# Default if none

Discussion in 'Python' started by Astley Le Jasper, Apr 29, 2010.

1. ### Astley Le JasperGuest

I realise I could roll my own here, but I wondered if there was an
inbuilt version of this?

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>.

def default_if_none(*args):
for arg in args:
if arg:
return arg
return None

x = None
y = 5
z = 6

print default_if_none(x,y,z)

>> 5
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

Astley Le Jasper, Apr 29, 2010

2. ### Chris RebertGuest

On Thu, Apr 29, 2010 at 3:16 AM, Astley Le Jasper
<> wrote:
> I realise I could roll my own here, but I wondered if there was an
> inbuilt version of this?
>
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>.

> def default_if_none(*args):
> Â  Â for arg in args:
> Â  Â  Â  Â if arg:
> Â  Â  Â  Â  Â  Â return arg
> Â  Â return None
>
> x = None
> y = 5
> z = 6
>
> print default_if_none(x,y,z)

If none of the potential values are considered boolean false:

print x or y or z

Cheers,
Chris
--
http://blog.rebertia.com

Chris Rebert, Apr 29, 2010

3. ### Astley Le JasperGuest

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

Thanks!

ALJ

Astley Le Jasper, Apr 29, 2010
4. ### Steven D'ApranoGuest

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

Steven D'Aprano, Apr 30, 2010