Method's default parameter values are evaluated *once*

Question

I\'ve found a strange issue with subclassing and dictionary updates in new-style classes:

Python 2.6.2 (r262:71605, Apr 14 2009, 22:40:02) [MSC v.1500 32 bit          


        

Answer 1

Your problem is in this line:

def __init__(self, props={}):

{} is an mutable type. And in python default argument values are only evaluated once. That means all your instances are sharing the same dictionary object!

To fix this change it to:

class a(object):
    def __init__(self, props=None):
        if props is None:
            props = {}
        self.props = props