bieff...@gmail.com wrote:
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
[snip]
It's not really 'compile' time. 'def' is a _statement_, not a
declaration. Its effect is to create a function and bind it to a name in
the current namespace. Any default parameters are evaluated when the
function is created and they are shared by all the function calls.
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