Weird Python behaviour

Benjamin Kaplan benjamin.kaplan at
Tue Aug 10 16:43:22 CEST 2010

On Tue, Aug 10, 2010 at 4:58 AM, Jonas Nilsson <jmnc at> wrote:
> Hello,
> Lets say that I want to feed an optional list to class constructor:
> class Family():
>        def __init__(self, fName, members = []):
>                self.fName = fName
>                self.members = members
> Now, lets add members to two different instances of Family:
> f1 = Family("Smith")
> f1.members.append("Bill")
> f2 = Family("Smithers")
> f2.members.append("Joe")
> Finally, lets look at the members in the Smithers family:
> print f2.members
> output: ['Bill', 'Joe']
> Why on earth is the output ['Bill', 'Joe']!? Is there a simple solution that
> separates f1 and f2 without forcing me to write code for the special case
> when you don't feed members to the __init__()-function?
> /Jonas

In python, a function definition is an executable statement, not a
declaration. Default args only get evaluated once- when the function
is first created. You have to use a sentinel value and create the
object inside the function if you want it to get executed every time.

class Family():
       def __init__(self, fName, members = None):
               if members is None :
                   members = []
               self.fName = fName
               self.members = members

If None is a valid argument to this function, then make a dummy object
and check identity (to make sure it's *that* object and not just
something that evaluates equal).

sentinel = object()
def something_that_accepts_none(foo = sentinel) :
    if foo is sentinel :
       foo = {}
    # other stuff

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