Rolf Wester wrote: > Hi, > > when defining: > > class A: > def __init__(self, l=[]): > self.l = l > a = A() > a.l.append(1111) > b = A() > print a.l > > I get the output > > [1111] > > instead of an empty list. I guess it's because the default value in > the constructor is constructed once and whenever the constructor is > called without l being specified.
Exactly right. See Python FAQ item 1.4.22 (http://www.python.org/doc/faq/general/#why-are-default-values-shared-be tween-objects) > My work around is: > > class A: > def __init__(self, l=None): > if l == None: > self.l = [] > else: > self.l = l > > Is there a way to take the first definition but force the constructor > to create a new empty list every time it is called? Not as far as I know. Worth reading the above FAQ as it also contains an interesting use of this side-effect. Dave. -- -- http://mail.python.org/mailman/listinfo/python-list
