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:
l1 = []
l2 = l1
l1.append(1)
print l2
See now? The same happens inside your 'nodes'.
Ciao
----
FB
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