bieff...@gmail.com writes: > On 14 Mar, 17:31, Dan Davison <davi...@stats.ox.ac.uk> wrote: >> I'm new to python. Could someone please explain the following behaviour >> of a recursive data structure? >> >> def new_node(id='', daughters=[]): >> return dict(id=id, daughters=daughters) >> > > Most probably, here is the problem : try this instead: > > def new_node(id='', daughters=None): > if not daughters: daughters = [] > return dict(id=id, daughters=daughters) > > This is one of the less intuitive points in python: default values are > evaluated only once, > at 'compile' time I think. So when you call twice 'new_node' without > specifying the daughters > parameters, both dict will have the _same_ list. Hence chaos > > In other words, it is exactly as if you wrote: > > EmptyList = [] > def new_node(id='', daughters=EmptyList): > return dict(id=id, daughters=daughters) > > See the problem now? If not try this:
Yes, that's very clear. Thanks for all the answers on this thread. Dan > > l1 = [] > l2 = l1 > l1.append(1) > print l2 > > See now? The same happens inside your 'nodes'. > > > Ciao > ---- > FB > -- > http://mail.python.org/mailman/listinfo/python-list -- http://mail.python.org/mailman/listinfo/python-list
