What is a function parameter =[] for?

[fl]

On Wednesday, November 18, 2015 at 5:38:45 PM UTC-5, fl wrote:
> On Wednesday, November 18, 2015 at 5:12:44 PM UTC-5, Ian wrote:
> > On Wed, Nov 18, 2015 at 2:08 PM, fl <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.
> 
> Thanks. The amazing thing to me is that the following two line codes:
> list1 = eList(12) 
> list2 = eList('a')
> 
> will have both list1 and list2 the same cascaded values:
> 
> list1
> Out[2]: [12, 'a']
> 
> list2
> Out[3]: [12, 'a']
> 
> I have known object concept in Python.
> 1. Why do they have the same list value?
>  Function eList must be for this purpose?
> 2. If I want to have two separate lists, how to avoid the above result?
>  Function eList is not for this purpose?
> 
> Thanks again.

After several trials, I find that the cascade list is caused by the second
function parameter absent. It is interesting. Thanks.