Is this unpythonic?

[Steven D'Aprano]

On Fri, 8 May 2015 06:01 pm, Frank Millman wrote:

> Hi all
> 
> I have often read about the gotcha regarding 'mutable default arguments'
> that frequently trips people up.
> 
> I use them from time to time, but I have never had a problem. I have just
> delved a bit deeper to see if I am skating on thin ice.
> 
> AFAICT my usage is safe. If I use a list as an argument, I only use it to
> pass values *into* the function, but I never modify the list. So if I omit
> the argument, the original default empty list is used, otherwise the list
> that I pass in is used.
> 
> 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

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.

Does z have to be a list? Could you use an empty tuple instead?

def x(y, z=()): ...



-- 
Steven