[Tutor] Question on implmenting __getitem__ on custom classes

[Steven D'Aprano]

On Tue, Apr 23, 2019 at 08:27:15PM +0530, Arup Rakshit wrote:

> >You probably want:
> >
> >      def __init__(self, list=None):
> >          if list is None:
> >              list = []
> >          self.list = list
> 
> That is really a new thing to me. I didn't know. Why list=None in the 
> parameter list is different than in the body of __init__ method? Can you 
> elaborate this?

Try running this code and see what happens:

def make_default():
    print("creating a new list")
    return []


def function(arg=make_default()):
    arg.append(1)
    return arg


Now call:


function()
function()
function()


and see if you can work out what is happening.

Hint: how many new lists are created? when are they created? 



-- 
Steven