Is this unpythonic?

[Frank Millman]

"Steven D'Aprano" <steve+comp.lang.python at pearwood.info> wrote in message 
news:554c8b0a$0$12992$c3e8da3$5496439d at news.astraweb.com...
> On Fri, 8 May 2015 06:01 pm, Frank Millman wrote:
>
>> Hi all
>>
[...]
>>
>> However, every time I look at my own code, and I see   "def x(y, z=[]):
>> ....."   it looks wrong because I have been conditioned to think of it as
>> a gotcha.
>
> It is a gotcha, and a code smell.
>
> http://www.joelonsoftware.com/articles/Wrong.html
>

Interesting read - thanks.

> You can use it, but with care: code smells aren't necessarily wrong, they
> just need to be looked at a little more carefully.
>
>
>> Would it be more pythonic to change them all to use the alternative
>> "z=None", or is it ok to leave it as it is? Or to phrase it differently,
>> how would an experienced pythonista react on seeing this when reviewing 
>> my
>> code?
>
> I would change it to z=None *unless* you actually required the list to be
> mutated, e.g. if you were using it as a cache.
>

Ok, you and Joel have convinced me. I will change it to z=None.

> Does z have to be a list? Could you use an empty tuple instead?
>
> def x(y, z=()): ...
>

That was Chris' suggestion as well (thanks Chris).

The idea appealed to me, but then I found a situation where I pass in a 
dictionary instead of a list, so that would not work.

Replacing them all with None is cleaner and, I now agree, more pythonic.

Thanks for the good advice.

Frank