg is not None:
            return arg

You don't need the "return None" at the end, because all Python functions 
do that automatically.

Here is a one-liner to do it:

(filter(lambda x: x is not None, args) or [None])[0]

but frankly the one-liner is ugly and I wouldn't use it. Just use the 
function.



-- 
Steven



More information about the Python-list mailing list