default value for __init__ doesn't work

Benjamin Kaplan benjamin.kaplan at
Sat Sep 11 07:14:54 CEST 2010

On Sat, Sep 11, 2010 at 12:38 AM, 人言落日是天涯,望极天涯不见家 < at> wrote:
> Please look at below code snippet:
> class test():
>    def __init__(self, a, dic={}):
>        self.a = a
>        self.dic = dic
>        print('__init__ params:',a, dic)

This is a pretty popular mistake to make. Default arguments aren't
evaluated when you call the method. They're created when the method is
created (meaning when you first run the file and the class itself is
defined), and that's it. Because you do self.dic = dic, this means
that every instance of the object will receive the same dict object.
Change it for one object, and the change will show up in all of them.
The solution to this is to use a sentinel value, like None

def __init__(self, a, dic=None) :
    if dic is None :
        self.dic = {}
    else :
        self.dic = dic

If None is a valid value for the parameter, make a sentinel object and use that

sentinel = object()
def __init__(self, a, dic=sentinel) :
    if dic is sentinel : #you want to use is here, not ==

More information about the Python-list mailing list