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From: "Frank Millman" <frank@chagford.com>
Subject: Re: Is this unpythonic?
Date: Sat, 9 May 2015 07:56:54 +0200
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"Steven D'Aprano" <steve+comp.lang.python@pearwood.info> wrote in message 
news:554cd119$0$12977$c3e8da3$5496439d@news.astraweb.com...
> On Fri, 8 May 2015 08:53 pm, Frank Millman wrote:
>
>>> 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.
>
>
> Why wouldn't it work? If it worked with an empty list, it will probably 
> work
> with an empty tuple instead.
>

Sorry, I should have been more explicit. In the case of a dictionary, I used 
'def x(y, z={}'

I have not checked, but I assume that as dictionaries are mutable, this 
suffers from the same drawback as a default list.

Unlike a list, it cannot be replaced by an empty tuple without changing the 
body of the function.

Dave's suggestion would have worked here -

    EMPTY_LIST = []
    EMPTY_DICT = {}

But as I have decided to use the None trick, I use it for a default 
dictionary as well.

Frank