What is a function parameter =[] for?

[Antoon Pardon]

Op 19-11-15 om 00:22 schreef Chris Angelico:
> On Thu, Nov 19, 2015 at 10:14 AM, BartC <bc at freeuk.com> wrote:
>> On 18/11/2015 22:11, Ian Kelly wrote:
>>>
>>> On Wed, Nov 18, 2015 at 2:08 PM, fl <rxjwg98 at gmail.com> wrote:
>>>>
>>>> Hi,
>>>>
>>>> I have tried the below function and find that it can remember the
>>>> previous
>>>> setting value to 'val'. I think the second parameter has something on
>>>> this
>>>> effect, but I don't know the name and function of '=[]' in this
>>>> application.
>>>>
>>>> Could you explain a little to me?
>>>> Thanks,
>>>>
>>>>
>>>> def eList(val, list0=[]):
>>>>      list0.append(val)
>>>>      return list0
>>>> list1 = eList(12)
>>>> list1 = eList('a')
>>>
>>>
>>> The list0 parameter has a default value, which is [], an initially
>>> empty list. The default value is evaluated when the function is
>>> defined, not when it is called, so the same list object is used each
>>> time and changes to the list are consequently retained between calls.
>>
>>
>> That is really bizarre behaviour.
>>
>> So, looking at some source code, a default value for certain types is only
>> certain to be that value for the very first call of that function?
> 
> On the contrary, it is certain always to be that exact object.

No, he is talking about the value. Since the object can be mutated
and thus have an other value, his statement seems correct.

-- 
Antoon.